Table of Contents
1. Executive Summary
2. Review of VAMOS Science Objectives
2.1 The American Monsoon Systems
2.2 Processes of the Monsonn Systems
2.3 Predictability and Prediction
3. Review of the Current Related Field Programmes
3.1 LBA
3.2 GCIP
3.3 PACS
3.4 EPIC
3.5 CORC
3.6 PIRATA
3.7 IAI
3.8 SACC
3.9 PACS-SONET
3.10 DEPROAS
3.11 PRECURSOR
3.12 WMO Tropical Meteorology Reseach Programme
3.13 ECLAT and ECOP
3.14 GOOS
3.15 GOOS-BRAZIL
4. Proposed Field Programmes
4.1 The Central American poleward Low Level Jet
4.2 Integrated Observing System
5. Reports of Workshop Working Groups on the Monsoon Systems
5.1 Summary of North and Centra American Monsoon
Systems Discussion
5.2 Summary of Workshop Working Group on the South
American Monsoon System Discussion
6. Short Reports
6.1 Lessons from the NCEP Reanalysis
6.2 Influence of Atlantic SSTs on precipitation
over South America
6.3 Moisture Flow over North America
6.4 SCSMEX
7. Plenary discussionof the monsoon systems
7.1 VAMOS Field Programmes Focusing on
Ocean-Atmosphere
Processes
7.2 VAMOS Field Programmes Focusing on
Land-Atmosphere
Processes
8. VAMOS: Thematic Foci and Working Groups
8.1 Process Studies - now underway and running
through
2001
8.2 Process Studies - planned to follow soon, up
to 2005
8.3 Process Studies - under discussion for the
future,
up to 2010
8.4 Data resources
8.5 Enhanced monitoring
8.6 Prospects for space-based observations during
VAMOS
9. Agency remarks
10. CLIVAR VAMOS Panel
11. Acknowledgements
12. References
1. Executive Summary
A joint VAMOS/PACS Workshop of Field Programmes and the first Meeting of the CLIVAR VAMOS Panel were held at the Instituto Oceanografico of the University of São Paulo, São Paulo, Brazil on March 30-April 2, 1998. The intent was to further focus the development of plans for field work to be done as part of VAMOS, to co-ordinate monsoon research programmes under consideration by the nations in the Americas, and to establish links between investigators planning to work in the ocean, the atmosphere, and on the land. The workshop was supported by International CLIVAR and drew about 60 participants from 10 countries. This document is a report of that workshop.
The workshop started with welcoming remarks by the chair of the CLIVAR VAMOS Panel, Prof. C. R. Mechoso, the co-chairs of the organizing committee, Drs. C. Nobre and R. Weller, and the local host, Dr. E. Campos. These speakers emphasized that the major goals of the workshop were to identify the scientific problems for a study of the American monsoon systems, to review the existing and already planned field programmes and process studies, to identify opportunities for and encourage collaboration among monsoon-related programmes in the Americas, and to initiate the planning of field programmes and process studies under VAMOS. The workshop continued with a series of talks that reiterated the science issues and objectives of the programme from various perspectives.
Three overview presentations about the American Monsoon Systems were given. For the North American Monsoon System (NAMS) the following basic questions were reviewed: What data is required to study the physical mechanisms for the system's life cycle, and to what extent is this life cycle captured in global and regional models? What are the relative roles of internal atmospheric dynamics, remote boundary forcing, and local and regional land surface forcing in determining the interannual and intraseasonal variability of the system? What are the dominant factors responsible for determining the interannual variability of the monsoon onset?
The Central American Monsoon System (CAMS) is not as well defined as NAMS in terms of calendar dates and distinct transitions, though the seasonal variations are quite distinct on the Pacific side of Central America. A basic question related to the mechanism of rainfall variability is whether the SST variations in the eastern Pacific warm pool exert a fundamental control on the fluctuations in Inter-tropical Convergence Zone (ITCZ) precipitation over the region. The available observational datasets are not suitable to address these issues.
The South American Monsoon System (SAMS) develops over a land mass characterized by a large area at the equator, very high mountains to the west that effectively block air transport, and surface cover that varies from tropical forests over Amazonia to high altitude deserts over the Bolivian altiplano. Plentiful moisture supply from the Atlantic maintains a precipitation maxima over central Brazil. The combination of this heat source and the orography results in seasonal evolution of convection unique to this region. Furthermore, there is an important influence of latitude systems on the organization of tropical precipitation. Many important questions remain on the relative roles of orography, the Bolivian altiplano, the Brazilian planalto, the Andes mountains and tropical heat sources on regulating circulation features over South America. Modelling and theoretical studies have given partial answers to these questions, but validation of these results require observational confirmation with more complete data sets.
These talks were followed by a number of presentations about the role and importance of different processes (e.g., in atmosphere, ocean, land). Thereafter, the relevant field work relevant to VAMOS was reviewed which enabled the participants to establish focal points for VAMOS field programmes. The workshop participants then separated into two working groups, one focusing on the specific problems of NAMS and CAMS and the other on the aspects of SAMS followed by a plenary discussion of the monsoon systems.
The ocean-atmosphere and land-atmosphere interactions were then discussed in depth. In these sessions, attempts were made to step back from the specifics of individual experiments and to address linkages such as those among regions, programmes, and between the land, atmosphere, and ocean. For example, an effort was made to diagrammatically summarize the large scale moisture transports in the Americas and the adjacent oceans. This was a particularly fruitful discussion, as it laid the groundwork for developing hypotheses that could be tested by VAMOS field programmes. One example drawn from the session is the discussion of why the warm waters of the Gulf of Mexico and the Caribbean do not act more like a traditional warm pool and the hypothesis that convection over the Amazon region regulates convection over the warm pool. Further discussion of this topic raised questions about the contrast between the influence of the warm water found on the west coast of Central America and that found in the Gulf of Mexico and about the moisture fluxes into North America from these two regions. A second example is the discussion of the stratus clouds found off Peru and Chile and the possibilities that the convection over the altiplano in South America contributes to the atmospheric subsidence in the stratus deck region and that there is some transport through the Andes between the altiplano and the eastern Pacific. In view of the links between ENSO variability and climate in South America, there may thus be a possible feedback to the equatorial eastern Pacific by the Altiplano's influence on the stratus region, which in turn influences the cold tongue-warm pool region.
In the last session a number of action items were identified to keep the momentum up and the development of VAMOS field programmes moving forward. In the near term, completion of a workshop report and communication of the results of the workshop was flagged as important. Groups to facilitate writing a workshop report were identified. While the science thrusts of, and plans for, work on the North American monsoon and on the eastern tropical Pacific are maturing as the Pan American Climate Study (PACS) and the Eastern Pacific Investigation of Climate processes in the coupled ocean-atmosphere system (EPIC), the discussion identified the need to encourage further developments associated with the SAMS and the stratus. After a review of planned and proposed process studies issues of data sets and enhanced monitoring relevant to VAMOS were discussed. For the further planning of the VAMOS programme, the workshop recommended to establish five small working groups with a primary task to collect, assess and integrate the information about (a) process studies, (b) data sets, (c) sustained measurements, (d) stratus, and (e) the SAMS. With the exception of the SAMS Working Group, these groups have a limited lifetime to prepare the workshop report. The SAMS Working Group was tasked to organize a workshop about the specific aspects of the South American monsoon later in 1998 or early 1999.
The VAMOS Panel met for its first session after the workshop. There was general agreement on a planning approach based on phases, each of which will address the problems of capacity building in data-void regions of the Americas. The Panel reviewed the results of the workshop and made the following recommendations:
2. Review of VAMOS Science
Objectives
One of the major elements of CLIVAR's study of climate variability on seasonal to interannual time scales will be a study of the Variability of the American Monsoon Systems (VAMOS). VAMOS is identified as element G3 in CLIVAR planning documents (WCRP, 1998 - the CLIVAR Implementation Plan). The 1998 scientific goals of VAMOS and preliminary plans for approaching those objectives have been developed and are documented in the CLIVAR Implementation Plan.
2.1 The American Monsoon Systems (J.Paegle, T.Ambrizzi)
This part of the workshop was devoted to discussion of the monsoon circulations over the Americas. There were three presentations on North-American, Central-American and South-American monsoon systems (NAMS, CAMS and SAMS, respectively). A common feature to all these three monsoonal circulations is the complexity of their time variability, with important modulations in the diurnal, intraseasonal and interannual scales.
a) NAMS (W. Higgins)
The evolution of the NAMS appears to be well established. There is a development phase during the months of May and June, characterized by a weakening and northward shift of the extratropical storm track, with the initiation of the Mexican monsoon, increasing frequency of occurrence of low level jets east of the Rockies and strengthening of the upper troposphere monsoon high. This is followed by a mature phase (July and August) that sees the Mexican monsoon extending into Arizona and New Mexico, establishment of the warm season continental precipitation regime and an upper troposphere ridge located over the west/central U.S. The decay phase (September-October) sees a reversal of the development phase and slow weakening of the system. Dates of monsoon onset latitudinal progression were given based on precipitation data, from June 5 at 15°N to mid-July at 35°N. Monsoons with long periods of heavy rainfall after onset average out to anomalously wet monsoons.
NAMS exhibits interesting regional variations. For example, rainfall over SW Mexico is found to be in phase with rainfall in much of the monsoon region and out of phase with precipitation in the Great Plains. The onset date for the SW Mexico monsoon is highly correlated with interannual fluctuations in rainfall over the entire monsoon region. It is also found that wet/dry monsoons in the SW often follow winters characterized by dry/wet conditions in the Pacific northwest. Questions remain on the extent to which this latter association is due to season-to-season memory of the coupled ocean-atmosphere-land system.
There are different research programmes (such as GCIP, PACS and the joint GCIP/PACS) that contribute to VAMOS. The joint GCIP/PACS programme is based on the premise that there is a deterministic element in the year to year variability of summertime precipitation and temperature over North-America. It is known that time scales of variability of the North-American hydrological cycle range from intraseasonal to centennial. Examples are the 1998 U.S. midWest drought, the 1993 floods and the Dustbowl era of the 30's and 40's.
b) CAMS (V. Magaña)
Sea surface temperatures (SSTs) have an important effect in modulating the seasonal cycle in the vicinity of Central America. The summer rainy season over Southern Mexico, Central America and the Caribbean occurs from May through October, with strong local modulations due to orography. The seasonal precipitation exhibits a double maximum with a relative minimum during July and August (referred to as the mid-summer drought). Possible explanations for this feature involve interactions between SSTs off the western coast of Mexico, solar insolation, convection and cooling effects due to intense trade winds. Questions remain on mechanisms that modulate convection over the Intra Americas Sea (IAS). In particular, reasons for weak convective activity and low precipitation rate over the IAS warm pool are not well known. There are well defined interannual variations in precipitation over Mexico and Central America. Drought over this region (associated with enhanced subsidence) is found with an equatorward shift in the mean position of the ITCZ during El Niño years.
c) SAMS (P. Silva-Dias)
The conventional definition of a monsoon circulation requires a full seasonal reversal of low level winds. This is nowhere observed over South America. Nevertheless, a high pressure system develops over elevated terrain over Bolivia (the "Bolivian high") during the austral summer. The South Atlantic Convergence Zone (SACZ) is a main feature of summer convection over this continent. Other north-west to south-east convergence zones, such as those over the South Pacific (SPCZ) and east of Africa, are conspicuous features during austral summer which do not appear to have counterparts in the Northern Hemisphere. The morphology of South America is unique with a large continental area at the equator, very high mountains to the west that effectively block air transport, and surface cover that varies from tropical forests over Amazonia to high altitude deserts over the Bolivian altiplano. Plentiful moisture supply from the Atlantic maintains precipitation maxima over central Brazil. The combination of this heat source and the orography results in seasonal evolution of convection unique to this region. Furthermore, there is an important influence of mid-latitude systems organizing tropical precipitation. Many important questions remain on the relative roles of orography, the Bolivian altiplano, the Brazilian planalto, the Andes mountains and tropical heat sources (both over South America and other continents) on regulating circulation features over South America. Modelling and theoretical studies have given partial answers to these questions, but of the results obtained in those studies require observational confirmation with more complete data sets.
2.2: Processes of the Monsoon Systems (I. Wainer, P. Aceituno)
The annual cycle of the monsoon, or the onset and demise of the rainy season has led the population of the monsoon controlled regions to divide their lives between the rainy and the dry cycles. The physical processes that govern the onset and demise of the wet and dry cycles associated with the monsoons were once believed to be local in origin. The advancement of global observing systems and the understanding of large scale ocean-atmosphere interaction processes and its global impact brought a new perspective into the nature of the problem. The physical processes that govern the coupled ocean-atmosphere-land system are very complex. As pointed out by Webster et al. (1998) the monsoon may be thought of as a circulation responding to the annual cycle of solar heating in an interactive ocean-atmosphere-land system. In addition to the interactive elements of the monsoon (discussed below), there are influences from other climate systems such as the El Niño Southern Oscillation (ENSO), the Atlantic dipole and the extratropical regime. The monsoon systems are driven by the differential heating of the land and the ocean and are affected by land-surface processes and orography and moist processes, which determine the strength, location and duration of the rainy season.
a) The role of atmospheric processes (V. B. Rao)
This presentation addressed the differences between the strength and duration of the SAMS and NAMS. SAMS behaves similarly to the East-Asian Monsoon. However, the land-sea contrasts leading to the differential heating which drives the Asian monsoon do not happen in South America. The importance of the SACZ, and its land versus ocean controls was noted. The determination of moisture budgets associated with monsoon process is fundamental to understand the variability of the system.
b) The role of ocean processes (R. Weller)
To advance modelling and prediction of the monsoon systems it is necessary to improve our knowledge of the seasonal to interannual variability of climate. To understand the monsoon system it is important to comprehend how the atmosphere and ocean communicate. This interaction is predominantly through the interfacial fluxes of heat, moisture and momentum.
This presentation emphasized that SST is a fundamental driver of the monsoon system and to access its variability one needs to know the complex interaction between the tropical SST and surface forcing on seasonal to interannual time-scales. To understand how tropical SSTs in the Americas affect climate variability in the American sector, one needs to improve our knowledge of the ocean processes that govern the SST variability: these include air-sea exchanges of heat, freshwater, momentum fluxes on broad ranges of time scales, plus three-dimensional advection processes from basin to local scale and horizontal and vertical mixing as well, on time-scales ranging from turbulent scale to interannual.
The presentation also addressed the ocean processes modelling questions within the PACS domain. The large scale questions associated with the connections of equatorial current system and the fluxes of heat, salt and freshwater between the ITCZ and the pacific cold-tongue were discussed. The discussion included the complexity of air-sea interaction and the role of high frequency forcing, coastal processes that modify SST and coupled processes studies such as the interaction of ocean currents with nearby orography forcing.
c) The role of land processes (R. Koster)
The basic elements of land-atmosphere interaction are the exchanges of moisture and energy between these two systems. Fluxes of moisture and heat from the land surface help determine the overlying distributions of atmospheric temperature, water vapour, precipitation, cloud properties, and hence even the downward radiative fluxes at the surface. This presentation emphasized the importance of interactive land surface parameterizations in a General Circulation Models (GCM). Inclusion of such processes leads to substantially greater interannual variability of precipitation over both tropical and mid-latitude land than does the inclusion of observed ocean temperature variations. The implication is that the understanding of continental precipitation variability including that associated with monsoon systems requires an understanding of how land surface energy and water balance anomalies influence local water recycling and the general circulation itself.
The nature of this feedback in a modelling system depends on the chosen representation of land and atmospheric processes and on the manner in which they are coupled. While some of these coupling issues are common to the study of ocean-atmosphere interaction, a unique difficulty in the treatment of land-atmosphere interaction is the extensive spatial heterogeneity found in land surface characteristics. Much current research is aimed at developing parsimonious representations of this heterogeneity that will allow operational land surface models to go beyond the standard one-dimensional representations of surface physics currently available.
d) The role of coupled atmosphere-ocean processes (P. Chang)
The importance of coupled ocean-atmosphere variability over the tropical Atlantic sector in monsoon rainfall over the neighbouring continents was already recognized in the early 1980s. In contrast to the Pacific, where equatorial SST anomalies dominate, off-equatorial SST anomalies in the tropical Atlantic appear to be most energetic on interannual-to-decadal time scales. These SST anomalies influence the temperature gradient near the equator, which has a strong impact on the position of ITCZ and consequently rainfall over the neighbouring continents. Changes in the SST gradient are accompanied by spatially coherent cross-equatorial surface wind anomalies and changes in the trade winds on both sides of the equator. The pattern of SST and wind anomaly suggests a positive air-sea feedback through surface heat flux. Although some modelling studies suggest that the air-sea interaction could lead to decadal oscillation of SST in the tropical Atlantic, major uncertainties remain in our understanding of the fundamental processes of air-sea interactions in the tropical Atlantic. For example, because of lack of reliable observations, we do not know the exact relationships between the surface fluxes and SSTs. We also do not understand how important the tropical Atlantic SSTs are in driving the atmospheric circulation, and how much tropical Atlantic SST variability can be attributed to local air-sea interactions and how much can be attributed to remote influence of Pacific ENSO. An improved understanding of the coupled processes in the tropical Atlantic is essential for the development of satisfactory predictive models for the region. Advances are most likely through multidisciplinary process studies that connect the upper ocean and lower atmosphere.
2.3: Predictability and Prediction (A. Moura)
The International Research Institute for climate prediction (IRI) has been established through a co-operative agreement between NOAA/Office of Global Programs, Columbia University/Lamont-Doherty Earth Observatory, and the University of California, San Diego/Scripps Institution of Oceanography. The mission of the IRI is to assess and continually develop seasonal-to-interannual climate predictions; to produce the best and most useful climate forecast and prediction information on a routine basis; and to apply such information to the benefit of affected societies throughout the word. The IRI will address all aspects of end-to-end prediction, including model and forecast system development, experimental prediction, climate monitoring and dissemination, applications research, and training, in coordination and collaboration with the international climate research and growing network of applications centres and activities. The IRI involvement in the CLIVAR/VAMOS programme is meant to support the development of the scientific agenda as well as providing a framework for modelling and climate simulation and data assimilation studies to justify the observational systems such as GOOS, GCOS, PIRATA, LBA among others. The "end-to-end" approach taken by the IRI provides stimulus for national resources allocated to the above mentioned programmes and projects, via practical application demonstration projects in crucial areas such as agriculture, public health, water resources, as related to climate fluctuations and their prediction.
3. Review of Current Related Field Programmes
D. Enfield and V. Barros chaired a review of current field programmes relevant to VAMOS, with the notion of looking for ways to build upon these efforts, to establish links between them, and to see how they address VAMOS scientific objectives. Table 1 gives brief summaries of these field programmes.
3.1 LBA (J. Marengo and C. Nobre)
New knowledge and improved understanding of the functioning of the Amazonian system as an integrated entity and of its interaction with the Earth system will support development of national and regional policies to prevent the exploitation trends from bringing about irreversible changes in the Amazonian ecosystem. Such knowledge, in combination with enhancement of the research capacities and networks between the Amazonian countries will stimulate land managers and decision makers to devise sustainable alternative land use strategies along with forest preservation strategies.
The Land-Biosphere-Atmosphere experiment in the Amazonia (LBA) is designed to create this necessary new knowledge to understand the climatological, ecological, biogeochemical and hydrological functioning of Amazonia (Nobre et al., 1996). The interactions between land use and the physical climate will be studied over a range of space and time scales. The natural ecosystems will be studied at undisturbed forest and cerrado sites. Conversion of primary tropical forests to agriculture or to secondary vegetation in combination with the inputs from agricultural burning and forest clearing will be studied in relation to altered carbon and biogeochemical cycles in vegetation, soils and atmosphere. The net sources of the key greenhouse gases, oxidants and aerosols, and their transport in the atmosphere will be quantified. Changes in hydrological regimes will be studied in relation to land use change in river catchments. Land use will be studied in both the context of its physical and socioeconomic causes. Models will be parameterised and used to extrapolate experimental results in space and in time, and to predict the future functioning of Amazonia as an entity and its interactions with the Earth system.
The tropical land and atmosphere form a highly coupled system. The surface fluxes not only control the inputs of water and energy to the atmosphere, but depend themselves on the dynamical and thermodynamical properties of the planetary boundary layer through a chain of processes involving cloudiness, soil water content, evaporation, sub-surface hydrology and vegetative cover. Observing and modelling these coupled land surface/atmosphere processes is the first task of the Physical Climate component of LBA:
3.2 GCIP (R. Lawford)
The GEWEX Continental-scale International Project (GCIP) is a large observational and modelling climate study being carried out in the Mississippi River Basin under the auspices of the Global Energy and Water Cycle Experiment (GEWEX) and WCRP. The project receives funding from NOAA and NASA and involves more than 35 principal investigators from universities and government laboratories. The overall mission of the project is "to demonstrate the ability to predict changes in water resources on time scales up to seasonal and interannual as an integral part of a climate prediction system."
In order to pursue this overall mission the work has been divided into scientific foci and objectives. The basin scale challenge is to close water budgets for the Mississippi River Basin to within 15% uncertainty. In the southwestern part of the basin the focus has been land surface effects and the control of the LLJ on summer precipitation and hydrology in the semi-arid areas. Research in the North Central part of the basin addresses the hypothesis that the atmosphere is relatively decoupled from the atmosphere in the winter but significant coupling occurs as surface changes (e.g. snowmelt) in response to the onset of spring. In the eastern part of the basin the focus is on assessing the degree to which precipitation and runoff variations and extremes are predictable at climate time scales in complex topography. The project addresses these science issues by pursuing the five following objectives:
GCIP has made considerable progress on all the above objectives with the exception of objective No.4. Consequently more emphasis has been given to this topic in recent calls for proposals. GCIP is currently developing plans for the period beyond 2000. Priority areas for this time period include working more closely with the Pan-American Climate Studies (PACS) programme on prediction problems, addressing process and prediction issues related to water and energy budgets over all of the continental U.S.A. and participating in the Co-ordinated Enhanced Observing Period (CEOP) being organized by GEWEX.
3.3 PACS (S. Esbensen)
The Pan American Climate Studies (PACS) programme is a component of the U.S. CLIVAR/GOALS programme in the 1995-2005 time frame, providing a phenomenological context for some of the U.S. CLIVAR/GOALS research. The overall goal of PACS is to extend the scope and improve the skill of operational seasonal-to-interannual climate prediction over the Americas. Particular emphasis is placed on warm season rainfall, for which a predictive capability does not yet exist. In the context of PACS, climate prediction is concerned not only with seasonal mean rainfall and temperature, but also with the frequency of occurrence of significant weather events such as hurricanes or floods over the course of a season or seasons.
The scientific objectives of PACS are to promote a better understanding and more realistic simulation of (1) the boundary forcing of seasonal-to-interannual climate variations over the Americas, (2) the evolution of tropical sea surface temperature anomalies, (3) the seasonally varying mean climate over the Americas and adjacent ocean regions, (4) the time-dependent structure of the cold tongue/Intertropical Convergence Zone (ITCZ) complex (CTIC), and (5) the relevant land surface processes.
PACS scientific objectives map very closely onto VAMOS scientific objectives. PACS will therefore seek to contribute to the VAMOS programme through its activities in field studies, modelling, empirical studies and enhanced ocean-atmosphere monitoring, and will benefit from VAMOS coordination and facilitation of collaborative research in North and South America.
PACS field studies focus on different regions of the Pan-American climate system in sequence. During the 1995-2000 time period, field activities will focus on atmosphere-ocean interaction in the tropical eastern Pacific, in association with the ENSO cycle and the climatological-mean annual march. At the present time, PACS is conducting several pilot field studies and enhanced monitoring activities in the eastern Pacific and adjacent land areas (see Figs. 1 and 2). These activities will culminate in the PACS EPIC field programme, described by Bob Weller in more detail below.
During the 2000-2004 time frame, the emphasis will shift toward the tropical Atlantic Ocean where the sea surface temperature anomalies are more subtle and more diverse in terms of horizontal structure than in the Pacific, but no less important in terms of their influence upon precipitation in the adjacent continental regions.
While it is premature to propose specific PACS process studies in the tropical Atlantic or the Inter-American warm pool region at this time, we anticipate that pilot monitoring efforts will be required to establish seasonal-to-interannual variability in upper ocean structure and its relationship to wind stress and the surface heat fluxes. Of particular interest to PACS is the PIRATA array of 14 moored air-sea interaction buoys in the tropical Atlantic, described in more detail below by Joel Picaut. When combined with ocean observations from planned profiling float arrays, tide gauges, and expendable profilers of temperature and salinity, plus observations from satellite and conventional meteorological in situ observations, the PIRATA buoy array can help to provide the context for developing more focused tropical Atlantic field activities.
PACS contributions to field activities over land will be primarily through collaboration with the Global Energy and Water Experiment (GEWEX) and its regional programmes, with PACS supplying atmospheric modelling expertise and GEWEX the hydrological expertise. Much of this research will require the use of meso-scale models, applied in a climatological setting. Direct PACS support of land-based observations is expected to be limited.
3.4 EPIC (R. Weller)
The Eastern Pacific investigation of Climate processes (EPIC) in the ocean-atmosphere is a five year process study to improve the description and understanding of the CTIC and the stratus deck region. It focuses on investigating the key physical processes that must be parameterised for successful CTIC and stratus deck simulation with dynamical ocean-atmosphere models. The pilot phase of the Pan American Climate Study (PACS) conducted field work in the equatorial Pacific at 125°, where the prevailing September-October surface winds are easterly. EPIC will be located further to the east in the CTIC, principally between 95°W and 110°W, where southerly surface winds prevail at the equator in September-October. EPIC will also work in the stratocumulus deck region off the coasts of Peru and Chile.
3.5 CORC (D. Rogers)
The main objective of the Consortium on the Ocean's Role in Climate (CORC) is to "observe, deduce, and model climatically important variations in the global oceans that take place on time scales of a century of less. CORC seeks to understand the causes of both abrupt changes and quasi-periodic changes over periods from years to centuries and to assess their predictability". This document outlines the programme and focuses on the observational part of the programme most directly relevant to VAMOS.
The present phase of CORC, which is supported through 2001, is organized around the two scientific themes: 1) The Southern Ocean and Global Climate and 2) Interannual and Decadal Climate Variability in the Pacific. Each theme has a clear geographical focus for fieldwork and a surrounding area where analysis and modelling studies will be carried out. The focus of Southern Ocean fieldwork is the Weddell Sea where air-sea interaction affects both the deep thermohaline overturning circulation and the shallower intermediate-depth circulation that is involved in overturning circulations in all three subtropical oceans. Fieldwork in the Pacific theme, which is most directly relevant to VAMOS, focuses on the eastern tropics where large interannual variations of sea surface temperature mark the oceanic influence on the El Niño/Southern Oscillation (ENSO) cycle.
The Pacific programme is part of a broad international effort to understand and predict ENSO and its decadal variations of form and predictability. In particular the CORC observations are designed to supplement those TOGA observations now transitioning to an operational NOAA observing system, while CORC data-sensitive modelling addresses, in a research setting, the questions of integrating observations and dynamics that are addressed in an operational framework at NCEP. A team of observationalists, analysts, and ocean and atmospheric modellers from universities and NOAA laboratories are working together to make progress on this complex and important problem. The objectives of the program are:
The overall scientific objective is to describe, simulate, and understand climate change in the ocean-atmosphere system of the Pacific basin on interannual and decadal time scales. To make significant progress toward achieving this objective, CORC has identified a number of scientific questions to be answered.
The second part of this programme focuses on the development of data-sensitive modelling. This can be thought of as using ocean dynamics to add to the information content of the datasets by adding constraints that the time evolution should obey known physics. The approach is complementary to corrective data assimilation techniques such as the one applied by NCEP. In the latter scheme, a dynamical model is initialized with data and run in a predictive mode with observed forcing. It is then corrected to match new data (a non-physical form of forcing) and run for a subsequent time interval. The objective is prediction and the internal dynamical inconsistency due to the corrections is not an issue. Our alternative approach will be to determine what initial and boundary conditions and forcing fields are consistent with ocean observations via the dynamical forward problem. It is a form of inverse problem, where the initial guess (with known error variance) of boundary and forcing fields is adjusted so that the model results reproduce the data. Model testing hinges on whether the model can be made to reproduce the observations within their error bars by altering the boundary and forcing fields within their error bars. Clearly, the ability to test the model depends on the amount and quality of constraining data. If the fit is successful, then the resulting dynamically consistent evolution is a 4-D interpolation of the observations, using the dynamical constraints to contribute additional information beyond what is contained in the observations.
The third part of the programme focuses atmospheric modelling and retrospective data analysis on the problem of atmospheric forcing by SST. Atmospheric modelling is used to determine how tropical (via teleconnections) and extra-tropical SST anomalies modulate storm track activity. Global datasets of atmosphere and ocean variables, extending from 500 m in the ocean to 200 mb in the atmosphere, are being constructed for the period 1955 to the present. These are analysed to determine whether ENSO evolved differently during different periods of the modern record and, if so, whether different feedback mechanisms operated at different times or whether the feedbacks were modulated by background decadal change. A further exercise tests various hypotheses of feedback mechanisms for the observed decadal variability. These analyses provide a valuable perspective as well as datasets for eventual extension of the data-sensitive models from their initial annual/interannual focus out to decadal scale.
The research strategy includes five projects:
There is presently a substantial array of Pacific Ocean observations targeting climate variability. The broad-scale XBT network provides temperature profile data to 760 m depth at approximately 2° (along track) by monthly resolution along a number of commercial shipping routes. These are supplemented by quarterly eddy-resolving transects along a subset of the routes. The total number of XBT profiles averaged about 1,500 per month for the entire Pacific Ocean during 1995. An additional set of temperature profiles comes from the TAO network, with 7 moorings between 8°N and 8°S every 15° of longitude. These moorings, which also measure velocity profiles at the equatorial sites, provide the high temporal resolution, which is required to observe rapid propagating features in the equatorial waveguide. An array of surface drifters is maintained in the Pacific to measure both velocity and sea surface temperature. While the ongoing observing system represents a valuable resource and has provided a wealth of information on climate variability in the upper ocean, it has a number of deficiencies which limit further study. It is these deficiencies, which are addressed by CORC:
Sampling bias. Temperature profiles are presently collected only along established shipping routes or at TAO mooring locations. Large gaps, thousands of kilometres across, are unsampled. This problem is acute in the eastern tropical Pacific. The new array of profiling floats approximately doubles the number of temperature profiles per month in the target domain. More importantly, the floats spread into the gaps, providing data where there has been none.
Lack of salinity. The eastern Pacific is a region where there is both large forcing and important advection of freshwater. Strong evaporative forcing in the southeast creates a high salinity water mass that is subducted as it flows westward in the South Equatorial Current. This gives rise to a strong meridional gradient in salinity in the equatorial thermocline. Strong hemispheric asymmetry in freshwater forcing results from the large precipitation in the northern ITCZ, about 5 meters per year, with low salinity waters carried mainly eastward by the North Equatorial Countercurrent. Finally, equatorward currents along the coasts of both North and South America carry cool, low salinity waters of subpolar origin into the tropics. Variability in the distribution of freshwater anomalies is likely to be a crucial diagnostic of interannual to decadal E-P and circulation patterns. This signal is unobserved in the present network. Moreover, the salinity anomalies in this region are a significant part of fluctuating density fields and hence important to the accurate calculation of steric height and geostrophic shear. The salinity problem is approached in three ways - with salinity sensors on autonomous profiling floats, with XCTD deployments on high density XBT/XCTD transects, and through the development of a recoverable sensor to profile surface layer salinity from merchant vessels.
Depth limitation. Ocean-atmosphere interaction occurs through air-sea fluxes of heat and water, which are characterized by anomalies of SST. At low frequencies, however, these fluxes would destroy the SST anomaly that causes them unless ocean dynamics, particularly mixing and advection, can maintain the anomaly. Therefore understanding air-sea feedbacks on interannual to decadal time scales depends on knowing the underlying stratification and shear that determine the vertical exchanges and horizontal advection which, together with air-sea fluxes, govern SST evolution. Broadscale XBTs typically sample to 760 m. An extended depth range is required for calculations of geostrophic advection and also for interpreting the altimetric height field in relation to subsurface temperature structure. Autonomous profiling floats collect data to 1750 m, and the high density XBT/XCTD network is being used to test new XBTs with 2000 m range and improved fall rate determination.
Lack of subsurface velocity. Geostrophic calculations of velocity generally provide upper ocean circulation estimates sufficient for climate purposes. Direct observations of velocity are, however, needed near the equator, where geostrophy becomes unreliable, and where the depth extent of frequent sampling does not capture all the shear. Where variability plays an important role in transport, current-following observations can elucidate the mechanism. For example, WOCE sampling at 1000 m revealed an unexpectedly strong field of zonally polarized variable flow. Profiling floats in the CORC array are set to follow flow on the 15° isotherm, providing both patterns of flow on that surface and a reference for geostrophic calculations.
Resolution limitations. Even with the observational enhancements that CORC is implementing, the extended array will still have sparse resolution - both spatially with regard to broadscale profiling, and temporally with regard to the quarterly transects along high density XBT/XCTD tracks. These resolution limitations are diminished by combining subsurface profiling data with satellite altimetry.
Ekman layer physics. Directly driven Ekman currents are an important component of the upper ocean meridional cell which advects heat salt and mass from the equator. Equatorward of 15° latitude, under the Trades, the meridional Ekman currents tend to be twice as strong as the meridional geostrophic currents. Yet today, ocean models lack the ability to produce Ekman currents correctly. Comprehensive observations of upper ocean currents in many seasons and in many locations are being made to analyse why this deficiency occurs and to correct it.
The combination of temperature and salinity profile data with satellite altimetry is a major thrust of the CORC observations. If the profile data are used to determine the correlation of surface height with subsurface density structure, then it is possible to exploit the superior spatial and temporal resolution of the altimeter by projecting surface height onto the subsurface density and temperature fields. In this way, a significant fraction of the unresolved subsurface variance can be recovered. Specific sampling plans in the observational programmes are as follows:
Profiling floats. The profiling float array is building toward an operating inventory of 75 floats by deploying 25 per year, each float profiling on 15-day cycle and operating for 3 years. Floats profile to 1750 m depth but, between profiles, track flow on the 15° isotherm. Profiling to 1750 m extends the depth of samples for geostrophic shear calculations and provide temperature/salinity samples in a relatively stable water mass with which to correct any drift in the conductivity sensor. While the 15° isotherm does not outcrop in the cold tongue it is likely among the densest waters involved in equatorial upwelling and is a candidate for possible subduction links from the subtropics to the tropics.
Sounding Oceanographic Lagrangian Observer (SOLO) floats, developed with earlier CORC funding, are being used to improve reliability and operational efficiency over the predecessor ALACE float. Specific efforts at both SIO and WHOI are directed at improving longevity of the FSI conductivity. The complete array will deliver 1800 temperature and salinity profiles and velocity measurements per year. These data supplement the existing NOAA broadscale XBT network by filling unsampled areas, extending coverage to greater depth and providing critical salinity profiles and direct velocity observations. The main objectives of broadscale profiling are: (i) to determine patterns of variability in temperature, salinity and velocity, (ii) to determine the correlation statistics between surface height and subsurface density fields for use with satellite altimetry, and (iii) to provide interior data for constraining data-sensitive models.
High density XBT/XCTD transects. A Pacific-wide network of high density XBT/XCTD transects has been initiated and continued under the auspices of WOCE, CLIVAR and the first phase of the NOAA/SIO/LDEO Consortium. The present effort is continuing the previously initiated Consortium transects across the South Pacific (Valparaiso to Auckland) and across Drake Passage, and we will begin a new transect between Valparaiso and Honolulu. For the eastern and central tropical Pacific, the total network then includes zonal lines along the northern and southern boundaries of the domain and meridional lines crossing the equator in the central and eastern Pacific. The eddy-resolving XBT measurements, with sparse XCTDs, will target patterns of variability in meridional advection across the northern and southern boundaries and zonal advection in the tropics. Since 3 of these 4 lines already include multi-year time series (10 years for the central Pacific line), it is possible to begin immediately considering seasonal-to-interannual variability in the study domain. Objectives of high density sampling are: (i) to measure patterns of transport variability, (ii) to close oceanic heat and freshwater budgets (together with storage and air-sea fluxes) over large ocean areas, (iii) to provide boundary value information for the data-sensitive modelling, (iv) to determine the adequacy of model resolution through comparison of integral transports in models to those from highly resolved data, and (v) to determine correlation statistics between surface height and subsurface density fields.
An additional project is developing and testing a tethered recoverable T/S profiling instrument, with depth capability of about 100 m at 20 knot speed. Such an instrument would be highly valuable deployed in the high resolution network or on other volunteer ships. Low cost of profiles will enable tight spatial resolution in areas of significant T/S variability.
Surface drifters. The objectives of the surface drifter are to gather observations on the scales which resolve the seasonal and interannual evolution of the near surface currents and SST. This objective can be achieved because during the onset of the 1991-92 ENSO sufficient numbers of drifters were deployed in the central and western Pacific to capture the anomalous movement of water parcels to the east during that event. In the eastern tropical Pacific there have been sufficient observations of surface velocity to define the space scales of the seasonal cycle, but not the interannual variations. A concentrated effort is thus being made there, as was done in the western Pacific during TOGA/COARE. Drifters are being deployed on the known scales of seasonal signals in the anticipation that the interannual evolutions will be comparable.
Two sources of drifters are being utilized. The CLIVAR drifter array is being augmented to a total of 170 drifters in the region 30°N to 30°S, east of 150°W. Those deployed in the eastern equator for CLIVAR (about 70 units per year) rapidly diverge from the equator and transit through the regions of our study. Secondly, CORC deployments are being made from the XBT/VOS network and the TAO deployment vessel (100 units per year). Special consideration is given to the regions near the continental boundaries, where with the aid of the Colombian and Peruvian oceanographers, additional drifters can be deployed. An effective conduit of SST and subsurface anomaly dispersion from the equator to higher latitude is through Rossby and Kelvin wave processes, so a concentration of drifter augmentation is initially being done along the eastern boundary of the tropical Pacific.
Drifter data are used as the principal test data of the processes of the seasonally evolving Ekman dynamics in CORC models. Once sensitivity to local surface forcing via the Ekman layer is achieved, assimilation of the drifter data into models can occur and will form an additional constraint on model performance. Also, drifter data are used to test hypotheses of upper ocean thermal balances. During the warm phases of the interannual evolution, abnormally weak tropical currents are expected. In that case, local air-sea interaction will tend to be the principal process in the SST evolution. During the cold phases, the three-dimensional circulation in all of the tropical currents will be amplified and significant horizontal SST gradients will be established. It is anticipated that the surface advection of heat will play an increasingly important role in the evolution of SST.
IMET sensors. A realistic description of air/sea fluxes is central to understanding how the ocean responds to atmospheric forcing, and to understanding how the ocean feeds back to influence the atmosphere. Previous work has demonstrated that the primary direction of this interaction is atmosphere-forcing-ocean on monthly-interannual time scales in the extratropics, and an ocean model (with mixed layer) forced by monthly fluxes produces quite realistic anomalous upper ocean temperature variability at seasonal-decadal scales over a few decades in the North Pacific. Ocean-forcing atmosphere becomes more important in the tropics. However, the relative weight of ocean feedbacks in the subtropics (equatorward of 20°) is not very well understood, nor is their importance generally well known as time scales increase from seasonal to interdecadal. Modelled ocean currents driven by estimated wind stress have not been validated using observations. The anomalous components of evaporation vs. precipitation over the oceans is not well described and has not been examined in comparison to surface salinity fluctuations. The CORC study provides an opportunity to better test these interactions using a suite of estimated fluxes together with observed and modelled ocean/atmosphere data relating to the heat and fresh water variations.
IMET sensors are being deployed on selected volunteer vessels to provide quality control and research quality ground truth for the marine observations employed above, and as a basis for improved flux formulations. The period of concentration of the historical data is 1992-1997, although earlier years may be included when data are available. Monthly averages will be produced, but shorter intramonthly periods (weekly and perhaps daily) are also be considered. Surface marine weather observations (ships and buoys: wind, temperature, pressure, humidity, etc.) and satellite remote sensed data (wind (ERS and NSCAT), precipitation (microwave from NASA and combination microwave/IR), temperature, short wave radiation are being used. Comparisons between various derived products from the observed data will be conducted.
3.6 PIRATA (A. J. Busalacchi and C. A. S. Tanajura)
The interaction of the atmosphere and the tropical oceans is a subject of both scientific interest and societal importance, as demonstrated through the accomplishments of the international TOGA programme (1985-1994). The primary focus to date has been in the Pacific sector, owing to the prominence of El Niño and the Southern Oscillation in global climate. At the same time, it is well recognized that atmosphere-ocean interactions throughout the global tropics are potentially important to the earth's climate variability on the time scales of years to decades. Among the regions of particular interest, and the focus of this project, is the tropical Atlantic.
The tropical Atlantic is characterized by a strong seasonal cycle, deriving ultimately from radiative forcing and land-sea contrast, but strongly modified by atmosphere-ocean coupling. Superimposed on this seasonal cycle, there appears to be two modes of interannual and longer-term variability. The so-called dipole mode, which operates primarily at decadal and longer time scales, involves north-south interhemispheric variations in SST. This mode has been linked to severe climate anomalies in Northeast Brazil (Nordeste) and in parts of Africa (Sahel and Sub-Sahel). A second equatorial mode, operating preferentially at seasonal and interannual time scales, has many similarities to the ENSO phenomenon in the Pacific, and involves trade wind variations and the excitation of equatorial Kelvin and Rossby waves. This mode has been associated with rainfall extremes in the Gulf of Guinea and marine ecosystem disruptions in the Benguela current area. It is not presently known whether the two modes are related. Our current understanding of these phenomena is limited, and to a great extent this is because of the lack of routine, quality controlled, oceanic and atmospheric observations in the region. The observation system that exists relies mainly on volunteer observing ships and occasional research vessels that pass through the area. The generally infrequent oceanographic cruises in much of the region are insufficient even for monitoring interannual variability. Recently, a new observational programme known as PIRATA (Pilot Research Moored Array in the Tropical Atlantic) has been initiated by Brazil, U.S., and France in part to remedy this crucial lack of oceanic and atmospheric data in the tropical Atlantic. The scientific goals of PIRATA are:
In addition to a better understanding of the local coupled dynamics in the tropical Atlantic, it is expected that PIRATA will help in determining the linkages between the Atlantic and ENSO. The data will also help in determining the relationships between regional climate variations and agricultural/fisheries impacts for South America and West Africa. Ultimately, it is expected that better data will aid the development of useful forecast models and forecast capability for the region.
This pilot observing system for the tropical Atlantic Ocean follows on the scientific successes of TOGA and on the proven technology that is operative in the Pacific Ocean, particularly in the in situ observational system of approximately 70 buoys that form the TOGA-TAO array. PIRATA has been funded to install and maintain an array of 12 moored ATLAS buoys during the years 1997 to 2000 for monitoring the surface variables and upper ocean thermal structure at key locations in the tropical via satellite (e.g. CLS-Argos) in real-time (once a day). The data is available to all interested users in the research or operational communities in the Web Site www.ifremer.fr/orstom/pirata/pirataus.html. The total number of moorings is a compromise between the need to put out a large enough array for a long enough period of time to gain fundamentally new insights into coupled ocean-atmosphere interactions in the region, while at the same time recognizing the practical constraints of resource limitations in terms of funding, shiptime, and personnel. The objectives of the PIRATA array then, are to demonstrate scientific success in a limited geographical region for a limited duration of time, as a guide to more serious long range planning.
It is expected that at the end of the pilot phase of PIRATA, other nations will join in the maintenance and possible expansion of PIRATA to constitute a tropical Atlantic Ocean observing system and component of GOOS and GCOS. The programme is appropriately multinational since variations in the tropical Atlantic affect many nations in the Americas and Africa. PIRATA will create a true partnership in the study of tropical oceanography and ocean-atmosphere interactions in the Atlantic by bringing key research institutions in the region to a continuing collaboration. The observations that PIRATA will acquire are in direct response to requests from the international scientific community that these data be obtained in order to investigate the predictability of the coupled climate system in the region. Thereby, PIRATA addresses directly the call from several international working groups dealing with tropical ocean climate studies, including those from TOGA, VAMOS and the OOSDP.
3.7 IAI (B. Wilcox)
In recognition of the importance of a regional approach to the study of global change, eleven countries of the Americas signed an agreement establishing the Inter American Institute for Global Change Research on May 13th, 1992, at Montevideo, Uruguay: Argentina, Bolivia, Brazil, Chile, Costa Rica, Dominican Republic, Mexico, Panama, Peru, United States, and Uruguay.
The IAI focuses on the 1) increased understanding of global change related phenomena and the societal implications of such phenomena, 2) increased overall scientific capacity of the region, 3) enhanced regional relationships, establishment of new institutional arrangements, and 4) promotion of the open exchange of scientific data and information generated by the Institute's research programmes, implementation of IAI Training and Education Programs.
The following research themes have been identified as initial properties of the IAI: 1) tropical ecosystems and biogeochemical cycles, 2) impact of climate change on biodiversity, 3) ENSO and interannual climate variability, 4) ocean/atmosphere/land interactions in the inter-tropical Americas, 5) comparative studies of oceanic, coastal and estuarine processes in the temperate zones, and 6) comparative studies of temperate terrestrial ecosystems and high latitude processes.
Although they are primarily concerned with the science of climate assessment and prediction on seasonal-to-interannual time scales, the regional programmes organised under the CLIVAR/GOALS contribute to a broader range of human endeavours. The participation of IAI is of special importance to VAMOS in view of the Institute's major thrust on the societal impacts of climate variability over the Americas. VAMOS will provide IAI researchers with opportunities for enhanced co-operation with other science programmes focusing on the American climate system, particularly with the large and regional scale modelling efforts developed in PACS and GCIP. It will also provide enhanced co-ordination of field activities in the American sector.
3.8 SACC (R. Matano)
The South Atlantic Climate Change programme (SACC) effort is an international and multidisciplinary programme to study the effects of climate change in the southwestern South Atlantic. The oceanic conditions in the Western South Atlantic Ocean affect the climate of a region inhabited by approximately 200 million people, whose economic resources are closely tied to agricultural and fisheries activities. The effects of climatic variations on the general well-being of this region have become evident during the last few decades as drought periods produced dramatic changes in Brazil's cattle population while precipitation excess produced significant expansions of Argentina's farming regions. While it is obvious that these issues are most relevant to communities in South America, such climate anomalies are also an inextricable part of variations in the global climate system. This region has a well documented mid-latitude climate anomaly pattern that has been linked to global phenomena such ENSO, while recent results point toward important contributions from the Antarctic Circumpolar Wave. The fairly simple coastal geometry of this region combined with the emerging international scale expertise in Argentina and Brazil make this an ideal region in which to explore the important IAI/CLIVAR goal of understanding meridional climate linkages. The multinational and multidisciplinary studies being developed or planned by the SACC will significantly expand our knowledge of oceanic variations in the region and how they relate to, and coupled with, variations in Earth's climate system on relatively short time scales.
The main goal of the SACC programme is to understand the interactive relationship of the southwestern South Atlantic SST and the larger scale climate behaviour. The specific objectives are to:
3.9 PACS-SONET (M. Douglas)
An atmospheric sounding network sponsored by PACS (PACS-SONET) has been operating in Central America and northwestern South America since mid-1997, as part of a research project to investigate rainfall variations over Central America during the warm season. This network has consisted of between 12 and 18 pilot balloon (wind only) stations, and continues to operate with daily soundings being made at most sites. Three stations are operating in southeastern Mexico, one in Nicaragua, two in Costa Rica, one in Panama, one in Colombia, 3 in Ecuador, and up to 5 in Peru. Soundings are made at San Cristobal Island in the Galapagos Islands and have been made on Cocos Island, Costa Rica. The sites are maintained through support of PACS, with contributions by various countries. (The network is designed to end observations on October 31, 1998).
3.10 DEPROAS
The program in the Dinamica do Ecossistema de Plataforma da Regiño Oeste do Atlantico Sul (DEPROAS; Dynamics of the Ecosystems of the Continental Shelf in the Southwestern Atlantic) has two goals:
3.11 PRECURSOR (Chris Mooers)
PRECURSOR is a proposed, combined meteorological and oceanographic observational and modelling pilot project focused on the Intra American Seas (IAS). IAS comprise the combined Caribbean Sea, Gulf of Mexico, Straits of Florida, plus the adjacent waters of the western North Atlantic between ca. 5° and 30°N and ca. 55°W to the coastline of the Americas. (Bruce Albrecht and Chidong Zhang are responsible for the atmospheric observations, and Kevin Leaman and Doug Wilson for the oceanic observations, while Shuyi Chen is responsible for the atmospheric model, and Chris Mooers for the oceanic model.) It plans to explore the meso-scale and synoptic scale atmospheric plus meso-scale and seasonal oceanic circulation processes, and their interactions, in the largely unexplored southwestern subdomain of the IAS over the course of two years (1999/2000). Here is the Panama-Colombia Gyre, a large cyclonic recirculation cell that overlies the Colombia Basin of the southwestern Caribbean Sea and interacts with the Caribbean Current and the continental margin. A pair of interacting and highly variable upper level cyclones are embedded in the larger cyclonic recirculation cell. They overlie a pair of anticyclones that interact with the bottom topography.
Participants are from U.S. (RSMAS/AOML), Panamá, and Colombia at this stage. PRECURSOR aims to determine the importance of meso-scale resolution and processes in estimating the impact of intense precipitation on the oceanic circulation in the Panama-Colombia Gyre, the air-sea fluxes, and the moisture flux from the region. It will also take other initial steps in examining land-air-sea interaction, including the regional hydrological cycle. Because the Gyre is the locus of strong air-sea fluxes according to climatology and is known to have strong diurnal (meso-scale), easterly wave (synoptic scale), and intraseasonal (TIO) variability, it is suspected to play a significant role in regional climate, which is a perspective that the pilot study should begin to bring into focus. Based on the results of PRECURSOR, it is intended to develop a more expansive programme of research, which is anticipated to contain components treating coastal ocean ecology; for example, the physical transport and dispersal of fish larvae between their spawning and nursery grounds. The initial observing strategy includes two moored current meter arrays, drifters with acoustic rain gauges, repeated CTD/ADCP/GPS rawindsonde transects, and additional VOS tracks. The meteorological strategy also includes diagnostic studies of satellite scatterometer winds, MSU precipitation, OCR data, NCEP/NCAR reanalysis and SST analysis, and the oceanographic strategy also includes use of coastal (over the horizon) radar-derived surface current data. The modelling strategy, for both the atmosphere and ocean, involves meso-scale resolution over the entire IAS (the atmospheric model covers a slightly larger domain; for example, extending inland over northern South America.) Issues of ocean-atmosphere coupling will proceed with the analysis of one-way coupling issues first and of two-way coupling progressively.
From the VAMOS perspective, PRECURSOR's hypotheses are:
3.12 WMO Tropical Meteorology Research Programme (A. Grimm)
The WMO Commission of Atmospheric Sciences (CAS), in its twelfth session, recommended that the proposed activities in the Tropical Meteorology Research Programme (TMRP) involving the study of the Asian and Australian monsoon should be extended to also include American monsoon studies.
The purpose of TMRP is to promote and co-ordinate research activities by Members in high priority areas of tropical meteorology. The emphasis is on weather system scale, except for monsoons and drought related studies, which emphasize variability and prediction at the regional and seasonal scale.
In the redefinition of the TMRP elements, a new project on American Monsoon Studies was included (M3). This new project aims to enable CAS support of activities developing in the Americas on monsoon studies, in co-ordination with the CLIVAR/GOALS components of the WMO/ICSU World Climate Research Programme on the monsoon, including the Pan American Climate Studies programme.
3.13 ECLAT and ECOP (J. Picaut)
The programmes ECLAT (Etudes Climatiques de l'Atlantique Tropical and ECOP (Etudes Climatiques de l'Océan Pacifique tropical) are the French contributions to the CLIVAR international programme in the tropical Atlantic and Pacific oceans.
The aim of the ECLAT programme is to improve our understanding of the role of the Tropical Atlantic in the climate variability, focusing on its effects on a regional scale. Among the several issues addressed by CLIVAR, ECLAT intends to contribute answering these two fundamental questions:
ECLAT is a multidisciplinary programme that includes all components of the climatic variability associated with the ocean/atmosphere/continent coupling. It is based on a comprehensive observations programme including in situ and satellite observations and calling on real time data transmission, complemented with oceanographic cruises. Process studies, to a large extent, will use numerical simulations, using a complete hierarchy of models.
Several elements of the observing systems have been handled for years by the ORSTOM Centre in Brest, mostly the Ship of Opportunity network, with measurements of winds, sea surface temperature, sea surface salinity and temperature profile (XBT) along several lines crossing the tropical Atlantic. The master piece in the observational network of ECLAT is PIRATA. Specialized research cruises will be done during the course of this long-term program. In particular, the EQUALANT cruise will study of the ocean circulation variability and the ocean-atmosphere interactions in the equatorial Atlantic. The main objectives of the cruise are to examine the: 1) large scale variability of the thermohaline circulation in the 6°N-6°S equatorial band 6° of the Atlantic, 2) redistribution of mass, heat and tracer, between surface water and deep layers, 3) role of equatorial processes such as upwelling and deep jets in this redistribution and in its high frequency variability, and 4) variability of heat fluxes at the ocean-atmosphere interface in the equatorial upwelling area.
EQUALANT represents a combination of French WOCE cruises and will take place between an American experiment in 1998 and a German experiment in 2000, in the same area, in order to study the large scale variability of the different parameter distributions. Hydrographic, current and tracer measurements will be done along six meridional sections across the equator. During the cruise, the high frequency variability will be studied through repeated stations at the location of PIRATA moorings. In addition to altimetric measurements and modelling work, PIRATA and EQUALANT are complementary ways to study the variability of the ocean, from the surface to the bottom, and the role of the deep ocean - surface ocean - atmosphere interactions in this variability.
ECOP (Etudes Climatiques de l'Océan Pacifique tropical) is a continuation of the French effort in the tropical Pacific during the 1985-94 TOGA (Tropical Ocean and Global Atmosphere) programme and its 1992-93 COARE (Coupled Ocean-Atmosphere Response Experiment) sub-program. As such it is part of the international CLIVAR/GOALS program. ECOP is divided into three components:
With the continuation of long-term measurements in the tropical Pacific (e.g., Ship of Opportunity network), the strong involvement of several research teams in the TOPEX/Poseidon mission, and the use of different classes of model, data assimilation is an important tool for this programme.
3.14 GOOS (http://ioc.unesco.org.GOOS)
The Global Ocean Observing System (GOOS) is intended to be a permanent global system for observations, modelling and analysis of marine and ocean variables needed to support operational ocean services worldwide.
GOOS will provide: (i) accurate descriptions of the present state of the oceans, including living resources; (ii) continuous forecasts of the future conditions of the sea for as far ahead as possible; and (iii) the basis for forecasts of climate change. GOOS is being implemented by national and international facilities and services
3.15 GOOS-BRAZIL (http://www.labmet.io.usp.br/~goos-br/ingles/)
GOOS-BRAZIL, much like its international counterpart, is to (i) use continuous, multidisciplinary monitoring of the oceans aims to predict phenomena and processes that exert direct impact on issues related to the marine environment, such as its preservation, conservation and sustainable use, (ii) identify priorities for operational oceanography based on what will bring the greatest socioeconomical benefits, (iii) assess the current state of systematic ocean monitoring already underway, and (iv) provide services, data products and necessary information for the management of the marine environment.
4. Proposed Field Programmes
4.1 The Central American poleward Low Level Jet (M. Nicolini, J. Paegle)
The LLJ in central South America plays an important role in the:
There is a generalized idea that this low level jet structure is similar to its North American counterpart (Great Plains LLJ), observed by Bonner (1968) and others (see Fig. 4). This LLJ has been characterized and its role in the continental-scale water vapour budget has been presented by Helfand and Schubert (1995) using a GCM and by Berbery and Rasmusson (1996) using the high resolution ETA regional model. Confidence in these results has motivated the use of the ETA forecast products with a horizontal resolution of 40 km to characterize the structure and diurnal variability of the Central South America LLJ during the 1997-1998 warm season. Again, this representation cannot be verified by observations. This work also identifies other low level jet cases in the region, describes the relation between the convergence and precipitation patterns, characterizes the water vapour flux pattern, its diurnal variation and estimate the contribution of the low level jet to the convergence of the vertically integrated total fluxes in a domain covering the Paraná- Río de la Plata Basin. Figure 3a displays the mean northerly wind field for December 1997 at the level and time corresponding to the occurrence of the maximum. The area that verifies the LLJ criterion is located in a data void region. Figure 5 includes tentative locations of the proposed sounding sites within this area, in order to enhance the current South American rawinsonde network. Figure 3b displays a monthly average longitudinal vertical cross section along 18oS of the meridional water vapour flux for December 1997. This vertical section shows a meridional water vapour flux with a strong magnitude and a dominant poleward direction downwind of the Andes in a latitude just to the south of the Amazon Basin. This is relevant to the analysis of the atmospheric branch of the hydrological cycle both over the Amazonia and the Paraná-Río de la Plata Basin. A question that persists is whether the LLJ is a real feature or an artifact of model deficiencies associated mainly with the representation of the steep Andes. Studies at Resistencia (Berri and Inzunza, 1993) and a recent short period of pilot balloon observations at Santa Cruz, Bolivia (Douglas, personal communication) give evidence of its existence. A field programme effort to describe adequately its structure and variability in different time scales is recommended. This field experiment should contribute to answer the questions posed in the VAMOS document (see section 3.4.7 of this document). It should also determine the evolution of the LLJ during intraseasonal wet and dry episodes over the subtropical plains and the remote and local influences associated with this evolution. As a monitoring system of the low-level circulation, a field experiment on this LLJ, should have linkages with other programmes centred in understanding the rainfall regimes in the Bolivian lowlands and the Altiplano and the quasi-permanent heat low over the Chaco region.
The spatial density of the stations and the frequency of observation per station (once a day) are inadequate over a large area between latitudes 10oN and 30oS to identify these wind speed maxima and their diurnal oscillation. Operational analyses produced by global forecast centres: ECMWF, NCEP, United Kingdom Met Office, have finer resolution than spacing of available atmospheric soundings. Then the LLJ just to the east of the Andes represented in these analyses cannot be verified by observations. LLJ representations are substantially dependent of the different data archives. (Wang and Paegle, 1996). Differences in NCEP/NCAR and ECMWF reanalyses of rainfall and low level currents over South America for a selected period of intense rainfall over Argentina (see Fig. 6), show lack of convergence of the two assimilation systems in regions of low data density. Interannual variability in the intensity and location of this LLJ is apparent in the NCEP/NCAR reanalyses of low level wind field.
The first step of this proposed programme could be to provide a coverage of the low level circulation as it emerges from the tropical Amazonia to quantify vertically integrated water vapour meridional flux. The field element would be based upon: (1) the current radiosonde network, (2) enhancements to the frequency of radiosoundings and /or pibal balloon soundings from once to twice/day at a northern Argentine station, (3) moving portable radiosounding units from Brazil to sites in the LLJ area, and (4) a wind profiler. The field effort would last for about 2 months and involve international collaboration from Argentina, Brazil, United States, Bolivia, and Chile.
A field programme could also contribute to calibrate analyses against observations, assess uncertainty in the wind analyses, and resolve possible discrepancies in the moisture flux divergence computed from gridded analyses. In addition, it would contribute to verify the capability of models to reproduce well the mean state and variability of the tropospheric flow over the region. This will allow to estimate confidence levels on the simulations of different scenarios of regional climate change over South America. Finally, it would improve the radiosonde network over the region through the evaluation of the impact in the forecasts initialized with enriched analysed fields.
4.2 Integrated Observing System (D. Halpern)
In-situ observations of "key" variables at "key" sites should be recorded for limited time intervals in order to determine acceptability of satellite measurements of corresponding variables. For example, scatterometer measurements of surface wind velocity may be influenced, vis-a-vis the model function relating electromagnetic radiation and wind velocity, by the total amount of water in the atmosphere. The question to be addressed is whether the same algorithm be used in the ITCZ and stratus regions, if the regional accuracy of wind speed is 0.5 m s-1 or 1.0 m s-1.
5. Reports of Workshop Working Groups on the Monsoon Systems
5.1 Summary of North and Central American Monsoon Systems Discussion (P. Nobre and M. Douglas, using material contributed by V. Magaña and W. Higgins)
a) NAMS
NAMS is more pronounced than SAMS, and exhibits some close similarities with the stronger Asian monsoon. The North American precipitation regime is dominated by a continental-scale pattern characterized by an out-of-phase relationship between precipitation over the Great Plains of the U.S. and that in northwestern Mexico. There is a weaker in-phase relationship between precipitation along the southeastern coast of the U.S. and that over northwestern Mexico. Wet (dry) monsoons in the southwestern U.S. and northwestern Mexico often follow winters characterized by dry (wet) conditions in the southwest U.S. and wet (dry) conditions in the northwest U.S. In addition, the onset of the North American monsoon rains, which exhibits strong regional characteristics, is highly correlated with interannual fluctuations in rainfall over the entire monsoon region for up to 2 months after the onset.
Satellite imagery reveals that the NAMS rainfall patterns possess much meso-scale structure associated with topography and diurnal circulations driven by land-sea and mountain-valley thermal contrasts. The rainfall patterns are modulated by variations in the large-scale flow patterns on intraseasonal and longer time scales, and the response of the local circulations to these variations in mean conditions is not presently known. Even regional climate simulations of precipitation over the region fall short of being considered reasonably successful.
Some basic questions related to the variability of the North American monsoon include the following:
The monsoon system over Central America is less-well defined in terms of calendar dates and distinct transitions, though the seasonal variations are quite distinct on the Pacific side of Central America. Summer rainfall is strongly affected by ENSO, with drought associated with warm SST anomalies in the eastern Pacific. A basic aspect of the climatology of the region is the mid-summer dry season (locally called "veranillo") during July and August, when rainfall is a minimum at stations along the Pacific slope. Monthly mean rainfall statistics show this feature to be most pronounced in northern Nicaragua, though it is well marked from southern Mexico to Panama. Rainfall along the Caribbean coast of Central America is out of phase with that on the Pacific side, with highest values during the months on July and August. This latter observation, together with the presence of the strongest trade winds during these months, suggests that the rainfall variation results from an interaction of the prevailing flow with the mountain barrier of the isthmus. However, the July-August rainfall minimum is observed well offshore in the Pacific and also over parts of the Caribbean Sea, and it is not entirely clear that such a simple explanation suffices. In any case, the large-scale intensification of the trades during July and August, and the rapid strengthening of these winds during June and their weakening in September are part of the large-scale variations in the American monsoon system needing a clearer explanation.
A basic question related to the mechanism of rainfall variability is whether the SST variations in the eastern Pacific warm pool control the fluctuations in ITCZ precipitation over the region. Continuous observations, similar to those of the TAO network buoys, have been lacking in the warm pool, and satellite and ship estimates of SST may not be sufficiently accurate for detecting the small variations in SST, or in the depth of the warm pool, that may be associated with interannual precipitation variations in the North and Central American monsoon system. Although the distribution of precipitation is of greatest concern, regions of low rainfall over the Caribbean and Gulf of Mexico are also of interest, since the SST is often high in these regions during much of the warm season.
The successful prediction of the interannual variability of tropical cyclones over the eastern Pacific Ocean is clearly a desirable objective, since landfalling storms have a serious impact on the coast of Mexico and can affect agriculture as far North as California and into the southern Great Plains. The variations in the intensity of the trade winds over the Caribbean Sea and far eastern Pacific Ocean may be related to variations in the frequency of tropical cyclones in the eastern Pacific. Prediction of such variability may depend on the identification of those large-scale circulation patterns that are most conducive to tropical cyclogenesis, and to the predictions of these patterns with coupled models. The mechanisms responsible for the generation of tropical cyclones over the eastern Pacific have not been identified, and the reasons for intraseasonal and interannual variations in cyclogenesis require further clarification.
The major control played by topography and the diurnal cycle in producing the large spatial variations in rainfall over Central America is apparent from satellite imagery. Useful predictions of rainfall variations over the region must account for this spatial variability. It is not clear is much of the rainfall varies in phase, or whether the phase of the seasonal and interannual variations differs on small spatial scales. In addition, there is strong variability in the SST patterns over the eastern Pacific downwind from Central America. This variability may depend at least in part, on the strength of meso-scale gap winds; the resulting SST variations may than induce rainfall variations over the region that might not be obvious from consideration of the mean flow and the orography alone. Clearly, the net effect of large scale conditions predicted by coupled models needs to be scaled-down for local and regional applicability.
c) Field observational needs for VAMOS
The needs for observations to better describe NAMS and the wet season variability over Central America fall into two general requirements - those needed for extended monitoring and those needed for intensive, shorter duration process studies. The extended monitoring activities will provide the background context for the process studies, and can be used to judge whether the process studies give a representative picture of the particular phenomena being investigated in detail during the process study.
d) Extended monitoring for VAMOS
The special observations deemed necessary for monitoring the evolution of NAMS and CAMS include:
5.2 Summary of Working Group on the South American Monsoon System Discussion (V. Kousky)
Many of the descriptive features of SAMS have already been included in the VAMOS chapter of the CLIVAR Implementation Plan. This summary focuses on additional features and research areas not previously emphasized.
The Amazon Basin contains approximately 20% of the world's fresh water. As such it is important to understand the rainfall variability of the region and the relevant land-air, ocean-air processes involved. The Paraná/Plata river Basin of central South America, although not as large as the Mississippi basin in the U.S., does contain a nearly equivalent amount of fresh water. Therefore, processes that govern variability in this region are also extremely important in order to obtain a comprehensive understanding of the South American Monsoon System.
Field experiments are necessary to generate data sets that can be used to validate regional and global numerical models, used in simulations or in operational prediction. Some of the main scientific issues that should be addressed include:
6. Short Reports
6.1 Lessons from the NCEP Reanalysis (V. Kousky)
The NCEP/NCAR Renalysis project has been completed for the period 1958-1995. To update the analysis archive, the climate data assimilation system (CDAS) has been run each month since January 1996. The combined CDAS/Reanalysis data set is free of inhomogeneities due to changes in model physics, model resolution and analysis techniques that previously plagued global analysis archives. However, the Reanalysis project revealed that inhomogeneities still remain due to variations in the observational data base used in the analyses. For example, a marked discontinuity in temperature in the upper troposphere and lower stratosphere is evident at the beginning of 1979, which contrasts the pre-satellite sounding / satellite sounding periods. Other temporal inhomogeneities arise when radiosonde stations exist for a certain period and then are discontinued. As a result, in data void regions the analyses drift toward the NCEP model climatology, thus adversely affecting the quality of the analyses.
6.2 Influence of Atlantic SSTs on precipitation over South America (A. Robertson)
Several multi-decade simulations have been made with the UCLA atmospheric GCM, in which various distributions of SST are prescribed over the Atlantic Ocean, with climatological values imposed elsewhere (Robertson, Mechoso and Kim, 1998). The results suggest that during austral summer, warm interannual SST anomalies over the South Atlantic are accompanied by a southward shift in the Amazonian convergence zone. Furthermore, this meridional displacement of the GCM's South American monsoon is reflected in the model's regional Hadley circulation across the equator. Interannual variations in the latter are found to ultimately affect the North Atlantic Oscillation in the model.
6.3 Moisture Flow over North America (W. Higgins)
A fundamental and necessary first step toward understanding warm season precipitation variability over North America is the clear documentation of the major elements of the warm season precipitation regime within the context of the evolving atmosphere-ocean-land annual cycle. During the spring and summer months, the low-level flow over the Great Plains of the U.S. is characterized by a nocturnal LLJ. This LLJ is a diurnally modulated southerly wind maximum in the lower troposphere that accounts for a large portion of the moisture flux into the central U.S. during the warm season. Although most prominent during spring and summer, it is frequently observed during the autumn as well.
Previous studies (e.g., Higgins et al., 1997a) have shown that the patterns of night-time rainfall and thunderstorm activity over the central U.S. during LLJ events are fundamentally different compared to those during nonjet events. Great Plains LLJ-related precipitation is found to be associated most closely with the strongest, least frequent LLJ events. More important, climatological features such as droughts and floods over the central U.S. have also been related to decreases and increases in the intensity of the LLJ. These and other results strongly suggest a link between the large scale flow related to the evolution of NAMS and the smaller-scale hydrometeorological phenomena (e.g. Higgins et al., 1997b).
Numerous authors have attempted to identify the primary source of moisture for the summer rains over southwestern North America. The extent to which water vapour sources from the Gulf of Mexico/Caribbean Sea -vs- Gulf of California/eastern tropical Pacific contribute to precipitation in the southwest remains an open question. At the present time meso-scale features of LLJs from the Gulf of California and from the Gulf of Mexico are not adequately resolved with the conventional radiosonde network and, thus, reliable estimates of moisture budgets are limited to continental scale areas.
6.4 SCSMEX (C. B. Emmanuel)
The South China Sea Monsoon Experiment [SCSMEX] is a multinational collaborative effort involving the coordinated activities of national weather services and research institutions of East Asian countries and regions, including research activities of several U.S. and Australian institutions. In addition, SCSMEX encompasses ongoing and planned international field experiments and research programmes such as GEWEX and CLIVAR. SCSMEX has three basic research components:
The Joint Office for Science Support [JOSS], Office of Programs, UCAR, was assigned the task of providing support to the International SCSMEX Organizing Committee, International Science Steering Committee, SCSMEX Project Office, and act as the intermediary for all U.S.-PRC collaborative efforts, including shipping of research systems and expendables. To work out the details of this collaborative effort, and to secure the proper agreements, initial contact with the proper authorities was made nearly 18 months prior to the field phase of the experiment. The necessary logistics covered, among others, are areas such as: letters of invitation (where appropriate), visas, passports and other travel related documents and requirements; shipping arrangements; customs permits/clearances; communications (phones, fax, cell phones, internet, etc.); computer support as required by investigators on land, on ships; medical arrangements/health information; financial information/lodging/transportation; field operations center management/functional organization; research platforms (ships, aerosondes)/schedules/personnel assignments; and research system(s) installation(s) aboard ships, as well as necessary contractual agreements with platform owner (s).
7. Plenary discussion of the monsoon systems
7.1 VAMOS Field Programmes Focusing on Ocean-Atmosphere Processes (Discussion leaders: S. Esbensen and D. Rogers; Contributors: D. Enfield, C. Eriksen, D. Halpern, V. Magaña, R. Matano)
The goal of this part of the workshop was to integrate scientific issues identified in earlier discussions regarding oceanic and atmospheric variability as a guide for the development of VAMOS field activities. The discussion was organized around the principle that VAMOS ocean-atmosphere field programmes should be motivated by hypotheses arising out of empirical and modelling studies of significant seasonal-to-decadal phenomena in the American monsoon system. Broad scientific issues raised by these hypotheses would then lead to specific scientific objectives and hypotheses for field activities. Based on this approach, discussion participants identified four high priority oceanic regions participating in seasonal-to-decadal climate variability of the rances; communication systems:
a) The cold-tongue/ITCZ complex and stratus regime in the eastern Pacific
To predict ENSO over the Pacific Ocean and its effects on the American monsoon system with quantitative accuracy, the annual cycle of the eastern CTIC and its interaction with seasonal-to-interannual climate anomalies must be well simulated in coupled ocean-atmosphere models. In addition, heat sources over the Amazon and elevated regions of western South America may be linked to variability of the atmospheric subsidence in the stratus regime, suggesting that there may be connections between land surface processes over South America and seasonal-to-interannual variability of the cold-tongue ITCZ complex.
Intercomparisons of state-of-the-art ocean-atmosphere models and observed data have identified several major deficiencies in our understanding of the processes that determine upper ocean thermal structure and the associated atmospheric patterns of heating and cloudiness over the eastern Pacific (Mechoso et al., 1995). Coupled models tend to produce an unrealistic symmetric circulation with ITCZs north and south of the equator, often associated with unrealistically warm equatorial sea surface temperatures and parameterised boundary layer cloud decks that are less extensive than in observations. The coupled models exhibit considerable sensitivity to changes in parameterization of physical processes in the stratus deck region. Further, the atmospheric components of these coupled models often produce unrealistic atmospheric boundary layer structures over the oceanic cold tongue and significant errors in the strength and location of the northeast Pacific ITCZ. We are therefore concerned that we do not have the basic understanding of eastern Pacific ocean-atmosphere coupling that is required to develop to reliable seasonal to interannual prediction systems for this region and the remote regions influenced by its variability.
Several eastern Pacific field programmes that are underway, or in an advanced state of planning, will contribute to VAMOS. The PACS programme is planning an eastern Pacific investigation of climate processes (EPIC) in the ocean-atmosphere system to improve the description and understanding of the CTIC and key physical processes that must be parameterised for successful CTIC simulation with dynamical ocean-atmosphere models.
A potential unifying theme for a VAMOS ocean-atmosphere-land interaction field study was discussed involving the interaction of radiatively cooled flow above 700 mb in the eastern tropical Pacific with the convection over the Amazon and teleconnections with the Atlantic and Pacific regions. Pressure gradients above 700 mb driven by cooling over the equatorial tropical Pacific may interact with the convection and precipitation regime over the tropical western Amazon, which in turn may feed back to both the Pacific and Atlantic Oceans.
It was recognized that VAMOS can play an important role in coordinating and facilitating collaboration between North and South American scientists involved in eastern Pacific field programmes. Ongoing studies of the stratus decks off the coast of Chile, and enhanced monitoring of air sea interaction off the Peruvian coast, will benefit from coordination with EPIC field activities. EPIC will benefit from enhanced monitoring of ocean structure in the eastern Pacific through the Consortium for Ocean Role in Climate (CORC; a contribution to CLIVAR Pacific BECS) and ERFEN, a programme to monitor ENSO in the coastal eastern Pacific.
b) The tropical Atlantic
At the present time, there is substantial observational and theoretical evidence supporting the hypothesis that tropical Atlantic variability is generated by air-sea interactions confined primarily to the tropics, rather than through remote influences from tropical/subtropical interactions or ENSO forcing from the Pacific. It is anticipated therefore that VAMOS field programmes in the tropical Atlantic will focus on developing long and accurate time series of surface wind stress, heat flux and freshwater flux, and measurements of the large-scale oceanic and atmospheric structure that may be influenced by these air-sea exchanges.
Of particular interest for understanding off equatorial SST anomalies is the following feedback process: a strengthened cross-equatorial SST gradient leads to an increased cross-equatorial flow of air which, due to the Coriolis force, tends to intensify (weaken) the tradewinds in the northern (southern) hemisphere. This positive feedback mechanism may form an essential component of an ocean-atmosphere oscillation or enhance the response of a remotely forced ocean-atmosphere anomaly.
The PIRATA observing system of moored buoys and associated measurements during the 1997-2000 time frame will contribute to VAMOS field programmes that are designed to investigate air-sea interaction hypotheses. Provided that the PIRATA array can be developed and maintained for the remainder of the VAMOS Programme, it may provide the backbone of an observational array that is capable of testing the air-sea interaction hypothesis for tropical Atlantic variability.
c) The South Atlantic Convergence Zone
Seasonal-to-interannual variability in the SACZ and precipitation over southeastern South America has been linked to ENSO. In addition, the SACZ may be involved in teleconnections with the tropical and subtropical North Atlantic. The working hypothesis is that the variability southwestern Atlantic SST affects the seasonal-to-interannual variability of southwestern Atlantic storm tracks and the moisture and atmospheric heat sources over the SACZ and adjacent land areas. A South Atlantic Climate Change (SACC) programme focuses on understanding the evolution of sea surface temperature in the SACZ and its interaction with the larger scale climate.
d) The warm pool region over the eastern Pacific and Interamerican Seas
On a regional scale, ocean-atmosphere interaction in the Interamerican warm pool region may play an important role in the seasonal march of precipitation and the moisture transported to surrounding regions in North and South America. For example, it was hypothesized that the midsummer precipitation minimum over Mexico and Central America may be due to the interaction of the circulation with SST, convection and radiation processes in the eastern Pacific warm pool region. The hypothesis depends on an ocean-atmosphere feedback mechanisms in which warm SSTs develop under relatively clear skies in subsiding air. When the SST exceeds a threshold temperature of about 29oC, deep convection develops and SSTs cool due to albedo and air-sea interaction. The divergent circulations associated with these feedback mechanisms can affect the moisture transport and convection over land areas in the region.
On larger scales, however, the Interamerican warm pool region appears to play a passive role in large-scale seasonal--to-decadal climate variability. Unlike the western Pacific warm pool region, the Interamerican warm pool region does not contain precipitation and the atmospheric heat source maxima. The interannual variability of SST in the Interamerican warm pool region is relatively small and no strong teleconnections with warm pool SST anomalies have been identified.
There is an opportunity for pilot field studies of Interamerican warm pool processes in the eastern Pacific in conjunction with EPIC and CORC field activities in the 1998-2002 time frame. Understanding the controls on SST, convection and water vapour transports may provide a focus for future VAMOS field activities over the eastern Pacific and the Interamerican Seas, possibly in conjunction with field activities focused on monsoonal circulations over adjacent land areas.
7.2 VAMOS Field Programmes Focusing on Land-Atmosphere Processes (Discussion Leaders: J. Shuttleworth and C. Nobre)
The purpose of this discussion was to identify the coupled hydrologic-atmospheric processes which may be important in the context of the North and South American monsoon systems, to explore the opportunity for collaboration between VAMOS and field programmes currently addressing the need for better understanding of these processes, and to identify additional regions where land-based process studies and observations are required to support VAMOS objectives.
The participants considered that improved understanding of coupled hydrologic-atmospheric process and their better representation in models will serve two critically important purposes in the context of VAMOS, namely:
The representation of hydrologic processes in coupled ocean-atmosphere-land models is less well developed than is their representation in hydrological models. Discussion participants noted that the topography of the land surface is known to have a marked influence on precipitation patterns and that topography also has a strong influence on how precipitation is subsequently partitioned between hydrological storage and river flow. Moreover, there is observational evidence that the presence of the Rocky Mountains and Sierra Nevada and the Andes respectively influence the precipitation and runoff associated with the monsoon systems in North and South America.
The participants also recognized that the need to provide latent heat to melt frozen precipitation means that snow and ice cover can persist for many months. The consequences of this is likely to be particularly important in the case of the North American monsoon because the presence of snow and ice cover has a marked effect on surface albedo and surface energy balance, and frozen precipitation is a major water resource in the western U.S.
Studies carried out under GCIP in the Mississippi River basin provide evidence that regional soil moisture fields can influence the stability of the atmosphere downwind and this may impact the strength of convective precipitation during summer months. Similarly, the inclusion of better representation of the vegetation covering the ground and of its growth cycle through the year has proved beneficial for predictive models developed in GCIP. There is also evidence from meso-scale meteorological models that heterogeneous vegetation cover, when present in patches with length scales of order 10 km or greater, can influence the growth of convective clouds and release of precipitation. Investigating this is a high priority for the LBA Experiment because land cover change from forest to pasture often results in heterogeneous vegetation with this length scale.
On the basis of the discussion summarized above, workshop participants defined four coupled hydrologic-atmospheric processes whose study merits priority in VAMOS because of their likely influence on the seasonal-to-interannual variability of the North and South American monsoons, namely the processes associated with: topography, frozen precipitation, and seasonal evolution of soil moisture, as well as vegetation type and its seasonal vigor and heterogeneity.
The primary functions of and research priorities for field programmes focused on land-atmosphere process within VAMOS just described are similar to those applied in the GEWEX Continental-Scale Experiments (CSEs) already ongoing in the Pan-American region, namely the GEWEX Continental-scale International Project (GCIP), the Large-scale Biosphere-Atmosphere (LBA) Experiment, and Mackenzie GEWEX Study (MAGS). These GEWEX CSEs are therefore beneficial for VAMOS. However, the geographical focus of the CSEs is not ideal from studying the North and South American Monsoon systems.
The discussion participants recognized that for VAMOS to understand the land-atmosphere component of NAMS and SAMS, it will be necessary to carry out field studies that apply the observational and modelling methods developed and tested for the GEWEX CSEs in new geographical regions, specifically in the
There is already strong coordination between the individual GEWEX CSEs. In particular, there are plans for coordinated observations in the period 2000-2003 to synchronize with several relevant satellite systems (see http://monsoon.nagaokaut.ac.jp/ceop/index.html). There has also been attempts to improve coordination between GCIP and PACS. However, the workshop noted that currently there is little overall coordination between the land-atmosphere and ocean-atmosphere field studies that will contribute to VAMOS. Providing this coordination between field studies and coordinating the deployment of resources between studies in the northern and southern hemispheres was identified as being a priority task for the VAMOS Panel.
8. VAMOS: Thematic Foci and Working Groups
To progress toward developing plans for VAMOS Field Programmes, five areas were identified as the foci for discussion in plenary. These were: Process studies; Data resources; Enhanced monitoring; Stratusric circulation eeds a threshold temper. Each of these was discussed as time permitted at the Workshop, and the establishment of five Working Groups, one for each area, was recommended as the means to follow up on these initial discussions and to develop contributions to this Workshop Report.
8.1 Process Studies - now underway and running through 2001
a) The Semi-Arid Land Surface Atmosphere (SALSA) Programme (J. Shuttleworth)
The SALSA programme seeks to understand, model and predict the consequences of natural and human-induced change on the basin-wide water balance and ecological diversity of semiarid regions at event, seasonal, interannual, and decadal time scales. It is a long-term programme whose current research and integrated measurement efforts are focused on the San Pedro River basin which originates in northern Sonora, Mexico and flows north into southeastern Arizona. The basin represents a transition between the Sonoran and Chihuahuan deserts and includes a Riparian National Conservation Area together with significant topographic and vegetation diversity, and a highly variable climate, and it spans the significantly different cross-border legal and land use practices at Mexico-United States Border. Major vegetation communities include desert shrub-steppe, riparian, grasslands, oak savanna, and ponderosa pine. In portions of the basin all of these vegetation types are contained within a 20 km span. The border supports the second highest known number of mammal species in the world and the riparian corridor provides habitat for more than 300 bird species. Groundwater sustains the riparian system in the U.S. and also much of the ranching industry in the Mexican portion of the San Pedro. From a socio-economic perspective, great concern exists regarding the long-term viability of the San Pedro riparian system and ranching in the face of continued population growth.
Initial SALSA emphasis is on the following objectives:
b) BISEC - The Bifurcation of the South Equatorial Current (BISEC) (E. Campos)
The oceanic circulation in the equatorial regions of the Atlantic Ocean is characterized by a complex of currents oriented mainly in the zonal direction. One of these currents, the South Equatorial Current (SEC), is formed by at least three easterly branches separated by regions of much less evident counter-currents (Stramma, 1991). It is widely known that the two western boundary currents along the Brazilian coastline, the Brazil Current (BC) and about isture and groundwater reresult from the SEC bifurcation when it impinges the South American Continent. However, in view of the discontinuous zonal structure of the SEC, the exact latitude where this bifurcation occurs is still an open question. The knowledge of the exact location of the SEC bifurcation is very important in the study of the impacts of variabilities in heat and mass transport by the two western boundary currents (BC and NBC) in the global climate.
In January of 1998, during a meeting the Miami, a group of scientists of several institutions decided to begin the structuration of a multinational project with the objective of studying this oceanic phenomenon. An important component of this envisioned study will be the deployment of an extensive array of Inverted Echo-Sounders (IES). As of April, 1998, a project to be submitted simultaneously to NSF (U.S.) and FAPESP (Brazil) was being finalized. The proposed effort intends to deploy a number between 20 and 25 IES covering an area of about 5° X 5° with an average spacing of 1° between them. The group led by S. Garzoli (NOAA/AOML) and W. Johns (RSMAS/UM) will contribute with the results of an ongoing study of aiming to map the variabilities of the NBC dynamic topography and also with the deployment of a number of IES. The Brazilian contribution will also add a few more IES, to be deployed to the south of the GSO array, making it possible to cover an area wider than 10° of latitude.
c) The ARM Programme (R. Lawford)
The Department of Energy's Atmospheric Radiation Measurement (ARM) programme focuses its North American activities in the Clouds and Radiation Testbed (CART) site centred in northeast Oklahoma and at a second ARM site on the north slope of Alaska. The site in Oklahoma provides detailed measurements of radiation balances, water vapour profiles, clouds and aerosols with periodic intensive data collection periods to support specific initiatives. The main focus of ARM science studies are: 1) measurement of instantaneous radiative fluxes and 2) single column climate modelling.
The goal of the radiation measurement programme is to develop radiation measurements for climate studies with 1% accuracy. To achieve this objective measurement systems capable of providing high spectral resolution are being tested and implemented. In the U.S., the ARM programme is supplemented by a GEWEX/GCIP SURFRAD (Surface Radiation) network which has radiation measurement sites at Boulder (Colorado), Fort Peck (Montana), Bondville (Illinois) and Goodwin Creek.
Single column models are being used to examine the initiation of convection and to model cloud radiation interactions. Special intensive observational programmes have been carried out at the CART/ARM site to provide inputs for these studies.
d) Status of the Atlantic Climate Change Experiment (ACCE) (B. Owens)
The WOCE Atlantic Climate Change Experiment (ACCE) observational programme began in October, 1996 with float deployments continuing through late 1998. It consists primarily of several basin-wide hydrographic sections, an extensive deployment of profiling autonomous (PALACE) floats, and a deployment of acoustically tracked (RAFOS) floats within the North Atlantic Current and its extension. The hydrographic sections were carried out in 1996 and 1997, with some further repeats along 24° and 48° N. We expect data from the floats to slowly decrease in volume but to continue through early 2000. Relevant to VAMOS are the following deployments in the subtropical. The interaction of the sub-tropical gyre and the tropical circulation is the focus of an array of 35 floats launched in November 1997 and May 1998 in the Western Atlantic, south of 25°N. These floats are the responsibility of the University of Miami (Leaman). An additional 40 floats will investigate the circulation in the eastern sub-tropical gyre, the formation of the sub-tropical mode water in the eastern North Atlantic, and the interactions between the sub-tropical gyre and the topical circulation. The first ten of these floats were launched in July and August 1997 with the remainder to be launched in October, 1998. These floats are the responsibility of the Woods Hole Oceanographic Institution. The final array of AOML in the tropical Atlantic distributed between the equator and 6°S. These floats transmit only profiles of temperature. They will examine the circulation within the tropical band and also monitor the heat content within the upper 700 m.
The majority of the temperature data gathered by these floats is now being placed on the GTS network after it has been processed by the responsible institution. The remaining temperature data is sent into the ocean thermal data centres by way of the delayed mode data stream. Since this was the first substantial deployment of floats with conductivity sensors, the salinity measurements were not transmitted onto GTS. Although a number of the conductivity sensors on these floats show very low drift, there are examples of significant drift. This suggests that future PALACE float deployments should profile to a sufficient depth so that historical potential temperature ó salinity correlations can be used to monitor sensor drift.
8.2 Process Studies - planned to follow soon, up to 2005
a) Numerical Study of the North Brazil Current Retroflection (RECONOB) (E. Campos and R. Matano)
NBC is a western boundary current in the Tropical Atlantic, which plays a crucial role in the interhemispheric mass and heat exchange. Throughout the year, the NBC presents an intense seasonal variation.
From March to June, most of the water transported by the NBC flows continuously along the western board, reaching the Caribbean Sea. In the remaining of the year, however, the current separates abruptly from the coast, at approximately 6-7°N, in a pattern usually described as the NBC retroflection, feeding the North Equatorial Countercurrent (NECC). During the retroflecting phase, it is common to observe the formation of strong meandering, with the shedding of highly energetic anticyclonic rings. These rings move to the northwest, towards de Caribbean. Although relatively well documented in the literature, these processes associated with the NBC are still not well understood. This project would consist of a concentrated effort to study the structure and dynamics of the NBC in a numerical framework. The idea is to use two models well tested by the scientific community: the Miami Isopycnic Coordinate Model (MICOM) and the Princeton Ocean Model (POM). The results of the two models will be compared to each other, and with observational data.
This project was submitted simultaneously to the Brazilian national funding agency (CNPq) and to the U.S. National Science Foundation (NSF), as part of an international cooperation agreement. It is undersigned by researchers from the Universidade de São Paulo (Brazil), and from the University of Miami and the Oregon State University (U.S.). The PIs of this project are E. Campos, R. Matano and E. Chassignet.
b) PACS Tropical Atlantic (S. Esbensen)
During the 2000-2004 time frame, PACS will shift the emphasis of its field activities from the eastern Pacific toward the tropical Atlantic Ocean where the sea surface temperature anomalies are more subtle and more diverse in terms of horizontal structure than in the Pacific, but no less important in terms of their influence upon precipitation in the adjacent continental regions. While it is premature to propose specific PACS process studies in the tropical Atlantic or the Interamerican warm pool region at this time, we anticipate that pilot monitoring efforts will be required to establish seasonal-to-interannual variability in upper ocean structure and its relationship to wind stress and the surface heat fluxes. Of particular interest to PACS is the PIRATA array of 14 moored air-sea interaction buoys in the tropical Atlantic, described elsewhere in this document. When combined with ocean observations from planned profiling float arrays, tide gauges, and expendable profilers of temperature and salinity, plus observations from satellite and conventional meteorological in situ observations, the PIRATA buoy array will help to provide the context for developing more focused tropical Atlantic field activities. PACS will develop its plans for Atlantic field observations in collaboration with other VAMOS programmes and with investigators who are planning the CLIVAR Atlantic Basinwide Extended Climate Study (Atlantic BECS).
8.3 Process Studies - under discussion for the future, up to 2010
a) South American low-level jet experiment (C. Vera)
Global analyses suggest that there is a southward LLJ to the east of the Andes mountains that contributes to the meridional moisture transport from the Amazon Basin into the subtropical regions of South America and modulates convective outbreaks in those regions. Although there is a generalized idea that the LLJ structure is similar to that of its North American counterpart, the supporting observational evidences is not strong because of the low density of the South America observing system and because of the LLJ itself has subsynoptic length scales. There is a need for a documentation of the location, timing, cross-stream and vertical scales of this LLJ and variability of its structure. Therefore, VAMOS will contribute to planning the realization of a pilot field experiment to characterize the South American low-level jet. Such experiment will provide a coverage of the low level circulation as it emerges from the tropical Amazonia to quantify vertically integrated water vapour meridional transport. This experiment may involve international collaboration from several countries, particularly Argentina, Brazil, Bolivia, and the U.S. It will be based upon current rawinsonde network, augmenting the frequency at some sites of this network. Also deploy rawinsonde and/or pilot balloon sounding sites and move portable units in order to increase spatial distribution of the observational network over the region where the LLJ has been detected: Bolivia, Paraguay and northern Argentina. The pilot field experiment should be conducted in coordination with other field activities in South America, primarily with LBA.
The proposed initiative is expected to establish an important precedent in the assessment of the role of the LLJ as a component of the low level circulation in the rainfall regimes in the Bolivian lowlands and the Altiplano. A long term field programme over the Altiplano will be necessary to assess the relative importance of the heat source associated with the Altiplano on the variability of the South American monsoon system.
8.4 Data resources
a) Data Set Development (V. Magaña)
The VAMOS Data Management will be based, as far as possible, on the principle of free and open access to data. Data access will be achieved through existing data centres, research institutions and universities to the fullest extent possible, rather than through the establishment of one centralized data centre. Data management links will be established through a World Wide Web Home Page for VAMOS, through assistance from the participants to co-ordinate a variety of data management activities. Specific tasks include further identification of existing VAMOS-related data, procedures for accessing them, and co-ordination of VAMOS-specific data needs for field studies.
The data collected during the VAMOS programme will be diverse, requiring a wide variety of data processing and archival methods. VAMOS will promote cooperation between and amongst scientists, private organizations, government agencies and international panels and organizations that result in useful data and information within the VAMOS science plan. As a guiding principle, all data collected during VAMOS should be made available as quickly as possible to all nations and investigators who participate in the program. Procedures and timelines for processing and delivering of data sets should be specified and agreed upon in the implementation plan for VAMOS. Archival and long-term management of VAMOS data sets should make efficient use of existing data management structures and technology. VAMOS will make use of previous experiences (e.g., TOGA) in terms of data formats, means of distribution, etc.
1. Existing sources of data. Historic data sets / data archeology
An initial review of data sets of interest to VAMOS investigators reveal that some data are already available through existing institutions and distribution mechanisms.
While VAMOS will be able to take advantage of a number of existing data centres activities, as well as the data sets discussed above, the programme will require a number of specific data sets defined in response to the VAMOS science and implementation plans. VAMOS will provide an opportunity to merge a number of data sets from several different data sources to provide a coherent description of the American monsoon systems to support diagnostic and empirical studies, model validation studies and prediction activities. In each case new data will go through a quality control process that will include the rigours of scientific research.
The development of historical daily precipitation and temperature data sets will be a key activity. This development will build on current activities in IAI, PACS, GCIP, LBA and other regional and national activities.
VAMOS will establish a close relationship with National Weather Services, particularly in Latin America, to encourage them to maintain and support valuable observation systems. VAMOS will also encourage these institutions to make their historical archives widely available.
2. The VAMOS data base
VAMOS will require development of a data base to serve the needs of the diagnostics, empirical studies, modelling and prediction communities in VAMOS. The requirement to link with several data centres, research institutions and regional projects strongly suggests the development of a distributed data base structure linked through a VAMOS Home Page. This page will provide an efficient mechanism for accessing information about data and for effecting data exchange. The distributed data base concept will allow VAMOS to interact efficiently with the Inter American Institute (IAI) for Global Change Research, as well as other regional centres such as CPTEC and regional projects such as GCIP and LBA.
b) Surface Radiation Budget (R. Koster)
Estimates of the surface radiation budget over land may be critical to understanding and predicting the interaction of the land surface with monsoon precipitation. A historical breakdown of the budget on a global scale is available from the Langley DAAC (web site: http://agni.larc.nasa.gov/SRB_homepage.html). The GCIP SRC project (http://metosrv2.umd.edu/~srb) will provide near-real-time budgets over the U.S. and northern Mexico.
c) South American Surface Observations (V. Barros, J. Paegle)
VAMOS will strongly encourage the recovery of precipitation and surface temperature time series that are currently stowed away on non-digital formats in various non-conventional archives. This is especially applicable to data from South America where long term time series of good quality (sometimes going back to the turn of the century) exist in national records or private holdings in paper form. Examples are national and state agencies involved in the management of water resources and landowners. There is an urgent need to develop funding opportunities to rescue this valuable resource which might otherwise be lost due to the frailty of the records.
The private sector has recently engaged in active measurements efforts in some South America countries. For example, one company has sold 800 automatic stations in Argentina only in the last 5 years. This figure should be compared with less of 100 sites currently available in the GTS. Policies need to be developed to access this important resource and make it available to the science community. A concern is the fact that many of the national weather services do not encourage the open exchange of data. This pressing issue, that threatens the integrity of key scientific data, needs to be addressed at the highest possible administrative levels.
d) South American Surface Observations-river Runoff (J. Marengo)
Surface observations from South America are available from several sources, while curiously the countries themselves are a bit reluctant to provide climatic and hydrological data. The meteorological services of the South American countries usually sell the data, unless there is some formal scientific collaboration with them, in that case they would provide some limited data as their in-kind contribution. In the general situation, long term observations of the South American countries for temperature, pressure, precipitation can be found at the web site of the National Climatic Data Center (NCDC). The countries submit this data to WMO, especially for the synoptic (airport) stations. For Brazil, rainfall data is available from the Agencia Nacional de Aguas e Energia Eletrica (ANEEL), same as for river data, while some rainfall archives can be found at the CPTEC. It is possible to get access to this data by request, with a low overhead cost.
For river data, data banks such as the Global Runoff Data Centre (Koblenz, Germany) have observations from several rivers in South America, while the Pacific Rim Streamflow Data Center from UCLA (U.S.) have data from the Pacific coast countries of South America Peru and Chile. The Home page of the EOS program, at the University of Washington in Seattle have also data from rainfall and rivers for the whole Amazon basin.
The NCAR data sets also included the WMO river data sets which have records from big South American Rivers: Paraná, Amazon, Orinoco, etc., but somewhat non updated to the 1990's. In Brazil, the ANEEL have good quality river data for the entire country, and other data banks include ELETRONORTE, ELETROBRAS, PORTOBRAS, and other private institutions that may be willing to provide their data if some formal collaboration is established with them. Most of this data is available from the archives at CPTEC, but since is subject to copyright it is not for free distribution.
The LBA experiment is going to generate plenty of data, including surface and upper air, as well as hydrometeorological data, that is going to be available to any scientist that works in LBA related research. Pre-LBA data sets are being complied on CD-ROM and will be freely available by the end of July.
e) Land Use and Land Cover Change (J. Marengo)
The predictions of future changes in the functioning of Amazonia rely on quantitative forecasts of the rates of change and spatial patterns of future land-cover, and our predictive understanding of the land-use practices that drive these changes.
The conversion of closed-canopy forests to agricultural fields can be (and is routinely) estimated using existing techniques, but statistics on other land-cover changes such as selective logging, the conversion of cerrado vegetation to agriculture, and the abandonment and regrowth of forests are not available, although with some additional research investment satellite imagery might be used to distinguish between these categories.
The Land Use and Land Cover component of the LBA experiment, will address the following questions:
The temporal and spatial scope of satellite data provides a unique tool that can be used to study the dynamics of vegetation communities (disturbance, succession, fire, etc.) over a wide range of scales. Remote sensing will help to place the intensive study sites in their correct ecoclimatological and geographic context. This is important to enable optimisation of the field sampling design at basin-wide and more local scales and will be necessary for correct interpretation of research results.
Finally, because biomass burning is an extremely important issue in Amazonia, remote sensing will be used to monitor the frequency of occurrence and extent of fires and subsequent distribution of atmospheric aerosols.
According to the results of the PRODES project (Deforestation Project) carried on by INPE and the Brazilian Institute of Environment (IBAMA), the rate of deforestation in Brazilian Amazonia decreased from 1988 to 1991, to later increase in 1995, with a tendency to diminish after that time (see Table 2).
f) Paleoclimate (J. Marengo)
There are few studies on the paleoclimatology of South America Those for the Amazon basin are mostly based on records of pollen or lake sediments in different parts of Brazil. Martin et al. (1997), used pollen records from Serra dos Carajas (Para) in Amazonia, to identify the occurrence of successive dry periods between 6,000 and 4,000 BP and the existence of try periods of drought in the last 2500 years. Martin et al. (1997), Ledru (1993) and Ledru et al. (1994), and Servant et al. (1993) used analysis of pollen and lake sediments in different sites in central and southern Brazil. They proposed that 11,000 years ago winters were more intense, and that cold air coming from the south (friagens) penetrated more northerly than on the present climate. This was consistent with a weak ITCZ.
During the last glaciation maximum, Clapperton (1993) indicates for South America that draw-down of water tables possibly impacted forest cover, enhancing the drying influence of reduced SST and atmospheric humidity. As forest cover diminished in extent, affecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation. Substantial changes in precipitation totals and incidence, and in grown water availability, probably eliminated modern time tropical rainforests except in areas currently receiving approximately 5,000 mm of annual rainfall.
The Amazon basin exhibits a warm and humid climate at the present time, but this behaviour has not been constant during the last 15,000 years. Variations on the orbital parameters, in relation to the sun produced changes in the amount of solar radiation received on the planet's surface, modifying the composition of the governing atmospheric systems, and consequently the climate. The reduced solar radiation determined that the sub tropical Atlantic high and the cold oceanic currents (Malvinas Current) moved towards the equator. With the cooling of the Atlantic, the trade winds were less intense and did not bring much moisture towards the continents, determining and increase in the aridity of the region. The main climatic changes occurred on the Quaternary, were the results of alternations between glacial and interglacial periods, determining sudden changes, such as the domain of savanna over the tropical forest during cold and dry glacial climates.
Analyses of pollen (Absy, 1985) found in lake sediments indicate that during the Holocene (between 5,000 and 3,000 years ago), large extensions of savanna existed in Amazonia, where currently there is forest. Associated with this cooling, their sea level was anomalously low, which in turn would affect the moisture transport into the Amazon region. Pollen information also indicated that there was no tropical forest at the end of the Pleistocene (approximately 11,500 years ago). Between 4,000 and 2,100 years BP, and near the year 700 (1200 AD), Abyss (1985) suggested the occurrence of large variations of rainfall in the basin, determining a reduction in the volume of water of the Amazon rivers, and sometimes their total drying, affecting significantly the flora and fauna.
Today, orbital parameters are such that earth is far from the sun in June and closest in December in the Southern Hemisphere (Martin et al., 1997). As a consequence, seasonal differences are strong, with warm summers, cold winters and strong seasonal shifts of the ITCZ. In contrasts, around 11,000 years BP, the earth was closer to the sun in June and farther from it in December, resulting in colder summers, warmer winters, and reduced seasonality in the Southern Hemisphere. The continent was not warming as much as today during the austral summer, and the ITCZ likely was located farther north. Today, polar advections penetrate the continent from the south on wintertime. A weaker ITCZ 11,000 years BP would have helped cold advections to penetrate farther north in spring and autumn, and perhaps in summer. Support for this comes from Ledru (1993) who acknowledge the presence of the 880 km northward expansion of Araucaria Forest into the Salitre area (southeastern Brazil) by about that time. Modern Araucaria forests are closely linked to precipitation occurring in the frontal zone of polar advections and can not tolerate more than one month drought. In addition, the strengthening of the polar advections would increase snowfall in the south of the altiplano. Ledru (1993, 1994), and Servant et al. (1993) using results from pollen analysis at Salitre and Serra de Boa Vista (in Santa Catarina, Brazil) indicate the localization of Araucaria forests during the Holocene. The forests have been detected in Salitre at 9500 years BP and in Serra de Boa Vista at 4000 years BP, and the development of Araucaria forest in Salitre and Serra de Boa Vista indicates that southern Brazil climate and vegetation are influenced by polar advections and that today the existence of relict forests is due to specific local climatic conditions. Based on 11 pollen records from central and southern Brazil, Ledru et al. (1997) indicates that between 7000 and 000 years BP a more seasonal climate is recorded, implying that polar air advections probably were weakened. Between 4000 and 0 years BP modern climatic conditions became established, shown by the development of cerrado vegetation to the north, semi-deciduous forest in the centre and Araucaria forest to the south. Cold fronts remained restricted to their present day position, between 25° and 30°S latitude.
Palynological studies on late Quaternary lake sediments from the region of the Amazonian estuary, 100 km north-east of Belem, Para State, Brazil, indicate the presence of Podocarpus which suggests a distinct climatic cooling (Behling, 1996). During the Last Glacial, cold climates have been thought to relate to increased frequency and intensity of Antarctic cold fronts in southeastern Brazil. Today, the occurrence of polar air reaches rarely as far equatorwards as during the last glacial. During the Holocene, it is quite possible that the present-day climate zone of a long dry season in southeastern Brazil extended into southern Brazil. Warmer and drier climates with a pronounces dry season can be explained by a stronger influence of dry tropical continental air masses, that would have blocked polar cold fronts. Initial expansion of Aracucaria forests, probably along the rivers, started about 3000 years BP, and suggests a cooler and somewhat wetter climate than before.
Behling and Lichte (1997) indicate that climatic conditions during full-glacial time were apparently too dry for large Araucaria populations, which required a cold and humid subtropical climate without significant dry periods. Furthermore, the climate was too cold, and there were too frequent frosts. Dry climate conditions with reduced cloud cover might have favoured frosts during winter nights. Cold climate during glacial times were caused, according to them by increased frequency and intensity of polar air outbreaks. These cold air associated to cold fronts reached farther north and had a significantly stronger influence in southeastern Brazil during the last glaciation than they do today. Northward migration of the frontal systems, currently observed in central Brazil, causes heavy rainfalls when cold air masses pass below the wet tropical air mass. For present climates, studies by Marengo et al. (1997) indicate that wintertime cold fronts do not produce rain as much as cooling, while summertime fronts tend to organize and produce convection in eastern Amazonia and central Brazil.
g) Upper Air (CARDS) --- Aircraft Data (P. Silva Dias)
Upper air data availability in South America is strongly limited by the frequency (in general only once a day) and by the low spatial density), primarily in certain areas such as in the tropical sector of the continent. Satellite data provides the major source of information in the tropical sector but its use is limited by the larger errors in comparison with the direct measurement of temperature, moisture and winds by the radiosonde network. Aircraft data is an important data source and the automatization of the measurement process and transmission to data collecting centres provides a unique source of reliable soundings and flight level temperature and wind information. However, availability of commercial aircraft information over South America is very limited in comparison with other areas, taking into account the air traffic density. VAMOS should encourage the operational sectors to enhance the data collection and to improve the communication links in order to augment the aircraft data flow to the numerical weather forecast centres.
In view of the relatively low cost of the instruments on board commercial aircraft as well as the cost for the necessary installation on board, VAMOS should consider the possibility of establishing agreements with commercial airline companies to provide them with the appropriate instruments as well as establishing the communication links. Recently developed instruments allow moisture measurement at low cost thus providing a complete temperature, moisture and wind profiles and flight level information. Enhanced aircraft data over South and Central America would certainly have a significant impact in the 4DDA systems.
h) Lightning Data (P. Silva Dias)
Lightning data provides a relatively low cost reliable information of the intensity and frequency of deep convection. Operational lightening detecting systems are already available in some tropical regions such as in Southeastern Brazil. Recently developed satellite based systems are also producing large spatial coverage over the tropical Americas as part of the TRMM programme.
i) The Brazilian National Oceanographic Data Set (C. Tanajura)
The Brazilian National Oceanographic Data Set (Banco Nacional de Dados Oceanograficos, BNDO) maintained by the Directory for Hydrography and Navegation (DHN) of the Brazilian Navy. It contains data on physical and chemical properties of sea water, as well as meteorological and geological conditions, obtained from Brazilian Navy vessels and from commercial vessels. Until the end of 1997, regarding pure oceanographic data, there was a total of 923 currentmeters, 1319 CTD stations, 14953 XBT stations, 13688 subsurface water samples, and some ADCP's. The data set focus on the Atlantic, and most of them (including XBTs data) are dense only in a broad strip along the South American east coast from the equatorial region until the mouth of the Plata River. New data is being continuously added to BNDO, and less than 1% of the total amount of data available needs to be digitalized.
DHN has signed international agreements (under the COI International Oceanographic Data Exchange system) to allow researchers and institutions access to BNDO by request. Other Latin-American countries might be also willing to collaborate with VAMOS providing access to data sets similar to BNDO. DHN is also a strong partner of the PIRATA project and to the Brazilian National Buoy Program. It has successfully deployed the first 3 Brazilian PIRATA ATLAS buoys with the collaboration of NOAA/PMEL, and it is committed to the missions of deployment and replacement of all PIRATA buoys under Brazilian responsibility.
BNDO can be reached in the web site http://www.mar.br/~dhn/dhn.htm (up to now, only a Portuguese version is available; English version is under construction) or through the e-mail bndo@dhn.mar.mil.br.
j) Pre-LBA data sets (J. Marengo)
The data collection efforts in field experiments previous to LBA included satellite imagery, micrometeorological observations, near surface and upper-air atmospheric conditions, surface biophysical and hydrological measurements obtained throughout the last 20 years. Data were collected by several intensive field campaigns, during the rainy and dry seasons, and other periods that varies from short intensive field campaign to several years worth of data, measured sometimes with a time resolution of 5 minutes and 1 hour. The following table shows the main Pre-LBA data sets collected in Amazonia, as well as the name of the experiment, the contact institutions. This data sets are part of the Vols. 1-3 of the Pre-LBA data sets CD ROM that will be released by CPTEC/INPE in September 1998. For more information, contact Dr. Jose A. Marengo, leader of the Pre-LBA data sets working group (marengo@cptec.inpe.br). In addition this data is available directly on CPTECís web site: http://yabae.cptec.inpe.br/lba/prelba/field.html
| Institution | Dataset |
| EOS/CSRC/SER | ABLE (Amazon Boundary Layer Experiment) |
| CPTEC/INPE | ABRACOS (Anglo-BRazilian Amazonian Climate Observation Study) |
| IAE/CTA | ARME (Amazon Region Micrometeorological Experiment) |
| DSR/INPE | FLOODAMA (Amazon Floodplains) |
| UFPA | FLUAMAZON (Amazon Humidity Flux) |
| NASA/GSFC | ISLSCP (International Satellite Land Surface Climatology Project) |
| CENA/USP | ADAMBRAZIL |
| IAE/CTA | RBLE (Rondonia Boundary Layer Experiment) |
| DCM/INPE | RONDONIA MAPS |
| CPTEC/INPE | SCAR-B (Smoke, Clouds and Radiation - Brazil) |
| University of Washington-Seattle | CAMREX (Carbon in the Amazon River Experiment) |
| INPE | TRACE-A (Transport and Atmospheric Chemistry Experiment - A) |
1. ABLE (Amazon Boundary Layer Experiment)
The ABLE missions have been designed specifically to study the rate of exchange of material between the Earth's surface and its atmospheric boundary layer, and the processes by which gases and aerosols are moved between the boundary layer and the 'free' troposphere. These expeditions are conducted in ecosystems of the world that are known to exert a major influence on global atmospheric chemistry. In some cases, these ecosystems are undergoing profound changes as a consequence of natural processes and/or human impact.
The ABLE-2 project consisted of two expeditions: the first in the Amazonian dry season (ABLE-2A, July-August 1985); and the second in the wet season (ABLE-2B, April-May 1987). The ABLE-2 core research data were gathered by NASA Electra aircraft flights that stretched from Belem, at the mouth of the Amazon River, west to Tabatinga, on the Brazil-Colombia border, from a base at Manaus in the heart of the forest. These observations were supplemented by ground based chemical and meteorological measurements in the dry forest, the Amazon floodplain, and the tributary rivers through use of enclosures, an instrumented tower in the jungle, a large tethered balloon, and weather and ozone sondes.
The ABLE was a collaboration of U.S. and Brazilian scientists sponsored by NASA and Instituto Nacional de Pesquisas Espaciais (INPE) and supported by the Global Tropospheric Experiment (GTE) component of the NASA Tropospheric Chemistry Program.
2. ABRACOS (Anglo-Brazilian Amazonian Climate Observational Study)
ABRACOS had the primary objective of providing the field data needed to calibrate and validate land surface models of Amazonian pasture. ABRACOS was carried out at three sites in Brazilian Amazonia between 1990 and 1994. The experimental strategy was to have continuous collection of climate and soil moisture data at three pairs of forest and pasture sites, complemented by a series of six experimental missions during which intensive micrometeorological, plant physiological and soil measurements were made. During the project it was possible to expand the scope of the work to include measurements of the atmospheric boundary layer above forest and pasture (a contribution to the RBLE campaigns described below), measurement of carbon dioxide fluxes and the use of remote sensing to estimate forest regrowth. The ABRACOS sites were established close to Manaus in central Amazonia, Marabá in the east and Ji-Paraná in the south west of the basin. The project is summarized by Gash and Nobre (1997, Bulletin of the American Meteorological Society) and many papers describing all aspects of the work are gathered together in a book, "Amazonian deforestation and Climate", edited by John Wiley and Sons 1996.
The data collected under ABRACOS are made available by the UK Institute of Hydrology and the Centro de Previsao de Tempo e Estudos do Clima (CPTEC) of the Instituto Nacional de Pesquisas Espaciais. ABRACOS is a collaboration between the Agencia Brasileira de Cooperacao and the UK Overseas Development Administration.
3. ARME (Amazon Region Micrometeorological Experiment)
ARME was the first experiment explicitly directed towards improving the land surface description in general circulation models of the atmosphere. The experiment was an Anglo-Brazilian collaboration and addressed the requirement to provide an initial point calibration of the energy-water interaction between tropical rainforest. And the atmosphere. Experiment studies took place over two years from September 1983 at a single site over undisturbed tropical forest near the city of Manaus in central Amazonia.
4. CAMREX (Carbon in the Amazon River Experiment)
The objective of CAMREX over the last decade has been to define by mass balances and direct measurements those processes which control the distribution of bioactive elements (C, N, P and O) in the mainstream of the Amazon River in Brazil. The CAMREX dataset represents a time series unique in its length and detail for very large river systems. The central sampling strategy has been to obtain representative flux-weighted water samples for comprehensive chemical analysis and to make rate measurements over 18 different sites within a 2000 km reach of the Brazilian Amazon mainstream, including major intevening tributaries. Samples have now been collected on 13 different cruises (1982-1991) during contrasting hydrographic stages.
Institutions involved in CAMREX are the Centro de Energia Nuclear Aplicada a la Agricultura (CENA) of the University of São Paulo, Brazil, the Departamento Nacional de Aguas e Energia Eletrica (DANEE, now ANEEL) from Brazil, and the University of Washington in Seattle, U.S.
5. FLOODAMA (Amazon Floodplains)
This data set includes the Digital Mosaic of the Amazon River Floodplain (MDPA) prepared using Landsat TM images. This mosaic was planned in July 1995 as an activity of the EOS-IDS Project that has been developed inside the cooperation among INPE, CENA, University of Washington in Seattle (UW), University of California in Santa Barbara (UCSB), and NASA. The MDPA is composed by 29 Landsat TM images that were selected with minimum cloud cover and within the high water season of Amazon river. These images were geometrically corrected using ground control points extracted from topographic charts and image charts in the 1:250,000 scale. In addiction, these images were radiometrically rectified to 231/062 (Manaus region) TM image using the method developed by Hall et al. (1991). The radiometric rectification produce different results for TM bands due to atmospheric effects and scene characteristics. The MDPA was then built using the best bands (rectified or non-rectified) of the TM images with a 100 m by 100 m spatial resolution.
6. FLUAMAZON (Amazon Humidity Flux)
The FLUAMAZON Experiment was designed to measure the moisture flux from along the northern coast of South America (near the mouth of the Amazon River) into the central Amazonia. This experiment took place from November 23 to December 21, 1989 during the period of transition between the dry and humid seasons in this region. During FLUAMAZON, radiosondagens were made simultaneously in five different places: Alcantara, Belem, Oiapoque, Manaus e Alta Floresta (see map for location of the stations). Some of the studies performed using data from FLUAMAZON were related to the atmospheric thermodynamic structure over Amazonia.
Institutions involved in the FLUAMAZON experiment were the Instituto Nacional de Pesquisas Espaciais (INPE), São Jose dos Campos, São Paulo-Brazil, Universidade de São Paulo (USP), São Paulo, São Paulo-Brazil, Universidade Federal do Para (UFPA), Belem, Para-Brazil, Universidade Federal de Alagoas (UFAL), Maceio, Alagoas-Brazil, Centro Tecnico Aeroespacial (CTA), São Jose dos Campos, São Paulo-Brazil. Centro de Energia Nuclear aplicado ` Agricultura (USP/CENA), Piracicaba, São Paulo, Brazil;
7. ISLSCP (International Satellite Land Surface Climatology Project)
The ISLSCP Initiative I data sets should provide modellers with many of the fields required to prescribe boundary conditions, and to initialize and force a wide range of land-biosphere-atmosphere models. All of the data have been processed to the same spatial resolution (1° x 1°), using the same land/sea mask and steps have been taken to ensure spatial and temporal continuity of the data. The data sets cover the period 1987-1988 at 1-month time resolution for most of the seasonally varying quantities and at 6-hourly resolution for the near-surface meteorological and radiative forcings.
The data sets on the CD-ROM sets are organized into five groups: 1) Vegetation, 2) Hydrology and Soils, 3) Snow, Ice, and Oceans, 4) Radiation and Clouds, and 5) Near-Surface Meteorology. The data within each of these areas were acquired from a variety of sources including model output, satellites, and ground measurements. The individual data sets were provided in a variety of forms. In some cases, this required the data publication team to regrid and reformat data sets and in others to produce monthly averages from finer resolution data. The specific handling for each data set is detailed in the documentation. The regridded, reformatted, integrated, and peer reviewed data sets are published on this five-volume CD collection.
8. RADAMBRASIL
The RADAMBRASIL project extensively mapped the Amazon soils on a pedological base. As a result, 1162 soil pits, distributed basin-wide, were described horizon by horizon, carrying out lab analyses of texture, major cations and other chemical elements. Combining soil pit information, aerial photographies and geological maps, the RADAMBRASIL project produced soil maps of the Amazon on a 1:1 000 000 scale.
9. RBLE Rondônia Layer Experiment)
During ABRACOS three atmospheric boundary layer (ABL) measurement campaigns were carried out. These campaigns were called the Rondônia Layer Experiment (RBLE) 1, 2 and 3 and were held as Ji-Paraná where the scale of the forested and deforested areas is large enough for each surface type to develop its own ABL. The campaigns were held during the dry season when the difference in evaporation between the two surfaces types, forest and pasture, is at its greatest. Measurements were made with both free-flying radiosondes which measure temperature, humidity and wind up to about 12 km and with a tethered balloon which makes more detailed measurements in the lowest 1km of the atmosphere. Measurements were made at both the forest and clearing sites. Profiles of potential temperature measured during RBLE2 show that the daytime ABL was deeper over the clearing than the forest. The data have been used to test several models of ABL development. It appears that the ABL over the pasture grows more rapidly than predicted by the models, possibly because of the increased turbulence generated by the strips of forest typical of this area. The data have also been used to initialise one dimensional climate models used in experiments to investigate the sensitivity of climate to land surface parameters, and to initialise a meso-scale model which can predict local effects on climate caused by the pattern of deforestation in this area.
10. RONDONIA MAPS
Surface parameter digital maps of vegetation, soil and topography were obtained over Rondônia, Brazil, covering the 5° x 5° region 8°-13°S, 65°-60°W. Numerical maps of the natural landscape structure have been achieved by digitizing existing 1:1,000,000 maps. High-resolution (LANDSAT) satellite data are employed to give information about the most recent modifications of the surface due to human activities. This database can serve as a basis for meso-scale meteorological modelling.
11. SCAR-B (Smoke, Clouds and Radiation - Brazil)
The SCAR-B Project was conducted in Central Brazil and southern August 15 to September 20, 1995. This describes the Brazilian meteorological contribution to SCAR-B mission, including the occurred climate study and the weather analysis and forecast. Twice daily forecast, based on Centro de Previsao de Tempo e Estudos Climáticos and National Centers for Environmental Prediction (NCEP) model outputs and conventional observations, provided special support for mission plans. Long periods of haze, low index of relative humidity and also little cloudiness and rain occurred during the mission due to frontal system's blocking at South Pacific Ocean. Backward trajectory calculations show three types of air particles which have come from Pacific Ocean, Amazonia and other continental regions. Some results demonstrate the strong sensitivity of the atmosphere-surface radiative budget to the optical parameters of smoke particles at Cuiaba site.
12. TRACE-A (Transport and Atmospheric Chemistry near the Equator-Atlantic)
The NASA TRACE-A field study was deployed in August 1992 to determine the cause and source of high concentrations of ozone that accumulate over the Atlantic Ocean between southern Africa and South America during the months of August through October. This pool of ozone was initially discovered in the mid 1980's as a result of the re-analysis of ozone measurements from two operational satellites using a newly developed mathematical technique to extract the concentration of ozone in the troposphere. The satellite data provided the first hints of ozone spread over thousands of square kilometres over the Atlantic Ocean at concentrations comparable to those found in many large cites around the world during the summertime. The fact that the enhanced levels of ozone over the Atlantic were observed to be the highest during the southern hemisphere's springtime, a period of intense burning of vegetation in both southern Africa and South America, suggested a link between the biomass burning and the ozone pollution.) As additional satellite data were analysed an alternative source was suggested to be the downward transport of ozone from the stratosphere linked to a sinking motion of air prevalent over the region during the southern hemispherical springtime.
13. Additional Pre-LBA data
Additional data sets on Amazonia have been compiled by the Woods Hole Research Center, in the USA, by Peter Schlesinger, Daniel Nepstad and Paul Lefebvre. This collection of Amazonian datasets was assembled with funding from the National Aeronautics and Space Agency (NASA).: http://www.wh rc.org/tropfor/humanimpacts/WHRClba.htm
8.5 Enhanced monitoring
a) Radiosondes and profilers (M. Douglas)
The current radiosonde network over much of South America is inadequate to describe the atmospheric variability at a spatial scale comparable with the important rainfall variations. Some key regions, such as the western Amazon Basin, are not currently sampled by any radiosonde systems. Other key sites, such as Lima, Peru, and San Cristobal (Galapagos Islands), do not have an assured supply of radiosondes. No sounding sites are located near the LLJ over eastern Bolivia and western Paraguay. Given this situation, it is important that VAMOS consider what sites are essential for the program's success, and attempt to ensure that long-term observations are made at these sites.
Some of the key current radiosonde sites that should be maintained as part of VAMOS include: Lima - Peru, Manaus - Brazil, Leticia - Colombia, San Cristobal - Ecuador. These sites, from which soundings have been made for a number of years, provide the framework for a rudimentary sounding network in northern South America. Additional sites where radiosonde soundings are desirable include both Santa Cruz and La Paz, Bolivia, Mariscal Estigarribia, Paraguay, and one or more sites in southern Venezuela. In Central America the radiosonde sounding network is generally adequate, but an additional site between San Jose, Costa Rica and southeastern Mexico is desirable.
Although the high frequency of wind profiler observations is usually not needed for climate studies, there may be locations where it is desirable to maintain such profilers. Such possibilities include a site along the core of the LLJ east of the Andes in Bolivia and one or more such sites in northern Mexico; both regions exhibit large diurnal variations in the low-level windfield. The exact characteristics of the profilers need to be determined; 915 mHZ profilers are relatively inexpensive for example, but do not have the capability to obtain winds to high levels.
The possibility of operating radiosonde systems from ships should be investigated. Some ships would provide only infrequent transects through the VAMOS region, whereas others, such as tuna boats, could provide much longer records over more limited regions. A critical constraint would be the size and simplicity of the sounding system, together with the availability of personnel to make the observations.
New technologies for making radiosonde soundings should be investigated, such as glidersondes and remotely controlled aircraft. These may be means of obtaining more affordable soundings at land or island sites. Over the ocean, it may be possible to operated remotely piloted or autonomous vehicles to make soundings at locations far from land. Such technology is maturing at the moment and should be investigated for VAMOS monitoring activities.
b) Air-Sea Fluxes (R. Weller)
Traditionally, the bulk of surface marine observations have come from merchant ships. There have been numerous attempts to make use of these observations to map the air-sea fluxes and understand the ocean's role in climate. However, these shipboard observations have significant errors associated with the sensors, sensor placement, and flow disturbance. Furthermore, few ships are equipped to measure the short-wave and long-wave radiative fluxes.
Work to improve buoy meteorological and air-sea flux measurements has met with great success. A major, international collaboration on flux sensors and algorithms and a specific focus on in-situ intercomparisons of methods and sensors during the TOGA Coupled Ocean-Atmosphere Response Experiment (COARE) led to monthly mean net heat fluxes with an accuracy of better than 10 Wm-2. Further work has continued and gains made in reducing error due to radiative heating of sensors. Work on aspiration, on humidity sensors, on anemometer performance, and on radiation sensors continues; and these gains help make possible an accuracy approaching that achieved in COARE even though in-situ intercomparisons are not being conducted.
The time series of accurate surface meteorology and air-sea fluxes acquired by such buoys deployed in these experiments can now provide the means to examine the performance of atmospheric models, the accuracy of climatological data sets, the calibration and performance of satellite sensors, and methods used to improve the data on Volunteer Observing Ships (VOS). Taylor and Josey at the Southampton Oceanography Centre in the UK have worked to correct biases and errors in the data from the VOS; and comparisons between the buoy data and the SOC climatology verify that they have made significant improvement. In contrast, comparisons between the buoy data and numerical weather prediction models reveals problems with the surface meteorology and fluxes from the models. These problems may not be apparent in the net heat flux, as the heat flux components from the models can have biases that cancel. The latent and short-wave fluxes from the Hadley Centre model, for example, have biases of opposite sign.
High quality in-situ data is essential to validating models, remote sensing, and climatologies. A comparison between one year of buoy data from the Arabian Sea and net heat fluxes from the ECMWF, NCEP, and Hadley Centre models revealed that model error can approach 100 Wm-2; during the Southwest Monsoon the NCEP model had the wrong sign of the net heat flux.
Such SOC and buoy comparisons indicate the need to make regional choices of the parameterizations used in the bulk formulae. There is both temporal and spatial variability in the fluxes due to factors such as atmospheric aerosols that make high quality in-situ observations essential. Taylor's earlier work documented the benefits of a modest investment in understanding and improving the sensors in VOS. Thus, a strategy for greatly improved surface meteorology and air-sea fluxes for VAMOS is to deploy a number of surface moorings as flux reference sites and to field improved VOS systems to fill in the regions around the reference sites. The flux reference sites provide the regional tie points, and the VOS calibrated by these sites provide the mapping capability.
c) Equatorial Andes Monitoring (P. Silva Dias)
The air mass exchange across the Andes is not well known in view of the limited amount of data available. Surface meteorological data in the upper Andes is frequently representative of local circulations that are not representative of the synoptic flow. There are short term measurements, based on upper air data collected over the northern Chile area and the Bolivian Altiplano which confirm the possibility of strong zonal transport. There is also some numerical modelling evidence indicating air mass transport across the Andes in association with the local mountain generated circulation (Dappozzo, 1995) and on air parcel trajectories (Freitas et al., 1996). The air parcel trajectory studies suggest the transport of smoke produced by biomass burning the southern part of the Amazon towards the East Pacific Ocean trough gaps in the Andes in Colombia. Measurement of trace gases at surface stations in the upper Andes would indicate the air mass origin thus avoiding the uncertainties related to the lack of representativeness of the local winds.
The long time series of river levels that are available in Brazil have not been very much used by most authors interested in interannual and interdecadal variabilty.of tropical South American land climate and possible non-local forcing processes. In two recent papers (Marengo, 1995; Marengo et al.,1998) examine some of the longest series, and discuss their relation to stream flow and precipitation over their drainage basins. For the northern Amazon area, Marengo (1995) suggests that variability of the Rio Negro levels is associated to El Niño events. However, the moisture flux convergence in Amazonia is in part due to the Atlantic Northeast Trades, which is suggestive that variability of the atmospheric circulation over the Tropical Atlantic could be also connected to interannual variability of Amazonian river levels. In fact, the anti-correlation between Tropical Atlantic Dipole Indices (e.g., Servain, 1991) and northern Amazonian levels is high (around 0.6 even for Rio Negro), as with Nordeste (north-northeast Brazil), while the opposite phase is observed for southern Amazonia. This is typified by the long positive anomaly in northern river flows during the 1972 El Niño and 1973-1974 Atlantic Warm Event, and the opposite situation during 1979-1983, as compared to the southern Amazonian rivers, when their flows attained record levels at the latter period. This is probably due to the fact that during Atlantic Warm Events, the northeast Trades are intensified, strengthening the low pressure cells up to the northern Amazon area during the rainy season. This explains why Nordeste and northern Amazonia river level and flow anomalies tend to be in phase, while southern Amazonia and eastern Brazil tend to have opposite phases. It seems to be the balance between the El Niño and the Dipole induced circulations what essentially controls interannual variations, which may couple into the observed strong interdecadal variability. It is therefore suggested that a network of coastal monitoring met-ocean buoys and met stations around both the Amazon river delta and the Caribbean coastline could in principle provide useful data for constraining the boundary conditions for studies of ocean-atmosphere-land interactions within VAMOS.
The hydrological state of the land surface (as characterized, e.g., by the root zone soil moisture) can strongly influence the evolution and final intensity of continental monsoons. A programme aimed at monitoring the land's hydrologic state in monsoon regions may thus prove valuable. The programme would not involve direct measurement of soil moisture, which would be very difficult given the spatial scales under consideration; rather, it could involve the continued forcing (with observed precipitation rates, radiation fluxes, and near-surface meteorology) of a gridded array of detailed land surface models to obtain a continuous updating of the current soil moisture state. Strategically located soil moisture sensors or flux towers could be used to calibrate and validate the modelling system's performance.
d) Selected Energy and Water Budget Sites (R. Lawford)
Both GCIP and LBA are giving priority to the installation of stations aimed to close the energy and water budgets at the regional spatial scales and to validate the meso-scale models being used in energy and water budget computations. These stations provide temperature, wind, and precipitation, and in the case of the better grade stations also provide downwelling short wave, upwelling long-wave and net radiation; profiles of ground temperature and moisture fluxes and tower data to provide data for the computation of latent and sensible heat fluxes. However, GCIP and LBA do not provide these stations in certain climate zones, nor are they providing data from the areas most central to understanding the South and North American systems. Given these considerations, sites outside LBA and GCIP areas are recommended for: 1) Altiplano, 2) Argentina steppe, 3) Mexico, along a north-south transect, 4) Northern Chile, and 5) New Mexico/Arizona.
Potential funders of these enhancements include IAI, U.S. and other national governments.
e) Enhancing monitoring of precipitation at high elevations (R. Fu)
In view of the high inhomogenity and strong topographic dependence of the precipitation field, the current ground based observations, largely through the raingauge network in South America are highly uncertain in a complicated topographic region in terms of adequately representing the amount and spatial distribution of the precipitation climatology. At high elevations, the uncertainty is much greater due to the sparse distribution of the raingauges and the sharp changes of topography. Since precipitation is one of the most important elements of the monsoon process, insufficient observations of ground "truth" precipitation seriously limit our ability to understand the precipitation processes, to validate modelled precipitation, and to develop the methods for the remote sensing of precipitation in the South America.
Precipitation on the top and the east slope of the Andes may provide an important latent force to drive the LLJ (e.g., Cohen et al., 1995; Silva Dias, personal communication) and hence moisture transport to the convective regions. In these regions, the topographic forced precipitation clouds may appear warmer so the GPI based on infrared radiance may underestimate the precipitation. How the complex topography affects the radar measurements, such as those by TRMM, also needs to be examined. To address these issues, we recommend putting two arrays of raingauges on the east slope of the Andes. One located in the equatorial sector, preferable between the equator and 5°S and the other located around 20°S. Considering the structures of convection and topography, and the resolution of GOES satellite images, the array should cover about a 40 km square with 5 km or less distance (1km would be ideal) between raingauges.
f) Tide Gauge (C. Mooers)
Coastal tide gauges provide coastal sea level (CSL) data after (diurnal and semidiurnal) tidal and higher frequencies (tsunamis, etc.) are filtered from the time series. When corrected for steric and inverse barometer effects, CSL is a dynamical variable which is rich in variability over a broad spectrum, including the "weather cycle" (weekly time scale); intraseasonal, seasonal and interannual variability; short-term climate signals; and long-term climate signals, e.g., sea level rise (or fall). With modern digital tide gauges equipped with satellite or other telemetry systems, CLS data can be available in near-real-time and broadly distributed. Some primary tide gauge stations are part of a high quality global network (GLOS); others may or may not be of a similar high quality. CSL data can be "integrated" with sea surface heights (SSH) data from satellite radar altimetry to extend basin scale SSH fields across the coastal ocean (RRZ) to the coastline.
VAMOS should consider the following activities: 1) survey the status of the permanent coastal tide gauges of the Americas; i.e., identify their locations and characteristics (quality, origin of time series, data availability, etc.), 2) present the information in graphical (map) and tabular formats, 3) assess their adequacy as a de facto network for monitoring intraseasonal, seasonal, and short-term climate variability, 4) identify tide gauge stations that need to be upgraded or strategic locations where additional tide gauges may be needed, 5) provide information on CSL data access for interested researchers, and 6) determine if there are any historical CSL data sets that need to be rescued.
g) Temperature/Salinity Profiles (C. Eriksen)
Most historical data of upper-ocean temperature and salinity structure is based on shipboard observations made by either dedicated research vessels or volunteer observing ships. Measurements are taken irregularly in time and space and are often aliased by unresolved variability. The most common measurement is a temperature profile made with an XBT. The vertical resolution of XBTs is limited to a few meters and temperature accuracy is several hundredths °C. Expendable CTD probes exist, but are not widely used because of cost (about 20 times that of XBTs) and modest salinity accuracy (a few hundredths psu). The diurnal cycle of upper-ocean structure, often trapped within a very few meters of the ocean surface, is unresolved in typical XBT profiles. Because of the importance of the diurnal cycle to contributing to evolution of upper-ocean structure on seasonal and longer time scales, an important enhancement to temperature and salinity profile monitoring would be to resolve diurnal fluctuations over vertical scales of a meter or so.
Two prospects for enhanced monitoring of temperature and salinity profiles at the necessary time and space scales exist. One is to use moorings with recording instruments at the desired depths. The other is to use autonomous profiling vehicles. The advantage of the moored approach is that surface fluxes can be measured from a surface buoy together with subsurface data. The advantage of the autonomous approach is finer resolution vertically at a per CTD profile cost comparable to that of an XBT (which measures temperature alone and requires a ship). The glider vehicles now under development have sufficient to carry out time series measurements at fixed locations for several months at a time without need of a ship for launch or recovery because of proximity of the regions of interest to islands and continents.
Both moored and autonomous techniques can take advantage of expected developments in telecommunications to make data available in near real time. Worldwide cellular telephone service is scheduled to begin in September 1998, making possible transmission of much more data than can be sent over the ARGOS system now used by oceanographers. The advent of two-way communication will make measurement systems more reliable and allow changes in sampling plans to be carried out as desired.
h) Volunteer Observing Ships (VOS) (C. Mooers)
VOS platforms occupy repeated tracks and provide XBT, surface meteorological, and associated data, e.g., surface thermal salinograph (STSG) and acoustic doppler current profiles (ADCP) in some cases. The data are recorded digitally and telemetered by satellite link. As instrumentation technology advances, additional sensors are likely to be incorporated. VOS data sets have proven to be invaluable for developing space-time data sets of the upper ocean that are used for the analysis of seasonal and interannual variability, and, in some cases, meso-scale variability. Because they cover long tracks, they have proven to be a useful complement to satellite remote sensing data sets, moored and drifting buoy data, etc.
A number of VOS lines exist for the Americas. They need to be reviewed and assess for their adequacy in monitoring the short-term climate variability of the ocean regions adjacent to (and influencing) the Americas. The opportunities for additional VOS lines needs to be assessed. For example, there are very few VOS lines in the IAS; however, there are several heavily utilized tracks which would lend themselves to VOS operations.
i) Submarine Cables (C. Mooers)
Submarine telephone cables can monitor ocean transport variations due to the electrical voltage induced in the cables by an ocean current (i.e., an electrical conduction moving in the geomagnetic field). The two submarine cables across the Straits of Florida have proven invaluable for monitoring variations in the transport of the Florida Current, an important component in the "global conveyor belt" over several decades. Periodic recalibration cruises (with mass and velocity profilers) are needed; otherwise, such opportunistic cable observing systems are very inexpensive. There are opportunities to monitor the highly variable flows through the Antillean Island passages of the IAS using existing submarine telephone cables for transport monitoring around the Americas where it makes scientific sense.
j) Over-the-Horizon Radars (C. Mooers)
Land-based radars have been proven effective for monitoring surface currents, winds, and waves through Bragg backscatter from ocean waves. Systems with various design and frequencies (and, hence, ranges) have been demonstrated and more advanced systems are currently under development. These radar systems uniquely provide synoptic maps of at least surface currents.
Synoptic surface currents, wind and wave maps have a variety of uses for operational oceanography. This information has bearing on air-sea fluxes, which are of particular concern for VAMOS. One particularly promising system (ROTHR, operated by the U.S. Navy) covers the Caribbean Sea though its utilization for oceanographic purposes is a secondary priority, that utilization is being encouraged.
VAMOS should evaluate the utility of ROTHR in air-sea interaction studies of the Caribbean Sea. Then, assess the potential of emerging advanced coastal radars for use in other regions of the IAS.
8.6 Prospects for space-based observations during VAMOS (S. Esbensen, contributions D. Chelton, W. Berg and J.Bates)
Space-based observations will play an important role in providing the large-scale context for VAMOS ocean-atmosphere field studies. Satellite data analyses provide some key atmospheric and oceanic fields that cannot be obtained by other methods. Moreover, satellites provide a spatial resolution that cannot be obtained from the sparse distribution of in situ observations and the limitations of present operational data assimilation models in data sparse regions of the tropical and southern hemisphere oceans and the overlying atmosphere. Methods for obtaining fields of sea surface temperature, near-surface wind velocity and sea level are well advanced. Methods for obtaining quantitative estimates of the components of the surface radiation budget and of precipitation are developing rapidly.
Satellite observations will be most useful if they can be continued uninterrupted during the entire VAMOS programme in order to provide simultaneous, dense observations in space and time. As summarized below, most of the satellite needs will be met during the first few years of VAMOS.
However, maintaining satellite programmes through the end of the VAMOS programme in 2010 and beyond poses serious practical difficulties. Long, continuous time series, exceeding the typical 3-year lifetime of individual satellite missions or instruments, are extremely difficult to acquire outside of operational programmes. While many of the needed satellite-based observations of key atmospheric and solar forcing variables are already incorporated into U.S. and international operational meteorological satellite programmes, the situation is far less secure for ocean variables. Of particular concern for applying the research results from VAMOS marine field studies is assuring long records of scatterometer observations of near-surface vector winds and altimeter observations of sea level since these are available only from research satellites.
The atmospheric and oceanographic variables of most interest to VAMOS are listed below, together with summaries of the histories and future of satellite instruments for measuring these variables.
a) Near-Surface Winds
1. Scatterometers
Scatterometers provide accurate, global,
high-resolution
measurements of near-surface wind velocity (both speed and direction)
under
all weather conditions. With the development of numerical atmospheric
general
circulation models containing realistic boundary layer parameterizations,
the operational meteorological community has found that assimilation of
scatterometer surface vector wind measurements can yield improved
operational
weather forecasts. For this reason, NOAA invested in the
telecommunication,
data reduction, and data assimilation hardware and software needed for
operational acquisition and utilization of scatterometer measurements
from
the research-mode NASA scatterometer.
History
of Scatterometry:
The Defense Meteorological Satellite Program (DMSP)
has launched an operational series of passive microwave radiometers (the
Special Sensor Microwave/Imager, SSM/I) since the late 1980s. The primary
application of the SSM/I data is to study polar sea ice processes.
However,
the instrument also measures all-weather wind speed over the ocean, but
with no directional information. An SSM/I is included on each of the
operational
DMSP Polar Orbiters, of which there are generally two in orbit at any
given
time.
History
of SSM/I:
Altimeter measurements of the precise distance between the satellite and the ocean surface include important climate signals such as steric heating and cooling of the upper ocean and upper-ocean geostrophic currents. Satellite altimeter measurements provide unprecedented coverage and accuracy of the long, baroclinic tropical Kelvin and Rossby waves associated with seasonal and intraseasonal variability and interannual in the tropical Pacific and Atlantic Oceans.
History of Altimetry:
Satellite infrared measurements of SST have been available from the NOAA operational satellites since 1973, with high quality SST estimates from the Advanced Very High Resolution Radiometer (AVHRR) available since 1979. The AVHRR measures SST in cloud-free conditions. An AVHRR is included on each of the two NOAA Polar Orbiters that are generally operational at any given time. There are also infrared radiometers on several European and Japanese satellites.
History of NOAA AVHRR
d) Ocean Color
Estimates of near-surface chlorophyll can be obtained from ocean color measurements. These data are good tracers of upper-ocean currents, as well as a measure of the biological response to wind forcing and ocean dynamics. The chlorophyll distribution is also often correlated with the geographical distributions of fish populations.
History of Ocean Color:
Since the lead time to incorporate a new satellite sensor into the U.S. operational satellite system is of the order of a decade or longer, a coordinated long-range plan must be developed at the earliest possible opportunity to assure continuity of the oceanographic satellite sensors for climate observations. In response to this need and to budgetary constraints, a new programme called the National Polar-orbiting Operational Environmental Satellite System (NPOESS) is under development. The mission of NPOESS is to provide a convergence of the NOAA, NASA and DMSP operational satellite programmes into a single programme to acquire, receive and disseminate global and regional environmental satellite data. Where appropriate, one of the goals of NPOESS is to transition technology from the NASA research satellite programme to an operational status. NPOESS-1 is expected to launch in 2009, followed by NPOESS-2 in 2010. The plan is to maintain two NPOESS satellites in orbit at all times.
Much of the instrument compliment on NPOESS will consist of the present instruments for operational weather forecasting. Many of these sensors also provide data that are useful for climate studies such as VAMOS. There are some instruments, however, that are very important for climate studies but have a less direct impact on weather forecasting. Examples include satellite altimetry and ocean colour measurements.
The NPOESS programme is presently under review by the U.S. National Research Council Committee on Earth Studies. The outcome of the NPOESS development process is scheduled to be implemented toward the end of the VAMOS period. However, NPOESS is likely to play a crucial role in the success or failure of satellite data acquisition and analysis efforts when VAMOS research results are applied to improve seasonal-to-interannual climate forecasts over the Americas.
9. Agency remarks
a) GCIP perspective on VAMOS (R. Lawford)
The GEWEX Continental-scale International Project (GCIP) views the VAMOS project as an important extension of its research. While GCIP has excellent relations with the LBA project through the GEWEX Hydrometeorology Panel there are a number of other areas besides the Mississippi and Amazon River Basins where intercomparison and transferability studies could be launched. For example, in the case of studies of the dynamics of the low level jet phenomena which are so critical for GCIP, lessons could be learned from the low level jets in South America. Understanding this LLJ could hold the key to the prediction of precipitation in the central U.S. on seasonal time scales. GCIP would also benefit from studies of the Caribbean Sea and the role of land- sea temperature contrasts in driving the regional circulation pattern. Through VAMOS, GCIP will be able to extend its dialogue beyond the meteorological and hydrologic communities to include oceanographers and large scale climate modellers. Accordingly, GCIP plans to participate in VAMOS to ensure that it derives the full benefit of these new collaborations. In addition, GCIP and PACS are working together to define a set of studies that will be beneficial to both their programmes and to VAMOS.
10. CLIVAR VAMOS Panel
a) Panel recommendations
The VAMOS Panel current planning approach is based on phases, and addresses the problems of capacity building in data-void regions of the Americas. Figure 8 shows a preliminary timeline for VAMOS programmes. At its first meeting, the Panel decided:
b) Terms of reference for VAMOS working groups and proposed membership
1. Process Studies Working Group
The VAMOS Process Studies Working Group shall assist over the next year in developing the initial plans for VAMOS process studies. It is responsible to the CLIVAR VAMOS Panel, will have a limited lifetime, and is charged with assisting in the preparation of the Report on the VAMOS/PACS Workshop on Field Programmes. Specifically, with the target date for a draft workshop report of July 1, 1998 and for a final workshop report of September 1, 1998 in mind the VAMOS Process Studies Working Group shall: 1) collect information on field programmes now underway and likely to occur in the next several years that will be part of VAMOS or relevant to VAMOS; 2) develop recommendations and initial plans for the next series of VAMOS process studies, those likely to occur in 2002-2005; and 3) develop a strategy for fostering the planning and consideration of future process studies. A procedure by which future process studies a policy for the exchange of data by VAMOS investigators. The working group will collate the information about planned process studies and summarize the scientific objectives, timing, location, and observational elements of planned process studies. It should also look for opportunities where cooperation and collaboration among planned programs and/or additional observational elements would enhance their value to VAMOS and include recommendations about such enhancements in their summary report. The working group will provide this report to the workshop organizers, C. Nobre and R. Weller, for inclusion the workshop report.
Membership: Eriksen, Campos, Lawford, M. Silva Dias, Weller (chair)
2. Data Working Group
The VAMOS Data Working Group shall assist in developing the initial plans and documents for VAMOS over the next year. It is responsible to the CLIVAR VAMOS Panel, will have a limited lifetime, and is charged with assisting in the preparation of the Report on the VAMOS/PACS Workshop on Field Programmes. Specifically, with the target date for a draft workshop report of July 1, 1998 and for a final workshop report of September 1, 1998 in mind the VAMOS Data Working Group shall: 1) identify data sets that will be of use in describing, understanding, and developing predictive skill for the variability of the American Monsoon System; 2) develop plans to make these data sets and other to be collected during VAMOS and related field programs readily available, including recommendations for digitization of previously collected data and for archiving the data sets; and 3) develop a policy for the exchange of data by VAMOS investigators. The working group will provide the results of their efforts to address these three task to the workshop organizers, C. Nobre and R. Weller, for inclusion the workshop report.
Membership: LBA person, Leese, Magaña (chair), Barros, Servain, Meitlin
3. Sustained Measurements Working Group
The VAMOS Sustained Measurements Working Group shall assist over the next year in developing the initial plans for the sustained measurements to be made during VAMOS. It is responsible to the CLIVAR VAMOS Panel, will have a limited lifetime, and is charged with assisting in the preparation of the Report on the VAMOS/PACS Workshop on Field Programmes. Specifically, with the target date for a draft workshop report of July 1, 1998 and for a final workshop report of September 1, 1998 in mind the VAMOS Sustained Measurements Working Group shall: 1) collect information on sustained observations now underway and likely to begin in the next several years that will be part of VAMOS or relevant to VAMOS and 2) develop recommendations and initial plans for new sustained measurements for VAMOS. The working group will collate the information about existing and recommended sustained measurements and summarize the scientific objectives, timing, location, and observational strategies of these programs. It should also look for opportunities where cooperation and collaboration among planned programs and/or additional observational elements would enhance their value to VAMOS and include recommendations about such enhancements in their summary report. The working group will provide this report to the workshop organizers, C. Nobre and R. Weller, for inclusion the workshop report.
Membership: Douglas, Wainer, Kousky (chair), Silva Dias, Weller, Enfield, Picaut, Mooers.
4. Stratus Working Group
The VAMOS Stratus Working Group is charged with developing cooperative, international research to investigate the role of stratus in the variability of the American monsoon systems. Over the next year it will assist in developing the initial plans for VAMOS field work and help in preparing the report on the CLIVAR/VAMOS Field Programmes Workshop. With the target date for a draft workshop report of July 1, 1998 and for a final workshop report of September 1, 1998 in mind the VAMOS Stratus Working Group will provide material for inclusion in the workshop report to co-chairs C. Nobre and R. Weller. After that it will continue to develop, organize, and foster cooperative stratus research within VAMOS. It is responsible to the CLIVAR VAMOS Panel, and plans for further development of stratus research initiatives should be approved by the CLIVAR/VAMOS Panel.
Membership: Soldi, Rogers, Albrecht (chair), Aceituno
5. South American Monsoon Working Group
The VAMOS South American Monsoon Working Group is charged with developing cooperative, international research to investigate the South American Monsoon system, including the low level jet. Over the next year it will assist in developing the initial plans for VAMOS field work and help in preparing the report on the CLIVAR/VAMOS Field Programmes Workshop. With the target date for a draft workshop report of July 1, 1998 and for a final workshop report of September 1, 1998 in mind the VAMOS South American Monsoon Working Group will provide material for inclusion in the workshop report to co-chairs C. Nobre and R. Weller. In 1998 it will develop plans for a workshop on the South American Monsoon and, with the approval of the CLIVAR/VAMOS Panel, organize and conduct that workshop. The goal of that workshop will be to develop the scientific objectives and plans for future research on the South American Monsoon. The working group will be responsible to the CLIVAR VAMOS Panel, and plans developed by the working group will need the approval of that panel.
Membership: Silva Dias (co-chair), Paegle (co-chair), Vera, Grimm, Aceituno, Kousky, Fu, Niccolini, Campos, Eriksen.
c) Critical dates and action items for VAMOS
11. Acknowledgements
The CLIVAR-VAMOS Panel acknowledges the superb work by the Workshop Organizers R. Weller and C. Nobre and that of their committee members: M. Kayano and T. Ambrizzi. Dr. E. Campos was a perfect local organizer, which was a particularly demanding task in view of the multiple institutions and committees involved. The University of São Paulo provided the meeting space. Andreas Willwock has been a superb representative of CLIVARís International Project Office (IPO). S. Andrews at the University Corporation for Atmospheric Research (UCAR) handled many travel arrangements with her usual efficiency and friendliness.
12. References
Absy, M.L, 1985: Palinology of Amazonia: The
history
of the forests as revealed by the palynological record. In: Amazonia. G.
T. Prance and T. E. Lovejoy (eds.). Pergamon Press, Oxford, United
Kingdom.
442 p.
Behling, H., 1996: First report on new evidence for the occurrence of Podocarpus and possible human presence of the Amazon during the Late-Glacial. Veget. Hist. Archeaeobot, 5, 241-246.
Behling, H., and M. Lichte, 1997: Evidence of dry and cold climatic conditions at Glacial times in tropical Southeastern Brazil. Quaternary Research, 48, 348-358.
Behling, H., 1998: Late quaternary vegetational and climatic changes in Brazil. Review of Palaeobotany and Palynology, 9, 143-156.
Clapperton, C., 1993: Nature of environmental changes inn South America at the last glacial maximum. Paleo, 101, 189-208.
Dapozzo, I. J., 1995: Influence of the Andes in the local circulations in Peru. Ms. Dissertation, Institute of Astronomy and Geophysics, University of São Paulo, Brazil.
Freitas, S. R., K. M. Longa, M. F. Silva Dias and P. Artaxo, 1996: Numerical modelling of the air mass trajectories from the biomass burning areas of the Amazon Basin. An. Acad. Bras. Ci., 68, 193-206.
Higgins, R. W., Y. Yao, E. S. Yarosh, J. E. Janowiak and K. C. Mo, 1997a: Influence of the Great Plains low-level jet on summertime precipitation and moisture transport over the central United States. J. Climate, 10, 481-507.
Higgins, R. W., Y. Yao and X. Wang, 1997b: Influence of the North American Monsoon System on the U.S. Summer Precipitation Regime. J. Climate, 10, 2600-2622.
Ledru, M. P. 1993: Late quaternary environmental and climatic changes in Central Brazil. Quaternary Research, 39, 90-98.
Ledru, M. P., H. Behling, M. Fournier, L. Martin and M. Servant, 1994: Localisation de la forêt díÁraucaria du Brésil au cours de l'Holocene. Implications paléoclimatiques. C.R. Acad. Sci. Paris, Sciences de la Vie/Life Sciences, 317, 527-521.
Ledru, M. P., M. Salgado-Labouriau and M. Lorscheitter, 1997: Vegetation dynamics in southern and central Brazil during the last 10,000 years BP. Review of Paleobotany and Palynology, submitted.
Marengo, J., 1995: Variations and change in South American stream flow. Climate Change, 31, 99-117.
Marengo, J., A. Cornejo, P. Satyamurty, C. A. Nobre and W. Sea, 1997: Cold waves in the South American continent: The strong event of June 1994. Mon. Wea. Rev., 125, 2759-2786.
Marengo, J., R. Victoria, V. Ballester, J. Tomasella, L. Campos, J. Cavalcxanti, H. Hoff, J. Newcomer, M. Padovan, M. dos Reis, R. dos Santos Alvala, N. Filizola, J. Guyot, M. Gracia and F. Gerab, 1998: Pre-LBA Data Sets Initiative CD ROMs, Vols. 1-3. CPTEC/INPE, Cacheoira Paulista, Sao Paulo, Brazil.
Marengo, J., J. Tomasella and C. Uvo, 1998: Trends in streamflow and rainfall in tropical South America: Amazonia, eastern Brazil, and northwestern Peru. J. Geogr. Research, 103, 1775-1783.
Martin, L., J. Bertaux, T. Cirrege, M. P. Ledru, P. Mourguiart, A. Sifeddine, F. Soubies, D. Wirrmann, K. Seguio and B. Turcq, 1997: Astronomical forcing of contrasting rainfall changes in tropical South America between 12,400 and 8,000 year B.P. Quaternary Research, 47, 117-122.
Mechoso, C. R., A. W. Robertson, N. Barth, M. K. Davey, P. Delecluse, P. R. Gent, S. Ineson, B. Kirtman, M. Latif, H. Le Treut, T. Nagai, J. D. Neelin, S. G. H. Philander, J. Polcher, P. S. Schopf, T. Stockdale, M. J. Suarez, L. Terray, O. Thual and J. J. Tribbia, 1995: The seasonal cycle over the Tropical Pacific in General Circulation Models. Mon. Wea. Rev., 123, 2825-2838.
Nobre, C. A., A. J. Dolman, J. H. C. Gash, R. W. A. Hutjes, D. J. Jacob, A. C. Janetos, P. Kabat, M. Keller, J. A. Marengo, R. J. McNeal, J. Melillo, P. J. Sellers, D. E. Wickland and S. C. Wofsy, 1996: The LBA Concise Experimental Plan. SCDLO - The Netherlands.
Robertson, A. W., C. R. Mechoso and Y.-J. Kim, 1998: The influence of Atlantic sea surface temperature anomalies on the North Atlantic Oscillation. J. Climate, submitted.
SALSA, Dr. Bruce F. Goff, Coordinator, Semi-Arid Land-Surface-Atmosphere (SALSA). USDA ARS Southwest Watershed Research Center. 2000 E. Allen Road. Tucson, AZ 85719, USA. phone:+520-670-6380x149/fax:+520-670-5550 email: bgoff@tucson.ars.ag.gov
Servain, J. 1991: Simple climatic indices for the Tropical Atlantic ocean with some applications. J. Geogr. Research, 96, 15137-15146.
Servant, M., J. Maley, B. Turcq, M. L. Absy, P.
Brenac,
M. Fournier and M.-P. Ledru, 1993: Tropical forests changes during the
late quaternary in African and South American lowlands. Global and
Planetary
Change, 7, 25-40.
| Presenter | Program | Countries | Time Frame | Resources | Funding Orgs. | Goals/Objectives |
| Nobre | LBA | Brazil + U.S. + Amazon countries | 1997-2003 | 25M TOTAL | NASA, EEC, BRASIL | (1) how does Amazon function as climate system; (2) how is it likely to change under anthropogenic forcings |
| Lawford | GCIP | U.S. | 1998-2004 | 6M/year | NOAA, NASA | Understand/predict spatial & S-I temporal variability of water resources over Mississippi River Basin |
| Esbensen | PACS | U.S. | 1995-2004 | 2.5-3M/ year | NOAA/OGP | (1) Understand seasonal to interdecadal climate variations of Americas; (2) extend scope & improve skill of climate prediction, including tropical storms |
| Weller | EPIC | U.S. | 2000-2001 | 2M/ year | NOAA/OGP | Understand processes of Pacific cold tongue, warm pool and stratus deck |
| Rogers | CORC | U.S. | 1998-2000 | 4M/ year | NOAA/OGP | Understand through observations & modeling the ocean variability on scales of years to centuries & predictability |
| Vianna | PIRATA | U.S., Brazil, France | 1997-2000 | 4M total | NOAA, ORSTOM, Brazil navy, INPE | Monitor the tropical Atlantic, describe seasonal to interannual variability of near-surface processes and fluxes |
| Wilcox | IAI | Americas | 1996 onward | 2-3M/ year | U.S. OGP, NSF | Promote multidisciplinary, cooperative climate research in Americas with emphasis on applications for human dimensions |
| Aceituno | N. CHILE | Chile | 1999-2001 | 100K/ year | FONDECYT, CONICYT | (1) understand climatology of Bolivian altiplano; (2) study causes aridity & PBL variations over western Andes & adjacent coast |
| Matano | SACC | Brazil, U.S., Argentina, Uruguay | 1996-2002 | 200K
initial
6M total |
IAI, NSF FAPESA, CONICET, | Describe & understand the Brazil-Malvinas confluence, its variability & influence on adjacent land climate |
| Douglas | UPPER AIR | U.S., cooperating countries | 1997-1998 | 200K total | NOAA/OGP | Improve the geographical coverage of land-based upper air observations during concurrent with PACS process studies |
| Castro | DPROAS | Brazil | 1997-2000 | 2M/ year | IOUSP, INPE | Understand mechanisms & ecological impacts of seasonal variability of coastal cold water intrusions (southern Brazil) |
| Mooers | IAS | U.S., Brazil, Cuba, Mex, Pan., Venez., Colom. | 1998 onward | 5M/ year (proposed) | US/ONR, NSF, NASA, NOAA, in kind | Understand the circulation, heat balance & ecological/climate impacts of the Intra-American Seas through observations and modeling |
| Grimm | MONSOONS | Americas | 1998-2002 | not known | WMO/CAS | Help to understand the American monsoon system |
| Picaut | ECLAT | France | 1995-2005 | 3M/ year | ORTSOM, LODYC, CNRS, CNES | Understand ocean-atmosphere interactions and climate variability in the tropical Atlantic sector on seasonal to interdecadal time scales |
|
|
||||||||
| 78/88* | 88/89 | 89/90 | 90/91 | 91/92 | 92/94** | 94/95 | 95/96 | |
| Amazonia | 21130 | 17860 | 13810 | 11130 | 13786 | 14896 | 29059 | 18161 |
| % | 0.54 | 0.48 | 0.37 | 0.30 | 0.37 | 0.40 | 0.81 | 0.51 |
Figure 1: PACS-funded pilot studies for the period 1995-98 superimposed on annual mean a surface temperature contours. Also shown are pre- existing TAO Array Automated Temperature Line Acquisition System (ATLAS) and current meter mooring sites, and a wind profiler site in the m Galapagos Islands. The location of pilot study activities is offset from the TAO mooring locations for clarity. Soundings will be made along 95oW and 110oW; Improved Meteorological Instrument (IMET) and radar measurements will be within 30km of one another, and within 30 km of the ATLAS mooring at 10oN, 125oW.
Figure 2: Upper-air sounding stations with enhanced monitoring for PACS.
Figure 3: a) Mean northerly wind field for December 1997 at 900 hPa and 06:00 UTC from the high resolution ETA forecast products, and b) monthly average longitudinal cross section along 18oS and at 06:00 UTC of the meridional water vapour flux for December 1997 from the high resolution ETA forecast products.
Figure 4: Schematic of the South and North American low-level jets.
Figure 5: Distribution of the rawinsonde
network
over South America over the geographical region where the low level jet
occurs, identifies current sounding stations, represents tentative
locations
of proposed sounding sites.
Figure 6: ECMWF (top) and NCEP/NCAR (bottom) reanalyses of rainfall and low level currents over South America for a period of intense rainfall over Argentina. Differences in analyses suggest lack of convergence of the two assimilation systems in regions of low data density.
Figure 7: High priority regions for VAMOS ocean-atmosphere field programmes. CTIC, STR, TAV, SACZ and WP refer respectively to the cold-tongue/ITCZ complex, the eastern Pacific stratus regime, the tropical Atlantic variability region, the South Atlantic Convergence Zone, and the Interamerican warm pool region. Arrows indicate hypothesized interactions with other regions.
Figure 8: Initial timeline for VAMOS.