The Intertropical Convergence Zone
A Street of Covection

by Chih-wen Hung

updated on 11/20/2001

NEW! NEW! NEW!

Tropospheric biennial oscillation over the southeast Pacific intertropical convergence zone during ENSO breaks

Submitted to Geophysical Research Letters

The Intertropical Convergence Zone (ITCZ) may be viewed as the ascending branch of the Hadley cell in the general circulation of monthly or seasonal time scale. The monthly or seasonal and planetary-scale ITCZ actually consists of a sequence of tropical convection with smaller time and space scales. My research interest is basically the thermodynamic coupling between convection and large-scale motions in the ITCZ, and the interannual variability of the ITCZ. A description of the cloud regimes associated with the ITCZ and adjacent tropical and subtropical regions in terms of thermodynamic and dynamic variables is also a part of my research.

Satellite observations of the SITCZ

In a recent paper (Halpern and Hung 2001*, submitted to J. Geophys. Res.),  we studied the satellite observations of the Southeast Pacific ITCZ (occurred in March and April in 8S-2S, 130W-90W region) during 1993-1999. In the normal years, the main ITCZ over the eastern Pacific is usually located north of the equator, and the Southeast Pacific ITCZ occurred only during the northern spring. However, in an El Nino year (1997-98), the main ITCZ moved back and forth between two hemispheres, and the Southeast Pacific ITCZ became part of the main ITCZ which is usually located in the Northern Hemisphere. In a La Nina year (1999), the precipitation in the Southeast Pacific ITCZ was much stronger than in the normal year. The reasons for these anomalous behaviors of the ITCZ are not well understood.


Using Q1 and Q2 for the ITCZ study

In the current study I am doing, in addition to the satellite-derived cloud and precipitation data, we are using Q1 (apparent heat source) and Q2 (apparent moisture sink) extensively. These are obtained as the residuals of heat and moisture budgets (Yanai et al. 1973). The vertical profiles of Q1 and Q2 yield information on the cloud regimes. The horizontal distributions of vertically integrated values <Q1> and <Q2> provide the location of heat sources and moisture sinks. Together with the radiation, precipitation and the surface fluxes, these quantities provide information on the spatial distributions of the leading heating processes.

Last year (2000), I made test calculations of Q1 and Q2 values using the NCEP-NCAR and ECMWF reanalyses during the Intensive Observing Period (IOP) of the TOGA-COARE (November 1992-February 1993) using the scheme described in Tung et al. (1999). The original motivation of this work was to compare and calibrate the Q1 and Q2 derived from the two reanalysis datasets with those from "dry-bias corrected" in situ COARE sounding data. However, the moisture corrected sounding data were released from NCAR only recently (Wang et al.2000) and this calibration has not been done.


Intercomparison of Q1 and Q2 calculated from the NCEP-NCAR and ECMWF reanalyses
 

Nevertheless, we made comparisons between Q1 and Q2 derived from the two reanalyses in the tropical region with the satellite observed precipitation and outgoing longwave radiation (OLR). We found that the <Q1> and <Q2> patterns from the NCEP-NCAR reanalysis sometimes do not match the precipitation and OLR observations in the Southern Pacific Convergence Zone (SPCZ) region, while those obtained from the ECMWF reanalysis match well. We also found that the <Q2> values in the Pacific ITCZ from the NCEP-NCAR reanalysis are much smaller than those from the ECMWF reanalysis. The weak <Q2> within the ITCZ was previously noted by Yanai and Tomita (1998) who studied the Q1 and Q2 fields (1980-1994) from the NCEP-NCAR reanalysis.

From these preliminary work, we felt a need to recalculate daily Q1 and Q2 values from ECMWF reanalysis data (1979-1993) as a basic tool for the investigation of the ITCZ. The 15-year Q1 and Q2 results will add valuable information to the studies of the cloud regimes, heat sources and moisture sinks within the ITCZ from daily, monthly, seasonal to interannual time scale.


Different ITCZs in the world

We calculate Q1 and Q2 globally, because the ITCZs have different features in different locations. Over the eastern Pacific Ocean, the ITCZ prefers the Northern Hemisphere, and only has double ITCZ structure in northern spring. Over the western Pacific ocean, the ITCZ has the double-zone structure which locates in both hemispheres throughout the year. Over the Atlantic Ocean, the ITCZ is usually located in the Northern Hemisphere, but the western portion connected to Brazil can cross the equator to the Southern Hemisphere in some years. The ITCZ over the Indian Ocean can move back and forth between 2 hemispheres. The heat released by condensation with deep cumulus convection is the major heat source over the tropical oceans within the ITCZs. We would like to use the 15-year Q1 and Q2 results to study the percentage of each ITCZ which contributes to the global Hadley circulation over the different oceans, and find out the relationship between the migration of the heat source from ITCZs and the subtropical climate.


The role of evaporation in the SITCZ

This is the work I am currently doing. In March and April of normal year (without El Nino, and La Nina), the SST in the region of SITCZ reaches the highest (above 27 C) and the air column becomes moist due to increasing evaporation. The upward motions induced by surface wind convergence bring the moisture up, and produce a moist atmosphere for convection to occur.The integrated water vapor and the cloud liquid water remarkably rise and cause rainfall in this season.

Because SST plays a very important role in triggering the SITCZ, it is necessary to analyze the mechanism which causes the SST maximum in each March andApril. With the <Q2>=L(P-E) method, we can obtain the 15-year evaporation data for the SITCZ region. We are using these data to look at the SITCZ, and the other ITCZs, too.
 

TBO in the region of SITCZ?

Using satellite-based observations (SST, surface wind, and precipitation rate), Halpern and Hung (2001) demonstrated that the SITCZ showed biennial fluctuations in 1994-1997. Recently, I am examining how this TBO in SITCZ evolves, and trying to analyze the possible links with its neighbouring belts including the equatorial cold tongue and the Intertropical Convergence Zone in the Northern Hemisphere. I am also  looking at the relationship between the phase of TBO over the different latitudinal belts in the eastern Pacific and that in the Asian monsoon region.
 
 

*Halpern, D., and C.-W. Hung, 2001: Satellite observations of the Southeast Pacific Intertropical Convergence Zone during 1993-1999. (on Nov 2001 issue of JGR [Atmosphere]).