As the dominant mode of interannual tropical climate system variability, the El Niño/Southern Oscillation (ENSO) has profound impacts on climates both locally, i.e., within ENSO's equatorial central-eastern Pacific source region, as well as remotely. The remote climate signatures—or teleconnections—of ENSO occur globally. Within the tropics, the features of teleconnection are well-known from observations, including widespread warming of the troposphere and land and ocean surfaces and reduced rainfall.

Summary of Prior ENSO Tropical Teleconnections Research

Previous studies of ENSO tropical teleconnections have focused principally on a suite atmospheric mechanisms, called the "atmospheric bridge" (Lau and Nath, 1996), as the mediators of the remote ENSO influence. It has been suggested that ENSO-induced changes to circulation can act through surface heat fluxes (e.g., the windspeed dependence of latent heating), radiative fluxes (e.g., cloudiness changes), or ocean dynamical processes (e.g., Ekman transports). On the other hand, Chiang and Sobel (2002; hereafter CS02) emphasized the potential "thermodynamic control" of the remote tropical response to ENSO: using a single column model framework, CS02 obtained a realistic response, i.e. surface warming and precipitation deficits, under the application of temperature anomalies characteristic of El Niño. My postdoctoral research at UC Berkeley centered on assessing the validity of the CS02 tropospheric temperature (TT) mechanism in more complex models and probing the extent of its applicability to the osbserved ENSO teleconnection.

Chiang and Lintner (2005) examined the details of the remote surface temperature response to El Niño conditions. Using NCAR CCM3 simulations of the 1997-1998 "El Niño of the Century", we showed that the remote tropical ocean warming is consistent with the TT mechanism, with equatorial wave dynamics spreading TT anomalies to the remote tropics and moist convective processes mediating the (free) troposphere-to-surface connection, with the free tropospheric and boundary layer moist static energies varying in concert. Latent heat flux serves as the principal regulator of the troposphere-surface connection over oceans. Land region warming appears to be regulated principally by sensible heat flux, although the underlying mechanism remains to be fully determined.

Complementing the analysis of the remote surface warming to El Niño, Lintner and Chiang (2005) explored the remote precipitation response using a model of intermediate level complexity, the Quasi-equilibrium Tropical Circulation Model (QTCM; Neelin and Zeng, 2000; available for downloaded here.) and the weak temperature gradient (WTG) approximation (Sobel and Bretherton, 2000). WTG assumes negligible temperature gradients, which allows the QTCM's governing equations to be cast in a simplified form, i.e., the prognostic horizontal momentum equations are replaced by a diagnostic expression for the vertical convergence field estimated from the vertically-averaged temperature equation. Relative to the "full" QTCM, the WTG version appears to produce a plausible remote precipitation response to ENSO, although with some marked differences in the regional-scale features.

Lintner and Chiang (2007) considered the remote teleconnection from the perspective of an adjustment problem, asking what the time-evolution of the remote tropical climate response looks like following abrupt onset of El Niño conditions in the equatorial Pacific. The adjustment consists of two parts: a rapid (5-15 day), atmospheric phase consisting of the propagating Kelvin-like wave response and a slower phase (~months) in which longer timescale components of the climate system (e.g., the ocean mixed layer and land surface moisture) come into equilibrium with the applied forcing. During the rapid phase, precipitation deficits are locally large (essentially co-located with the Kelvin wavefront) and thereafter recover, with the details of the recovery depending on the underlying surface characteristics. The recovery over oceans, with a timescale related to the depth of the ocean mixed layer, supports the view of surface flux disequilibrium as a significant component of the precipitation teleconnection.

Future Directions

An emergent theme in my prior work is that an understanding of the remote response to El Niño conditions, especially the structure of rainfall anomalies manifest at regional scales—or those scales characteristic of human-environment interactions like agriculture and water resource management—requires understanding the role of moist (convective) dynamics, including horizontal moisture transport, feedbacks between convection zones and large-scale wave dynamics, and the details of ocean and land surface fluxes. With this perspective in mind, anticipated future directions of my ENSO tropical teleconnections work fall into three categories: (i) diagnostic intercomparisons of simulated ENSO teleconnections and observed event-to-event similarities/differences, especially at regional scales; (ii) use of El Niño events as possible analogues for global warming; and (iii) continued development of the paradigm of ENSO as a climate adjustment phenomenon.