Model Diagnoses of El Nino Teleconnections to the Global Atmosphere-Ocean System

A summary is offered of the principal findings based on a comprehensive series of model experiments aimed at simulating and diagnosing the impacts of El Nino on atmospheric and oceanic variability in different parts of the world. The tools being deployed include global, fully-coupled atmosphere-ocean models, atmosphere-only models with sea surface temperature (SST) conditions being prescribed in certain strategic locations, and hybrid models allowing for SST prescription in some regions but air-sea coupling in others. The global atmospheric response to El Nino episodes in the deep tropical eastern Pacific (DTEP) leads to notable changes in the surface wind speed and cloud amount over many sectors of the World Ocean. Such changes modulate the surface heat and radiative fluxes in these regions, and thereby generate local SST anomalies. The contribution of this 'atmospheric bridge' mechanism to the formation of distinctive SST patterns in the North Pacific, North Atlantic, and Indo-western Pacific (IWP) during El Nino events is described. The SST anomalies induced by the atmospheric bridge in various ocean basins could in turn exert feedback effects on the atmospheric circulation. For instance, the warming of the IWP following the mature phase of El Nino is seen to force zonally extended midlatitude upper-level pressure ridges in both hemispheres. These features are accompanied by prominent seasonal changes in the surface climate. The interplay between the circulation patterns associated with oceanic forcings at DTEP and at IWP is a critical factor for the onset timing of the summer monsoons over East and South Asia, prolonged planetary-scale temperature and precipitation anomalies, and secular trends in the strength of El Nino and La Nina signals over North America. A brief discussion is given of the results by other investigators on the role of SST conditions at DTEP in the recent hiatus in global warming, as deduced from experimental setups similar to those described above.