Distinct Surface Warming Response over the Western and Eastern Equatorial Pacific

Department of Urban and Environmental Engineering (Environmental Science and Engineering)Most climate models show an ???El Ni??o-like??? warming pattern in response to global warming, which is characterized by larger sea surface temperature (SST) warming in the eastern equatorial Pacific compared to the western equatorial Pacific. Although the zonal SST gradient is generally projected to decrease, there exists a pronounced model uncertainty in the projection of the tropical Pacific warming pattern owing to the various mechanisms working in combination on different time scales in the tropical Pacific. In this study, to analyze regional characteristics of atmospheric and oceanic feedback processes in the western and eastern tropical Pacific we employ localized perturbation experiments with a hierarchy of general circulation models, including the fixed SST, slab ocean, and fully coupled ocean configurations. The response to regional forcing can be regarded as an initial response to a global greenhouse gas increase before any remote impacts substantiate. We reveal that the effect of radiative forcing in the western Pacific is largely offset by localized negative shortwave cloud radiative effect (SWCRE) and the wind-evaporation-SST (WES) feedback, resulting in a zonally flat SST response across the equatorial Pacific. In contrast, radiative forcing in the eastern Pacific, while being partially compensated for by the temporary ocean dynamical thermostat mechanism, results in an ???El Ni??o-like??? warming pattern due to only weak local radiative damping. These distinct regional response suggests that an ???El Ni??o-like??? pattern is more probable in response to uniform greenhouse gas increase. The same conclusion is drawn using a bias-corrected model via flux adjustments, implying that the results are independent of the present-day model bias. We further confirm that the same mechanisms operate in shaping the inter-model spread of the future tropical Pacific surface warming pattern on both fast and slow timescales but with the dominant role played by ocean heat uptake response in the eastern equatorial Pacific. In the transient response, for example, the models with a more El Ni??o-like warming pattern are associated with a weaker eastern Pacific cooling due to anomalous ocean heat uptake. A better representation of ocean processes is the key to better constraining the future tropical Pacific surface warming pattern.ope