Distinct Surface Warming Response Over the Western and Eastern Equatorial Pacific to Radiative Forcing

Abstract

This study examines regional characteristics of atmospheric and oceanic feedback processes in the western and eastern equatorial Pacific, by applying a localized surface heating in the respective region in a hierarchy of climate models. A western Pacific forcing is largely offset by a negative shortwave cloud radiative effect and damping via wind-evaporation-SST feedback. In contrast, an eastern Pacific forcing, while being partially compensated for by ocean dynamical adjustments, induces an amplified warming extending to the central Pacific due to weak local damping mechanisms. As for the inter-model spread of the future tropical Pacific surface warming pattern, the ocean heat uptake response in the east can explain much of the spread both on fast (<5 years) and slow (>100 years) timescales. Our results suggest that an "El Nino-like" warming pattern is probable in response to increasing greenhouse gases owing to the strong negative feedback intrinsic to the western Pacific. Plain Language Summary It is of question whether Earth&apos;s surface warming in response to increasing greenhouse gas concentrations will be more amplified in the western (La Nina-like) or eastern (El Nino-like) part of the equatorial Pacific. This response is uncertain across climate models since multiple atmospheric and oceanic feedbacks operate in combination with their relative importance varying in time. We thus prescribe radiative heating either to the western or to the eastern tropical Pacific in a hierarchy of climate model simulations, varying in the degree of atmosphere-ocean coupling, in order to systematically examine different feedback mechanisms at work in the two regions, respectively. Our hierarchical model experiments reveal that strong negative atmospheric feedback mechanisms are intrinsic to the western Pacific while atmospheric feedbacks are less effective in the eastern Pacific with a partially offsetting effect from climatological oceanic upwelling. Therefore, the surface warming is likely to be amplified in the eastern rather than the western equatorial Pacific under uniform radiative heating. In addition, we invoke similar mechanisms to explain the inter-model spread of the future tropical Pacific surface warming pattern, in which uncertainty in ocean dynamical processes turns out to be important on both fast and slow timescales.