Contrasting Tropical SST Warming in Two Climate Models: The Roles of Winds, Clouds, Evaporative Cooling, and Ocean Dynamics

Abstract

To investigate why climate models simulate diverse tropical Pacific sea surface temperature (SST) changes under greenhouse-gas forcing, we utilize two configurations of the Community Earth System Model (CESM2) that differ in their atmospheric component (CAM6 and CAM4). Under abrupt CO 2 quadrupling, CAM6 simulates over double the weakening of the east-west equatorial SST gradient compared to CAM4 by end-of-century. To understand this contrast, we conduct overriding experiments that replace wind stress, or wind stress and shortwave fluxes, with values from the preindustrial control experiment. In CAM6, low clouds amplify climate warming and act as a forcing and positive feedback on reductions in the east-west SST gradient. In contrast, in CAM4, low clouds act as a negative climate feedback and provide little contribution to the SST gradient reduction. In both models, interactive wind stress leads to a strong positive feedback (i.e., the Bjerknes feedback). The two models also diverge in their short-term SST responses: CAM6 exhibits an immediate weakening of the zonal SST gradient, while CAM4 initially shows a steepening. Our findings highlight the importance of shortwave radiative and Bjerknes feedbacks, evaporative cooling, and ocean dynamics, all of which control tropical Pacific SST pattern formation.