Evaluating the effect of low-cloud feedback strength on the transient response of Pacific sea surface temperature patterns under CO2 Warming

Abstract

General circulation models (GCMs) are complex models used to predict future climate under various greenhouse gas emissions scenarios. However, when different GCMs simulate the same scenario, there is widespread variability in the final climate. One of the leading causes of the spread has been attributed to differing low-cloud feedback between the GCMs. To address this, Erfani & Burls (2020) conducted a suite of idealized, fully-coupled, climate model simulations under abrupt doubling of CO₂ concentrations with varying strengths of low-cloud feedback. This study uses the output from the simulations in Erfani & Burls to study the transient response of the sea surface temperature (SST) gradient between the west and east Pacific, known as the zonal gradient, which has a strong impact on global atmospheric conditions as exemplified by El Nino and La Nina events. We find an initial increase in the zonal gradient which resembles the thermostat mechanism. This mechanism proposes that the eastern Pacific will warm more slowly than the western Pacific under uniform warming as a result of the upwelling of cold waters, thus increasing the zonal gradient. In the long term, however, the equilibrium zonal gradient response driven by subtropical cells becomes more important, causing SSTs to increase across all feedback parameters, with more the positive parameters experiencing a greater reduction of the zonal gradient. A simplified box-model that simulates the essential coupled ocean-atmosphere dynamics is compared with the CESM results to determine if the box model is able to capture the behavior seen in the CESM simulated temperature gradients. This hierarchy of models approach seeks to improve our understanding of the processes controlling the response of the Tropical Pacific to global warming over the coming decades to centuries.