Understanding the Ability of the Southern Ocean to Influence the Southeastern Tropical Pacific

May 09, 2025

Yiyu Zheng

Committee: Maria Rugenstein (Advisor); Peter Jan van Leeuwen; James Hurrell; Daniel McGrath (Geosciences)

Abstract

The tropical Pacific plays a central role in the climate system and is linked to two major challenges in climate modeling: persistent biases in simulations and large inter-model spread in projections. The Southern Ocean (SO) has a remote influence on sea surface temperatures (SST) in the southeastern tropical Pacific (SEP) through a teleconnection involving cloud feedbacks, oceanic upwelling, climatological winds, and wind-evaporation-SST feedback. This teleconnection has been primarily explored in previous studies using perturbation experiments imposed on climate model simulations, leaving open questions about how it manifests in observations and fully coupled model outputs.

We first analyze this relationship using pre-industrial control simulations and abrupt-CO2-forced simulations from 53 CMIP6 models. We find a robust positive SO–SEP relationship both within and across models, regardless of whether the climate system is forced by external CO2 The inter-model spread of the positive SO-SEP relationship is attributed to the strength of shortwave cloud feedback and ocean heat uptake off the west coast of South America.

We then analyze 30-year SST trends over the historical period (1985–2014) using 42 CMIP6 models and multiple observational products. Most models simulate delayed warming trends in both the SO and SEP, failing to capture the observed cooling. These warming trends are positively related across models, even after removing the global-mean trend. Models underestimate both shortwave cloud feedback and ocean heat uptake variability off the west coast of South America, leading to opposing constraint effects: strengthening cloud feedback would enhance the SO–SEP relationship, while strengthening ocean heat uptake variability would weaken it. Furthermore, the strength of the SO–SEP relationship is positively associated with equilibrium climate sensitivity, linking this teleconnection to the "hot model" problem.

Finally, we assess the SO–SEP relationship on interannual timescales using 26 CMIP6 models and observations. Both models and observations show robust positive correlations, even after removing the effects of El Niño-Southern Oscillation-related variability, tropical decadal variability, and the forced response. The constraining effects of shortwave cloud feedback and ocean heat uptake variability remain consistent with the previous results. The observed SO–SEP correlation suggests that the SO-SEP relationship is underestimated in models, pointing to a dominant role of cloud feedback over ocean heat uptake variability.

Together, these findings demonstrate that the SO–SEP relationship is a robust feature of the climate system. They underscore the importance of accurately simulating both shortwave cloud feedback and ocean heat uptake variability to improve this relationship in climate models, with broader implications for climate simulations and projections.