Understanding the ability of the Southern Ocean to influence the southeastern tropical Pacific

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. Emerging studies show that the Southern Ocean has a remote influence on sea surface temperatures (SST) in the southeastern tropical Pacific through a teleconnection involving cloud feedbacks, oceanic upwelling, climatological winds, and wind-evaporation-SST feedback. This teleconnection has primarily been explored using perturbation experiments imposed on climate model simulations, leaving open questions about how it manifests in observations and fully coupled model outputs. This dissertation investigates the relationship between SSTs in the Southern Ocean (SO) and the southeastern tropical Pacific (SEP) using Coupled Model Intercomparison Project phase 6 (CMIP6) coupled model outputs. In Chapter 2, I analyze this relationship using pre-industrial control simulations and abrupt-CO2-forced simulations from 53 CMIP6 models. I find a robust positive SO-SEP relationship both within and across models, regardless of whether the climate system is forced by external CO2 or not. 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. In Chapter 3, I 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: if I strengthen cloud feedback in climate models, it would enhance the SO-SEP relationship; if I strengthen ocean heat uptake variability, it would weaken the SO–SEP relationship. Furthermore, the strength of the SO-SEP relationship is positively associated with equilibrium climate sensitivity, linking this teleconnection to the higher climate sensitivity in CMIP6 models--the "hot model" problem. In Chapter 4, I 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 (ENSO)-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 chapter. The observed SO-SEP correlation shows that the SO-SEP relationship is underestimated in models, pointing to a dominant role of cloud feedback over ocean heat uptake variability in affecting the strength of such a relationship. Together, these findings demonstrate that the SO-SEP relationship is an intrinsic and 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 implications for reducing the SST trend biases in climate simulations and for a warmer climate in projections.