Phase Transition of CO2-Driven Equatorial Pacific Warming Pattern Governed by Ocean Dynamics and Remote Forcing

Abstract

Climate models robustly project an El Niño–like warming under rising CO₂, yet recent observations show the opposite with enhanced zonal SST gradients. Whether the long-term projection is relevant for interpreting historical trends remains questionable. Using a large-ensemble of abrupt 4xCO₂ experiment, we identify a two-phase transition in equatorial Pacific warming: an initial gradient strengthening followed by long-term weakening. To isolate the underlying mechanisms, we prescribe CO₂ forcing over discrete latitudinal bands in a model hierarchy that varies the degree to which ocean dynamics are represented. While the initial strengthening is commonly attributed to ocean dynamical thermostat associated with climatological upwelling, our equatorial forcing experiment highlights the role of ocean circulation changes. We further reveal another mechanism involving atmospheric feedbacks via Hadley cell shifts under remote Northern Hemisphere off-equatorial forcing. These fast responses transition to weakened gradients through slow ocean adjustments. In contrast, Southern Hemisphere forcing induces a persistent gradient weakening. Our results suggest that observed SST trends may reflect a fast, forced response to CO₂.