Drivers of CMIP Tropical Pacific Warming Pattern Diversity

Abstract

Anthropogenic changes in sea surface temperature relative to the tropical mean (relative SST) play a pivotal role in influencing atmospheric stability and circulation. In the tropical Pacific, CMIP5/6 multi-model mean (MMM) projections by the end of the 21st century show a southeastern relative cooling and a reduced equatorial SST gradient, although individual models exhibit considerable diversity. Using a simplified heat budget framework, we analyze the processes driving these relative SST changes across 63 CMIP5/6 models under historical and most pessimistic future scenarios. In the southeastern tropical Pacific, MMM relative SST cooling is driven by intensified winds that enhance latent heat flux, with inter-model diversity explained by variations in clouds and winds. Conversely, the MMM equatorial SST gradient reduction arises from reduced evaporative cooling efficiency in the climatologically cold eastern Pacific. A heat budget covariance analysis reveals that inter-model diversity in equatorial Pacific warming is predominantly driven by ocean dynamical processes, challenging previous studies that emphasized cloud feedback mechanisms. Clouds instead mitigate inter-model spread. The inter-model spread in ocean dynamics is linked to two factors: trade wind relaxation and the cold tongue bias. Stronger trade wind relaxation amplifies western Pacific warming, while a weaker cold tongue indicates a less effective ocean thermostat, enhancing eastern Pacific warming. During the present-day period, only a subset of models captures the observed equatorial SST gradient strengthening, but the mechanisms vary across these models, complicating the identification of robust drivers of this observed trend.