Despite global warming, SSTs in the Southern Ocean (SO) have cooled in recent decades largely as a result of internal variability. The global impact of this cooling is assessed by nudging evolving SO SST anomalies to observations in an ensemble of coupled climate model simulations under historical radiative forcing, and comparing against a control ensemble. The most significant remote response to observed SO cooling is found in the tropical South Atlantic, where increased clouds and strengthened trade winds cool the sea surface, partially offsetting the radiatively forced warming trend. The SO ensemble produces a more realistic tropical South Atlantic SST trend, and exhibits a higher pattern correlation with observed SST trends in the greater Atlantic basin, compared to the control ensemble. SO cooling also produces a significant increase in Antarctic sea ice, but not enough to offset radiatively induced ice loss; thus, the SO ensemble remains biased in its sea ice trends.

No abstract

Since the beginning of the satellite era, Southern Ocean sea surface temperatures (SSTs) have cooled, despite global warming. While observed Southern Ocean cooling has previously been reported to have minimal impact on the tropical Pacific, the efficiency of this teleconnection has recently shown to be mediated by subtropical cloud feedbacks that are highly model-dependent. Here, we conduct a coupled model intercomparison of paired ensemble simulations under historical radiative forcing: one with freely evolving SSTs and the other with Southern Ocean SST anomalies constrained to follow observations. We reveal a global impact of observed Southern Ocean cooling in the model with stronger (and more realistic) cloud feedbacks, including Antarctic sea–ice expansion, southeastern tropical Pacific cooling, northward-shifted Hadley circulation, Aleutian low weakening, and North Pacific warming. Our results therefore suggest that observed Southern Ocean SST decrease might have contributed to cooler conditions in the eastern tropical Pacific in recent decades.