Is there a tropical response to observed Southern Ocean warming and cooling?

Abstract:

SSTs in the Southern Ocean (SO) have displayed low-frequency internal variability in the past decades, namely a warming phase from 1950 to 1978, a cooling phase from 1979 to 2009, followed by another warming phase since 2009. Idealized experiments have shown that Southern Hemispheric extratropical forcing leads to a robust tropical response in the Pacific and the Atlantic. Here we probe the global impact of the observed SO SST warming and cooling trends in two fully coupled GCMs. Pacemaker experiments are conducted by nudging evolving SO SST anomalies to observations in ensemble simulations under historical radiative forcing. Both the CESM1 and GFDL-CM2.1 SO pacemaker experiments show a significant remote response to observed SO cooling in the tropical South Atlantic. Increased low clouds and strengthened trade winds cool the sea surface via radiative and wind-evaporation-SST feedbacks, as well as Ekman advection, partially offsetting the radiatively-forced warming trend. The inclusion of SO SST cooling produces a more realistic tropical South Atlantic SST trend, and a higher pattern correlation with observed SST trends in the greater Atlantic basin. The same mechanisms appear to operate in the earlier SO warming phase as well, where the model ensembles show a corresponding warming trend in the tropical South Atlantic SSTs. Our results suggest that the future trajectory of tropical South Atlantic SSTs may depend on the SO SSTs, especially given that SSTs have recently warmed in the SO. If this SO warming trend continues, the tropical South Atlantic may experience accelerated warming due to the combined influence from SO SSTs and increasing greenhouse gas concentrations. Implications for Sahel rainfall changes are also discussed.