Processes Controlling the Southern Ocean Temperature Change

Abstract Observations show that since the 1950s, the subsurface Southern Ocean has been rapidly warming in the south and cooling in the north. However, what drives these opposing latitudinal patterns of change is not well understood. By analyzing the outputs of atmosphere–ocean general circulation models, we examine and quantify the processes controlling the temperature change in the Southern Ocean. We find that subsurface cooling in the north results from the combined effect of weak warming driven by greenhouse gases and cooling forced by aerosols. Contrary to previous work, we find that the influence of stratospheric ozone forcing is small and unnecessary to explain the observed patterns. Analysis of heat budget diagnostics shows that ocean heat content change between 400 and 1200 m in the Southern Ocean is primarily driven by slight imbalances between the large and opposing changes in residual mean advection and isopycnal diffusion. We show that while surface heat flux anomalies are the main contributor to the warming in the Southern Ocean, changes in wind stress are also required to produce the observed pattern of temperature change. These results highlight the importance of ocean dynamics in subsurface ocean heat content change. The counterbalancing effects of aerosols and greenhouse gases on ocean heat content change in the Southern Ocean will change with aerosols declining in the future, while our results suggest that changes in stratospheric ozone forcing will have only a minor effect on future patterns of Southern Ocean thermohaline structure.