Processes Responsible for the Southern Ocean Heat Uptake and Redistribution under Anthropogenic Warming

The Southern Ocean absorbs the majority of the excess heat stored in the climate system due to anthropogenic greenhouse gas emissions. We find that in climate model future projections, about two thirds of the surface net heat gain in the high-latitude Southern Ocean from anthropogenic warming is redistributed northward, leading to enhanced warming at middle latitudes with most of the deep-reaching warming located near the boundary between the subtropical gyres and the Antarctic Circumpolar Current. In a theoretical framework by Bindoff and McDougall (1994), the subsurface ocean temperature and salinity changes are associated with three distinctive processes on the temperature-salinity diagram: pure heave, pure warming, and pure freshening. The enhanced mid-latitude warming and isopycnal deepening are attributed to both pure heave and pure warming processes, which might be related to the wind-driven heat convergence and the accumulation of extra surface heat uptake by the background ocean circulation, respectively. The equatorward and downward subductions of the surface heat and freshwater input at high latitudes, i.e., pure warming and pure freshening processes, result in cooling and freshening spiciness changes on density surfaces within the Subantarctic Mode Water. The correspondence and distinction between each process and atmospheric forcing as well as the implications and limitations of this theoretical framework are further explored by comparing the results with model perturbation experiments, using flux perturbations of momentum, heat, and freshwater from the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP).