Mechanisms and consequences of delayed Southern Ocean warming

Delayed Southern Ocean (SO) surface warming is a robust feature of the climate response to greenhouse gas forcing. It is found in surface and satellite-based observations, and is simulated within coupled atmosphere-ocean general circulation models. The warming delay has been widely attributed to deep mixed layers and efficient deep ocean heat uptake processes within the SO. While these mechanisms do play a role, we argue here that delayed SO warming is primarily a consequence of the mean SO circulation: Ekman upwelling of unmodified water from depth acts to anchor sea surface temperatures over multi-centennial timescales; equatorward transport of surface waters acts to advect the anomalous temperature signal out of the SO to midlatitudes, where it is subducted, driving an anomalous northward ocean heat transport that largely balances the increased air-sea heat flux into the SO under global warming.

We explore here the relative roles of the mechanisms of delayed SO warming within a variety of general circulation models and observations. We further consider the consequences of delayed SO warming for (i) changes in atmospheric meridional energy transport with global warming, and (ii) the global top-of-atmosphere energy budget, via the delayed activation of destabilizing SO radiative feedbacks.