An observational assessment of feedbacks on Southern Ocean SST variability

Coupled model studies consistently predict an initial cooling, but equilibrium warming, response of the polar Southern Ocean (SO) in response to forcing by stratospheric ozone depletion and a concurrent shift of the Southern Annular Mode (SAM) towards its positive phase. Yet the time scale of this warm equilibration shows large inter-model discrepancies. The strength of the feedback of air-sea heat fluxes on SO SST anomalies, which is a major factor in setting adjustment time scales, is thereby a key source of uncertainty.

Here we present a new observational assessment of the air-sea feedback as a function of spatial scale, across the SO up the Antarctic sea-ice edge. This reveals systematic regional contrasts in the air-sea feedback strength: feedbacks of 20-30 W/m²K observed over the Southern Hemisphere's subtropical gyres and their boundary currents, are seen to transition to feedbacks of typically only 10 W/m²K along the Antarctic Circumpolar Current. In particular in the summer time SST anomalies on large spatial scales adjacent and within the zone of seasonal sea ice are subject to feedbacks as weak as 5 W/m²K. The mechanisms at the origin of these variations - with region, season and scale - of the air-sea feedbacks are isolated observationally.

The implications of these new observational results for the persistence of SO SST are explored, placing particular emphasis on anomalies at large spatial scales adjacent to the sea-ice edge. We thus reassess the observed response of SO SST to SAM forcing, and in particular examine whether, contrary to previous observation-based results, departures from the classical ‘AR-1' paradigm of large-scale extratropical SST anomaly dynamics, such as seen in the models, need to be evoked to explain observations in the presence of the weak observed air-sea feedbacks.