On the Relationship between Antarctic Sea Ice Extent and the Southern Ocean Overturning Circulation.

The importance of the Southern Ocean for the exchange of heat and carbon with the atmosphere motivates attempts to understand the overturning circulation in this region. Recent literature has noted a spatial correspondence between the Antarctic sea ice extent and the latitude at which the divide between the upper and the lower overturning cells in the Southern Ocean outcrops at the surface. This has given rise to two hypotheses: firstly, that the sea ice extent controls the location of the latitude dividing the two overturning cells via its effect on surface buoyancy fluxes. Secondly, that the latitude of the overturning divide limits the sea ice extent due to its influence on sea surface temperatures. These hypotheses are tested by running a series of perturbation experiments in a coupled GCM. It is found that no directly causal relationship exists between the Southern Ocean overturning and the Antarctic sea ice extent, but that there are complex interactions between the two. In particular, changes to the overturning circulation may indirectly influence the sea ice extent via modification of the Southern Ocean stratification and upwelling of heat. Perturbations to the sea ice extent, meanwhile, have a highly seasonal influence on surface buoyancy fluxes, whose effect on the overturning is not immediately apparent in the annual average.