Impacts of far field forcing on the Southern Ocean Residual Overturning Circulation.

The overturning circulation in the Southern Ocean consists of a residual overturning circulation (ROC) formed by opposing mean and eddy circulations. Although the ROC is part of the global overturning circulation, previous studies of the Southern Ocean ROC have explained this circulation from local surface wind and buoyancy forcing. Using a series of idealised MITgcm channel model runs we shown that the ROC cannot be maintained without incorporating a representation of diabatic processes to the north of the domain such as North Atlantic Deep Water formation. By relaxing to a set temperature profile at varying timescales in a sponge layer in the north of the domain a collapse of the ROC occurs when these processes become weaker, invoking a response in the diabatic eddy heat fluxes in order to modulate the ROC. We outline an updated Transformed Eulerian Mean scaling for the ROC to include its dependence on the stratification at the northern boundary and forcing by diabatic eddy fluxes. The robustness of this dependency on far field foricng is tested in a series of larger simulations incorporating the effects of topography, where the changes in the ACC due to far field forcing are also considered showing large changes in the baroclinic transport with changing relaxation timescales.