Dependence of Southern Ocean Overturning on Wind Stress

The Southern Ocean MOC has recently been proposed to respond strongly to the strength of the southern-hemisphere surface westerlies. (Toggweiler & Russell 2008). In this context, an eddy-resolving numerical model of a zonal flow, meant to resemble the Antarctic Circumpolar Current, is described and analyzed using the framework of Marshall and Radko (2003). In addition to wind and buoyancy forcing at the surface, the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation to exist at depth. The strength of the residual MOC is diagnosed for different values of surface wind stress. It is found that eddy circulation compensates for the changes in Ekman circulation to a large degree. The extent of the compensation, and thus the sensitivity of the MOC to the winds, depends on the surface boundary condition. A fixed-heat-flux surface boundary severely limits the ability of the MOC to change. An interactive heat flux leads to greater sensitivity. The change in eddy circulation is decomposed into a part due to changing isopycnal slopes and a part due to changing eddy diffusivity. The contribution from changing eddy diffusivity is found to dominate. A scaling theory for the buoyancy diffusivity, based on the mechanical energy balance, is developed and tested; the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress.