The Force Balance of the Southern Ocean Meridional Overturning Circulation

The Southern Ocean limb of the meridional overturning circulation is characterized by three vertically stacked cells, each with a transport of about 10 Sv. A Southern Ocean state estimate for the years 2005 and 2006, which reproduces observed eddy statistics, is used to determine the forces balancing this overturning circulation. Diagnosing the zonal momentum budget in density space allows an exact determination of the adiabatic and diapycnal components balancing the thickness-weighted (residual) meridional transport. At midlatidues, there is a balance between wind-driven and geostrophic flows consistent with Sverdrup theory for gyre circulations. In the Antarctic Circumpolar Current (ACC), the leading-order balance is between a directly-wind driven flow and an opposing eddy-driven flow. The eddy-driven flow is associated with the diabatic stationary eddy buoyancy fluxes generated by the meanders of the ACC. This is in contrast with theories and idealized models of the Southern Ocean where eddy fluxes are associated with adiabatic transient interior potential vorticity fluxes. Implications of these results for parameterization of eddies in ocean models will be discussed.