Equilibration of the Southern Ocean Stratification with and without Topography

The processes that determine the depth of the Southern Ocean thermocline are considered. From the perspective of heat transport, the thermocline depth is determined by the efficiency of the geostrophic circulation at transporting heat poleward. Existing conceptual frameworks focus on the importance of transient eddies, assuming that standing eddies can be neglected through transformation to a "streamwise" coordinate. Using numerical simulations of a simple circumpolar channel, we show that, when topography is present, standing eddies are the primary mechanism of poleward heat transport by the geostrophic circulation. The standing eddies are more efficient at transporting heat than the transient eddies, meaning that a flat-bottomed experiment with identical forcing has a significantly deeper thermocline. Analysis of the variance budget shows that this meridional heat transport by standing eddies is sustained by a term related to the transient eddy heat flux. Thus the transient eddies remain important for the equilibration, but play a different role. The model is also analyzed in streamwise coordinates; from this viewpoint, the heat balance with topography more resembles the flat-bottomed case, in which transient eddies dominate the geostrophic heat transport across the front. However, unlike the zonally symmetric flat-bottomed case, the transient eddy fluxes are highly localized downstream of topography. Overall, the cross-stream transport is more efficient with topography than without.