Northern Hemisphere influence on stratification and transport of ACC

We shall show that the surface boundary conditions on buoyancy in the Northern Hemisphere strongly influence the transport and stratification of the Antarctic Circumpolar Current (ACC). To demonstrate this, we use an ocean general circulation model in an idealized single-basin configuration with a high latitude circumpolar channel. A decrease of the northern hemisphere SST meridional gradient reduces production of deep water in the Northern Hemisphere, affecting the Meridional Overturning Circulation (MOC) and deepening the thermocline in both hemispheres. The induced change of stratification in the Southern Hemisphere circumpolar region increases the zonal volume transport of circumpolar current because of an increase in the local meridional density gradient and associated thermal wind shear --- the dominant baroclinic part of the total volume transport.

The effects are strong and robust to changes in parameters and subgrid parameterization schemes (e.g., parameterization of mesoscale eddies, and of diapycnal diffusivity). For example, a 4 C increase in SST in the high latitudes of the Northern Hemisphere can lead to a change in the ACC transport of approximately 40 Sv. We will also discuss the implications of this result for the response of the ACC to climate change, which is predicted to be ammplified in high latitudes in the Northern Hemisphere.