The response of Southern Ocean eddies to increased midlatitude westerlies: a non-eddy resolving model study

The midlatitude westerlies of the southern hemisphere have intensified since the 1970s. Non-eddy resolving general circulation models respond to such wind intensification with steeper isopycnals, a faster Antarctic Circumpolar Current (ACC), and a stronger Atlantic Meridional Overturning Circulation (AMOC). However, hydrographic observations show little change in the slope of the Southern Ocean isopycnals over the past 40 years. This insensitivity seems to result from a compensating mechanism whereby an initial increase in the slope of the isopycnals causes eddy activity to intensify and forces the isopycnal slopes down. Climate models do not yet resolve ocean eddies, and the eddy parameterizations included in them do not capture well the compensation mechanism mentioned above. We present simulations with a non-eddy resolving model incorporating an eddy parameterization in which eddy compensation is greatly enhanced by the use of a non-constant, spatially varying thickness diffusivity. The sensitivity of the simulated ACC and AMOC to increased southern hemisphere westerlies is greatly reduced compared to simulations using constant and uniform diffusivities