Southern Ocean transformation in a coupled model with and without eddy mass fluxes

A coupled air–sea general circulation model is used to simulate the global circulation. Differentparameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnalplus eddy advective flux, are compared from the perspective of water mass transformation inthe Southern Ocean. The different mixing physics imply different buoyancy equilibria in thesurface mixed layer, different transformations, and therefore a variety of meridional overturningstreamfunctions. The coupled-model approach avoids strong artificial water mass transformationassociated with relaxation to prescribed mixed layer conditions. Instead, transformationresults from the more physical non-local, nonlinear interdependence of sea-surface temperature, air—sea fluxes, and circulation in the model’s atmosphere and ocean. The developmentof a stronger mid-depth circulation cell and associated upwelling when eddy fluxes arepresent, is examined. The strength of overturning is diagnosed in density coordinates using thetransformation framework.