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. Different parameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnal plus eddy advective flux, are compared from the perspective of water mass transformation in the Southern Ocean. The different mixing physics imply different buoyancy equilibria in the surface mixed layer, different transformations, and therefore a variety of meridional overturning streamfunctions. The coupled-model approach avoi ds strong artificial water mass transformation associated with relaxation t o prescribed mixed layer conditions. Instead, transformation results from t he more physical non-local, nonlinear interdependence of sea-surface temper ature, air-sea fluxes, and circulation in the model's atmosphere and ocean. The development of a stronger mid-depth circulation cell and associated up welling when eddy fluxes are present, is examined. The strength of overturn ing is diagnosed in density coordinates using the transformation framework.