Vorticity balance in coarse-resolution global ocean simulations

From an ocean model that was constrained by WOCE data sets as part of the NOPP-funded ECCO assimilation project, we analyze the barotropic vorticity budget of the global ocean. It will be shown that the data impose important changes in the mean state circulation as compared to unconstrained runs. For the depth-integrated circulation, it is found that the bottom pressure torque (BPT) dominates over the curl of friction in the western boundary currents and in the Southern Ocean. This important role played by BPT in the dynamics of the western boundary currents and the Antarctic Circumpolar Current (ACC), as found in this 2 degree-resolution simulation, is consistent with the recent theoretical argument and the analyses of eddy-resolving simulations. In the upper layer of the model, the linear Sverdrup balance holds in the interior of the basins; and the vorticity stretching/compressing associated with the vertical velocity is important at high latitudes. For the deep ocean, it is the combination of BPT and the vertical velocity that balances the advection of planetary vorticity.