THE ZONAL MOMENTUM BALANCE IN AN EDDY-RESOLVING GENERAL-CIRCULATION MODEL OF THE SOUTHERN-OCEAN

The momentum balance in the zonally unbounded region of the Southern O cean is examined using an eddy-resolving ocean general-circulation mod el (namely FRAM). Momentum, which is input at the surface and accelera tes the Antarctic Circumpolar Current, is transferred down the water c olumn and removed by topographic form stress. Bottom friction and late ral eddy viscosity are found to be negligible. The poleward flux of ea stward momentum has a small effect in redistributing momentum. In spit e of this, below the wind-driven surface layer and above the level of topography, the poleward momentum-flux divergence provides the main ba lance along with the ageostrophic nux of planetary vorticity (although the magnitude of these terms is an order of magnitude smaller than th e wind stress). Below the Ekman layer, standing eddies produce a drag on the now whilst transient eddies accelerate the flow. However, the i mpact of transient eddies is smaller. The downward transfer of momentu m is achieved by interfacial form stress. This can be understood in te rms of a poleward density (heat) nux. The main contribution comes from standing eddies, with a smaller contribution from transient eddies. B oth contributions assist the transfer. The flux of density (heat) from the neighbouring oceans (to the north and south of the Drake Passage latitudes) influences the depth penetration of zonal momentum, particu larly in the upper 1000 m. The Johnson-Bryden theory is generalized to give an additional term which is proportional to the stream function for the residual circulation associated with Eliassen-Palm cross-secti ons.