Effects of Wind, Density, and Bathymetry on a One-Layer Southern Ocean Model

Steady solutions from a one-layer, wind-driven, primitive equation model are analyzed to determine the importance of wind forcing, pressure gradient force due to the climatological density distribution and bottom form drag on circulation in the Southern Ocean. Five simulations are discussed: three wind-forced simulations, with differing bathymetry (flat bottom, 15% bathymetry, and full bathymetry), one case with full bathymetry forced with the density-induced pressure force, and one case with full bathymetry forced by both wind and density-induced pressure gradients. The simulations presented here confirm the previous speculation (Munk and Palmen, 1951) that form drag is effective in balancing the driving force due to the surface wind stress. In fact, it has such a strong effect that bathymetry with only 15% of the true amplitude reduces the transport from over 480 x 106 m3 s-1 to about 190 x 106 m3 s-1. If the true bathymetry is used, the total transport is reduced to a value around 20 x 106 m3 s-1. Analysis of the zonally integrated momentum in the unblocked latitudes of the Southern Ocean shows that the bottom form drag balances the surface forcing, even for simulations that have viscosities that are in the upper range of acceptable values The vertically integrated pressure gradient due to the climatological density distribution produces a body force that accelerates the Antarctic Circumpolar Current, producing a transport of about 250 x 106 m3 s-1. Therefore the pressure gradient produced by the density structure of the Southern Ocean is an integral part of the dynamics of the Antarctic Circumpolar Current. It forces the flow across bathymetry that would, in the absence of the spatially varying density field, block the circulation. This result is in contrast to mid-latitude gyres in which the steady, wind-driven circulation is insulated from the influence of bathymetry by stratification (Anderson and Killworth, 1977)