Vorticity Dynamics of Seasonal Variations of the Antarctic Circumpolar Current from a Modeling Study

A one-layer numerical model was developed to analyze the vorticity dynamics of the seasonal variations of currents in the Southern Ocean. The model includes the continental geometry and bathymetry of the Southern Ocean and is forced by monthly climatological wind stress. Five cases are considered that compare (i) circulation over a flat bottom to that with bathymetry, (ii) effects of zonally averaged wind stress forcing versus the climatological forcing and (iii) anomaly wind stress (winds with the annual mean removed) versus the full stress. The individual terms in the vorticity conservation equation are calculated from the model solution along two latitude lines; 57.5-degrees-S, which passes through Drake Passage, and 43.5-degrees-S, which is in the subtropical gyres. In the zonal part of the flat bottom simulation, the curl of the surface stress balances bottom stress curl. However, in Drake Passage, beta (advection of planetary vorticity) balances bottom loss-the western boundary balance. Such vorticity interactions depend on the partial barrier of South America and, thus, do not occur in zonal channel models. The removal of vorticity occurs throughout the Southern Ocean for the seasonally varying winds but the mean circulation is balanced mainly by losses near Drake Passage. The location of the Antarctic Circumpolar Current (ACC) is controlled by the tip of South America rather than the structure of the wind. The seasonal changes in the model surface elevation in Drake Passage occur largely in the southern part of the passage, in agreement with pressure observations. The calculated ACC transport is similar for climatological and zonally averaged winds but the structure of the forced circulation is rather different for the two cases. Bottom topography changes the vorticity interactions so that the largest effects occur where the flow is forced over bathymetry creating relative vorticity by stretching, which is then removed by bottom friction. The major loss in the model occurs near Drake Passage, although there are smaller losses at other locations along 57.5-degrees-S. Bathymetry provides a strong counter-force to the wind stress and the transport is reduced by a factor of ten compared to the comparable uniform depth simulation. Friction plays a secondary role by determining the width of the currents and the spinup time but has only a weak effect on the total transport