Eddies in Numerical Models of the Antarctic Circumpolar Current and Their Influence on the Mean Flow

Abstract

The dynamics of the Southern Ocean have been studied using two high-resolution models, namely the Fine Resolution Antarctic Model (FRAM) and the Parallel Ocean Program (POP) model. Analysis of these models includes zonal averaging at Drake Passage latitudes, averaging along streamlines (or contours of constant sea surface height), and examining particular subregions of the flow in some detail. The subregions considered in the local analysis capture different flow regimes in the vicinity of the Crozet Plateau, the Macquarie?Ridge Complex, and Drake Passage. Many aspects of the model results are similar, for example, the magnitude of eddy kinetic energy (EKE) in the ?eddy rich? regions associated with the large-scale topography. An important difference between the two models is that away from the strong topographic features the level of EKE in POP is 2?4 times greater than in FRAM, giving values close to those observed in altimeter studies. In both FRAM and POP instability analysis performed over ACC jets showed that baroclinic instability is likely to be the main mechanism responsible for generating EKE. In the case of FRAM this view is confirmed by regional energy budgets made within the ACC. In contrast to quasigeostrophic numerical experiments upgradient transfer of momentum was not found in the whole ACC, or over large subregions of the Southern Ocean. The only place it occurred was in localized tight jets (e.g., the flow northeast of Drake Passage) where the transients are found to transfer kinetic energy into energy of the mean flow. The transient eddies result in a net deceleration of the ACC for the streamwise averaging.