Jet Structure and Scaling in Southern Ocean Models

Abstract

The jet structure of the Antarctic Circumpolar Current (ACC) simulated by two general circulation models (GCMs), FRAM (Fine Resolution Antarctic Model) and POP (Parallel Ocean Program), is examined in relation to the bottom topography field. Despite differences in configuration both GCMs display similar behavior: the model ACC consists of a number of distinct current cores superimposed on broader-scale flow. The jets display temporal and spatial (including vertical) coherence with maximum velocities occurring at the surface. It is shown that multiple jets can arise in wind-forced baroclinic quasigeostrophic flow. The main factors influencing the number and spacing of jets are found to be the bottom topography and the proximity of lateral boundaries. The meridional spacing of jets on a flat-bottomed ? plane is consistent with the Rhines scaling criterion for barotropic ?-plane turbulence with a small modification due to baroclinicity and the presence of meridional boundaries. When a zonally oriented ridge is present, the meridional spacing decreases. This is explained by postulating that the ? effect is augmented by a factor related to the topographic slope. Smaller-scale roughness alters the magnitude of the mean flow and mass transport but does not necessarily alter the meridional scaling. The number and meridional spacing of multiple jets in FRAM are also found to be broadly consistent with this hypothesis, although other effects such as topographic steering may also be important. The POP model generally exhibits shorter length scales than would be expected from the topographically modified Rhines scaling alone, and it is likely that other factors are present.