Nonlinear vorticity balance of the Antarctic Circumpolar Current

Abstract

Using a recently published global ocean mean surface dynamic topography, based predominantly on drifter data, the near‐surface vorticity balance of the Antarctic Circumpolar Current (ACC) is calculated. With a little smoothing, the dynamic topography is found to produce good estimates of even such highly differentiated quantities as the relative vorticity advection. Two clear modes of flow are found in the ACC: meanders of wavelength 300–500 km, in which the nonlinear term is important, resulting in a balance between advection of relative and planetary vorticity, as in a stationary equivalent‐barotropic Rossby wave (this implies a surface divergence which is at least partly balanced by an opposite divergence at depth); and a flow with divergence associated with topographic features. It is tentatively concluded that the divergence of this latter flow is a scaled measure of bottom pressure torque. The inferred bottom pressure torque shows large‐scale topographic interactions, as well as a strong influence of some sharp topographic features such as the fracture zones and Macquarie ridge system south of the Tasman Sea, and the narrow ridges in Drake Passage.