Abstract

Jets are a well-known feature of the Southern Ocean’s Antarctic Circumpolar Current. Evidence from satellite altimetry and numerical models suggests that zonal jets are also a robust feature of the mid-latitude ocean basins. The characteristics of mid-latitude and Southern Ocean jets differ significantly, with the latter having narrow, ribbon-like appearances and a greater tendency to merge, migrate and meander. Topography, lack of meridional boundaries, and variations in the strength and vertical structure of mean flows all contribute to the dissimilarity of mid-latitude and Southern Ocean jets.

This study considers the influence of simple topography—sinusoidal ridges and bumps—on the formation and transport properties of coherent structures (jets and eddies) in forced-dissipative, quasi-geostrophic turbulence. The experimental framework is a series of two-layer, baroclinically-unstable simulations in a doubly-periodic domain. Transport and mixing properties are diagnosed using the Nakamura effective diffusivity. In simulations with zonal ridges, the upper layer, in particular, feels a competition between the imposed topographical scale (ridge separation) and the Rhines scale. This can lead to unsteady jet structure and vertical misalignment of transport barriers. Three-dimensional topography with a sufficiently large amplitude may induce periodic bursts of high eddy kinetic energy related to baroclinic instability acting on topographically steered non-zonal mean flows. These episodes allow large-scale reorganization of the jet structure. It is likely that these features play a key role in the dynamic nature of Southern Ocean jets.