Geometric Stability Analysis of Antarctic Circumpolar Current Meanders

The Antarctic Circumpolar Current (ACC) is unique in the global ocean as it permits large-scale tilting of isopycnal surfaces and the ventilation of a range of density classes at the surface Southern Ocean. The isopycnal tilt involves a complicated balance between surface wind forcing and mesoscale turbulence. The generation of mesoscale variability is generally thought to be related to baroclinic instability, extracting potential energy from the isopycnal tilt. However, the distribution of baroclinic instability is not zonally symmetric, but intensified downstream of topographic features, where the instability processes are more efficient. Using an idealized channel model, the stability of the flow downstream of a zonally-symmetric Gaussian ridge spanning the channel is examined by considering the local energy budget as well as the geometric stability properties of the flow. We consider the linear instability properties as a function of zonal position and find that large barotropic shear suppresses growth immediately downstream of the ridge and relaxation of this shear coincides with a peak in eddy kinetic energy. This examination of wave-mean flow interaction leads to a discussion of the applicability of the barotropic governor mechanism in the ACC.