Evidence for Deep PV Mixing in the ACC

Satellite altimetric observations show that Rossby wave phase speeds in the core of the Antarctic Circumpolar Current (ACC) are directed downstream but are typically only one quarter the speed of the surface current, implying a steering level at significant depth. Detailed linear stability calculations of gridded hydrographic data indeed yield steering levels at a depth of typically 1 km in the ACC, significantly deeper than other regions of the global ocean (see figure). Moreover, our calculations show that the primary baroclinic instability is due to a growing mode with maximum amplitude and phase at a similar depth, near the base of the jet, implying preferential eddy generation at the steering level.

Calculations using a nonlinear model forced by the mean shear and stratification at a particular location in the ACC (chosen to coincide with a position where mooring data and direct eddy calculations are available) confirm this picture, and show that PV mixing occurs primarily at the depth of the steering level. Despite an instability structure which is broadband and complex, with multiple zero-crossings in the mean PV gradient, we find that the mixing level is dominated by a particular growing mode, that which can lead to the most efficient conversion of available potential to eddy kinetic energy.

The resulting structure of the eddy PV flux is consistent with an eddy velocity which acts to flatten tilted isopycnals. An important result of our study is that the vertical structure of the PV diffusivity is strongly depth-dependent, and so the diffusivities for PV and buoyancy are not the same. Implications for the dynamics of the meridional overturning circulation of the ACC and for the parameterization of eddies, are discussed.