Evidence for Enhanced Eddy Mixing at Middepth in the Southern Ocean

Satellite altimetric observations of the ocean reveal surface pressure patterns in the core of the Antarctic Circumpolar Current (ACC) that propagate downstream (eastward) but slower than the mean surface current by about 25%. The authors argue that these observations are suggestive of baroclinically unstable waves that have a steering level at a depth of about 1 km. Detailed linear stability calculations using a hydrographic atlas indeed reveal a steering level in the ACC near the depth implied by the altimetric observations. Calculations using a nonlinear model forced by the mean shear and stratification observed close to the core of the ACC, coinciding with a position where mooring data and direct eddy flux measurements are available, reveal a similar picture, albeit with added details. When eddy fluxes are allowed to adjust the mean state, computed eddy kinetic energy and eddy stress are close to observed magnitudes with steering levels between 1 and 1.5 km, broadly consistent with observations. An important result of this study is that the vertical structure of the potential vorticity (PV) eddy diffusivity is strongly depth dependent, implying that the diffusivity for PV and buoyancy are very different from one another. It is shown that the flow can simultaneously support a PV diffusivity peaking at 5000 m s or so near the middepth steering level and a buoyancy diffusivity that is much smaller, of order 1000 m s, exhibiting less vertical structure. An effective diffusivity calculation, using an advected and diffused tracer transformed into area coordinates, confirms that the PV diffusivity more closely reflects the mixing properties of the flow than does the buoyancy diffusivity, and points explicitly to the need for separating tracer and buoyancy flux parameterizations in coarse-resolution general circulation models. Finally, implications for the eddy-driven circulation of the ACC are discussed.