Evidence for deep eddy mixing 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%. We 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 at a particular location near the core of the ACC (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. The simulated eddy potential vorticity (PV) flux is consistent with an eddy velocity that acts to flatten tilted isopycnals. An important result of this study is that the vertical structure of the PV diffusivity is strongly depth-dependent, and so the diffusivities of eddy PV and eddy buoyancy are not the same. In fact, it is shown that one can simultaneously have a large PV diffusivity that is peaked near the mid-depth steering level (as expected from linear theory), and a buoyancy diffusivity that is much smaller and less structured. An effective diffusivity calculation, using an advected and diffused tracer transformed into area coordinates, confirms that the PV diffusivity more closely reflects the Deep eddy mixing in the Southern Ocean 2 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 ACC circulation are discussed. Deep eddy mixing in the Southern Ocean 1