Enhancement of Mesoscale Eddy Stirring at Steering Levels in the Southern Ocean

Abstract

Meridional cross sections of effective diffusivity in the Southern Ocean are presented and discussed. The effective diffusivity, Keff, characterizes the rate at which mesoscale eddies stir properties on interior isopycnal surfaces and laterally at the sea surface. The distributions are obtained by monitoring the rate at which eddies stir an idealized tracer whose initial distribution varies monotonically across the Antarctic Circumpolar Current (ACC). In the absence of observed maps of eddying currents in the interior ocean, the advecting velocity field is taken from an eddy-permitting state estimate of the Southern Ocean (SOSE). A three-dimensional advection?diffusion equation is solved and the diffusivity diagnosed by applying the Nakamura technique on both horizontal and isopycnal surfaces. The resulting meridional sections of Keff reveal intensified isopycnal eddy stirring (reaching values of ?2000 m2 s?1) in a layer deep beneath the ACC but rising toward the surface on the equatorward flank. Lower effective diffusivity values (?500 m2 s?1) are found near the surface where the mean flow of the ACC is strongest. It is argued that Keff is enhanced in the vicinity of the steering level of baroclinic waves, which is deep along the axis of the ACC but shallows on the equatorial flank. Values of Keff are also found to be spatially correlated with gradients of potential vorticity on isopycnal surfaces and are large where those gradients are weak and vice versa, as expected from simple dynamical arguments. Finally, implications of the spatial distributions of Keff for the dynamics of the ACC and its overturning circulation are discussed.