Critical-layer isopycnal mixing and PV homogenization in the Southern Ocean

The vigorous mesoscale eddies in the Southern Ocean mix tracers horizontally along isopycnals in the ocean interior. We diagnosed the intensity of this mixing process and its variation with depth and latitude in the Southern Ocean State Estimate (SOSE) numerical model. The model is constrained tightly to observational data and its high resolution permits the formation of mesoscale eddies. The SOSE velocity fields were used to simulate the advection / diffusion of a passive tracer, and Nakamura's "effective diffusivity" diagnostic was applied to the resulting tracer distributions along isopycnals. This analysis reveals enhanced mixing near the surface on the equatorward flank of the the Antarctic Circumpolar Current (ACC) as well as deep below the core of the current. Where the mean flow was strongest, mixing was suppressed. This band of enhanced mixing (~1500 m2/s) corresponds roughly with the steering level of eddies with apparent phase speeds between 2 and 4 cm/s at the surface, as diagnosed from sea-surface height anomalies. Furthermore, the isopycnal gradient of potential vorticity (PV) reverses sign in the vicinity of the enhanced mixing region. This coincidence of the steering level, the reversal of the PV gradient, and enhanced isopycnal mixing are consistent with the classical picture of baroclinic instability familiar from quasigeostrophic theory.