Suppression of Eddy Diffusivity across Jets in the Southern Ocean

Geostrophic eddies control the meridional mixing of heat, carbon, and other climatically important tracers in the Southern Ocean. The rate of eddy mixing is typically quantified through an eddy diffusivity. There is an ongoing debate as to whether eddy mixing in enhanced in the core of the Antarctic Circumpolar Current or on its flanks. A simple expression is derived that predicts the rate of eddy mixing, that is, the eddy diffusivity, as a function of eddy and mean current statistics. This novel expression predicts suppression of the cross-jet eddy diffusivity in the core of the Antarctic Circumpolar Current, despite enhanced values of eddy kinetic energy. The expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations. This work suggests that the meridional eddy diffusivity across the Antarctic Circumpolar Current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current.