Abstract

A statistical analysis of surface drifter observations is used to compute eddy length and time scales and eddy diffusion in the Southern Ocean. Eddy diffusion values of the order of 104 m2 s–1 are found in the energetic western boundary currents north of the Antarctic Circumpolar Current (ACC) and secondary peaks occur where the ACC negotiates topography. The diffusivity shows an increase from the Antarctic continent to the core of the ACC, then a slight decrease or a stable plateau within the ACC. North of the ACC, diffusivity generally decreases into the interior of ocean basins, except in the western boundary regions where values are maximum. Diffusivity is also calculated from simulated trajectories based on altimetric geostrophic velocities, with and without mean flow, as well as with simulated trajectories based on Ekman currents. Ekman currents at the drogue depth (15 m) have only a small impact, and the geostrophic currents dominate the eddy diffusivity. Complementary statistical analyses confirm these results. The surface drifter cross-stream eddy diffusion is used to test a simple parameterization based on satellite altimetric observations of eddy kinetic energy (EKE). For EKE ≥ 0.015 m2 s–2, κ = 1.35√EKELd m2 s–1, where Ld is the first baroclinic Rossby radius. This parameterization holds in the energetic ACC, consistent with an eddy field in the “frozen field” regime. Over the broader areas of weaker eddy fields, mixing is fairly uniform and stable at about κ = 1800 ± 1000 m2 s–1.

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