Abstract

Theories of the Southern Ocean circulation generally assume that flow is along rather than across isopycnals (surfaces of constant density) in the ocean interior, with diapycnal (across density surfaces) processes limited to the ocean’s mixed layer. Recently, though, observational studies have found evidence of large diapycnal mixing near topographical features, which suggests that the assumption of purely isopycnal flow is invalid. In this study, temperature and salinity profiles obtained with expendable CTD (XCTD) probes throughout Drake Passage, Southern Ocean between February 2002 and July 2005 are analyzed to estimate turbulent diapycnal eddy diffusivities to a depth of 1000 m. Diffusivity values are inferred from density/temperature inversions and internal-wave vertical strain. Both methods indicate that mixing in Drake Passage is characterised by strong spatial variability with the Polar Front separating two dynamically different regions. Mixing rates typically exceed open ocean levels, especially north of the Polar Front. Wind-driven near-inertial waves, strong mesoscale eddy activity and double-diffusive convection are suggested as possible factors contributing to observed mixing patterns. Both the spatial variability and the magnitude of the mixing rates across Drake Passage suggest that new models are needed to describe the meridional circulation in the Southern Ocean and its role in the global overturning circulation.