Three Dimensional Structure of Diapycnal Mixing in Mesoscale Eddies in the Southern Ocean South of Australia

Abstract:

Diapycnal mixing in the ocean can be enhanced by mesoscale eddies. In-situ tracking of mesoscale eddies is difficult, observations of diapycnal mixing are sparse in the Southern Ocean, and eddies and mixing intensities have strong spatial and temporal variability. We, therefore, tried to quantify the three-dimensional distribution of the diapycnal mixing caused by mesoscale eddies with a statistical approach. Here, we combined satellite altimeter and high-vertical-resolution Argo profiles over 20 years, referenced to a gravest empirical mode (GEM) climatology, to estimate diapycnal mixing using composite analysis and finescale parameterization methods. We found that intensified diapycnal diffusivity exceeding the order of 10^-3 m²/s occurred within twice the radius of anticyclones and at the periphery of cyclones between the mixing layer depth and 2000 m. Horizontally, on a given neutral density level, the diapycnal diffusivity varies along the eddy radius. This is especially true for densities between 26 and 28 kg/m³. Vertically，the diapycnal mixing varies more strongly with latitude than longitude. This study illustrated the relationship of mesoscale and small-scale spatial structure. These results highlight the enhancement of diapycnal mixing by mesoscale eddies, which is important for the global thermohaline circulation. They also suggest a possible route for energy cascading from balanced mesoscale dynamics to unbalanced submesoscale in the upper ocean.