The vertical structure of geostrophic turbulence in the ocean

Understanding the mechanisms responsible for the structure and non-linear evolution of mesoscale eddies remains a major challenge. We address here the question of the vertical structure of mesoscale oceanic eddies, by analyzing the results from experiments with a comprehensive high resolution primitive equation ocean model and interpreting the results using linear stability analysis, idealized quasi-geostrophic simulations and geostrophic turbulence theory. This methodology provides a powerful way of studying the turbulence in a quasi-realistic context, complementing observational work.

In our numerical work we take mean shear and stratification at various locations as given, from the PE model, and compare the energy levels and structure produced by a quasi-geostrophic model with the primitive equation results. This allows us to interpret of the role of local non-linear baroclinic instabilities and cascade phenomena in the generation of those eddies. Although the footprint of the linear instability is in general visible on the eddy vertical structure, their non-linear evolution show a tendency toward grave vertical scales. We explain the role of bottom friction and stratification, and relate these results to the freely evolving case. More generally, we investigate under which conditions barotropization (the tendency of geostrophic turbulent flows to reach the gravest vertical mode) can occur using both numerical simulations and analytic (statistical mechanical) arguments, and relate our results to recent diagnostics emphasizing the role of surface quasi-geostrophic modes in the ocean.