Submesoscale dynamics in the open ocean

TL;DR: In this article, the seasonal cycle of submesoscale flows in the upper ocean is investigated through a series of numerical simulations in an idealised model domain, and the role of flow curvature is considered and it is shown that the angular velocity of mesoscale eddies, as well as relative vorticity, contributes to their stability.

Abstract: The seasonal cycle of submesoscale flows in the upper ocean is investigated through a series of numerical simulations in an idealised model domain. Subme-soscale processes become stronger as the resolution is increased from 4 km to 0.5 km. There is a seasonal cycle in the magnitude of horizontal buoyancy gradients and horizontal wavenumber spectra consistent with stronger mixed layer insta-bilities developing as the mixed layer deepens and energising the submesoscale. Up to 30% of the mixed layer volume in winter has negative potential vorticity and this negative potential vorticity is found in both mesoscale vortices and filaments. Further investigation of the filaments that arise in anticyclonic eddies show that they are generated in regions of negative potential vorticity and lead to the upwelling of fluid from the thermocline. Symmetric instability is found to be the most likely mechanism. The role of flow curvature is considered and it is shown that the angular velocity of mesoscale eddies, as well as relative vorticity, contributes to their stability. A numerical tracer release experiment shows that the filament generation process leads to downwelling of surface layer flows and a simplified biogeochemical experiment shows that the instability gives rise to much higher primary productivity in mesoscale anticyclones. Observations of a mixed layer front observed in the North Atlantic are presented which has a more diffuse cross-front buoyancy gradient at the surface than lower in the mixed layer and a strong ageostrophic cross-front velocity. The observations are used to motivate two dimensional simulations of Rossby adjustment. These simulations show that as the frontal strength increases the Rossby adjustment phase accounts for an increasing proportion of the restratification. The simulations become much more turbulent over the parameter range for frontal strength. The vertical profile in the observations can be re-produced when the adjustment is simulated with a thermocline, though definitive conclusions are difficult to draw.