Vertical transport due to submesoscale processes in the upper ocean

Higher resolution numerical studies reveal a new class of motions in the upper ocean that exhibit O(1) Rossby and Richardson numbers. Such motions arise in the presence of lateral density gradients (at fronts), exhibit large strain rates and vorticity, and a horizontal length scale of O(1-10km) that is set by the buoyancy gradient and the boundary layer depth. They are associated with intense vertical velocities and affect the exchange of potential vorticity, density, and tracers between the surface layer and pycnocline. Thus, they enhance the flux of nutrients for biological productivity and the affect the stratification of the upper ocean on both short and long time scales.

There are multiple mechanisms that can generate submesoscale motions: quasi-geostrophic frontogenesis, finite amplitude mixed layer instabilities, secondary circulation forced by wind or buoyancy mixing, as well as nonlinear Ekman effects. Using a non-hydrostatic model we demonstrate how submesoscale motions redistribute buoyancy and tracers in the upper ocean under varying wind conditions. We separate the along- and cross-isopycnal fluxes of buoyancy, and diagnose the eddy induced streamfunction using the Reynolds fluxes. The residual mean theory can be used to understand the competition between various processes.