The role of mixed layer instabilities in submesoscale turbulence

Recent observations of submesoscale turbulence in the upper ocean reveal that it undergoes a strong seasonal cycle: submesoscale flows are strongly energized in winter and weak in summer. These observations point to the importance of mixed layer instabilities in energizing the submesoscale range, because these are active when mixed layers are deep in winter. Here, we formulate a simple quasi-geostrophic model to explore the dynamics of submesoscale flows driven by mixed layer instabilities and how they interact with the mesoscale eddy field. The model consists of two interacting layers of constant potential vorticity representing the mixed layer and the thermocline. This allows the development of both submesoscale mixed layer instabilities and mesoscale thermocline instabilities. We explore both linear and nonlinear dynamics of this model and compare them to mesoscale-driven surface frontogenesis, as described by surface quasi-geostrophic dynamics, which has previously been suggested to energize the submesoscale range.