Turbulent dynamics and energetics of anticyclonic submesoscale headland wakes

Both submesoscale flows and interactions with topography have been posited to be important factors driving dissipation and mixing in the ocean. However, since their in-situ measurement is difficult and numerical approaches do not typically resolve the turbulent processes responsible for the irreversible mixing and dissipation, the dynamics and energetics of submesoscale flows generated by topography are not currently well-understood.

In this work we attempt to clarify the topic by investigating a series of realistic Large-Eddy simulations of submesoscale flows past a headland where the turbulence is fully resolved, allowing us to probe into the small-scale processes responsible for the energy cascade. Consistent with previous studies, we find strong evidence of submesoscale centrifugal-symmetric instabilities (CSIs) in the wake, with most of the wakes being energized mainly via horizontal shear production (i.e., centrifugal instabilities). We also find that the mixing efficiency (the fraction of total energy extracted from the flow spent mixing buoyancy) within CSI regions in the wake varies between approximately 0.1 and 0.3, consistent with previous studies that found similar variability in CSI mixing efficiency values.

Finally, despite our simulations spanning a wide range of parameter space and at least three different dynamical regimes (namely regimes with detached eddy formation, attached boundary layers and tridimensional wake turbulence), we show that some quantities of interest can be predicted by simple scalings. As examples, the kinetic energy dissipation and buoyancy mixing rates scale with the Slope Burger number, and the vertical eddy diffusivity scales with the Rossby number times the Froude number.