The structure of geostrophic stirring in the oceanic submesoscale

Oceanic submesoscales straddle the regimes characterized by quasigeostrophic dynamics, and the zoo of unbalanced turbulent processes occurring on the mixing scales. The interactions between these processes are not well understood, and in particular, it is not clear which processes drive the observed variance at these scales. The work described here addresses the baseline problem: how much of the observed structure can be generated by mesoscale stirring alone? We present new results on the three-dimensional stirring of tracers, buoyancy and potential vorticity by the geostrophic mesoscale field, motivated by a recent analysis of temperature-salinity (T-S) profiles from the region in which the North Atlantic Tracer Release Experiment (NATRE) was conducted. These T-S profiles show large isopycnal excursions at depths just below the thermocline. It is found that mesoscale stirring acting on climatological gradients of temperature and salinity generates a direct cascade of thermohaline variance to small scales. Because stirring in the ocean's interior is primarily along isopycnal surfaces, it is very ineffective at generating isopycnal gradients, resulting in the production of thermohaline fronts that are compensating in their effect on density. A high-resolution quasigeostrophic simulation, driven by baroclinic instability in the presence of observed mean gradients, is used to verify this picture quantitatively.

The geostrophic field develops strong vertical shear on small scales, resulting from the three-dimensional forward cascade of potential enstrophy. It is shown that the combined action of strain and shear generates tracer filaments that, on average, maintain a scale-independent aspect ratio proportional to N/f. The result is a submesoscale coupling between vertical mixing and horizontal stirring that allows vertical diffusion to effectively absorb the laterally driven cascade of tracer variance.

Nearer the ocean's surface, geostrophic turbulence changes its character: the turbulent dynamics becomes dominated by a forward cascade of buoyancy, causing the energy spectrum to flatten, and the tracer variance spectrum to steepen. This occurs in an environment of weak upper-ocean stratification, and the result is a ubiquitous generation of lateral density fronts. These dynamically active fronts are primed by mesoscale stirring, but provide a pathway to interaction with mixed-layer processes.