The atmosphere and ocean interior communicate through the ocean surface mixed layer. Turbulence acts to vertically-homogenise this upper-most layer of the ocean down to typical depths of 10 to 100 metres. The specific properties of the mixed layer determine how permeable this transport buffer is to exchanges of heat, momentum, carbon, oxygen, and other important biogeochemical tracers, and so these dynamics in the upper ocean can have an important influence on the Earth’s climate system.
In particular, we focus on dynamics at the submesoscale, which can greatly enhance communication across the mixed layer. The submesoscales in the ocean range from 0.1 km to 10 km with time-scales on the order of hours to days. This range of scales falls between the small, nearly-isotropic microscale of turbulence and the Rossby deformation radius, above which motions become quasi-geostrophically balanced. In non-dimensional terms, both the Rossby and Richardson numbers are order unity so that inertial, rotational, and stratification effects are all important. Of these submesoscale features, frontal regions, or local areas with large lateral density gradients are relatively common. These lateral density gradients may be set up through frontogenetic strains by mesoscale eddies, or by coastal upwelling, intrusions into intermediate waters, or river discharges. Submesoscale fronts are particularly susceptible to symmetric instability — a form of stratified inertial instability (or “slantwise convection”) which can occur when the potential vorticity is of the opposite sign to the Coriolis parameter. We are interested in how symmetric instability instigates and is coupled with large-scale inertial shear oscillations responsible for restratifying the frontal region. In particular, we are exploring the influence the front strength and finite width has on the linear and nonlinear dynamics of equilibration, and how symmetric instability may influence larger-scale motions related to baroclinic or barotropic instability.