Simulation of Inertial Currents and Enhanced Mixing in Ocean Fronts

Ocean fronts have taken the center stage in recent research on small-scale mixing and transport in the ocean. Here we examine a simple, isolated front using a turbulence resolving large-eddy simulation (LES) model to better understand the interaction between baroclinicity and surface fluxes in generating both vertical transport and horizontal spreading of fronts. Both surface cooling and surface wind stress are considered. In general, we find that frontal instabilities begin with 200-300 m scale coherent roll structures that are aligned with the front. With weak surface cooling and no wind, the alignment is nearly perfect, yielding results that are equivalent to previous constant gradient symmetric instability cases. Over time, the symmetric modes transform into mixed modes with an off-axis orientation. Vertical mixing of the geostrophic shear also leads to an inertial oscillation that modulates the strength of the overall shear. After ~48 hours, the inertial current decays as traditional baroclinic instability develops along the front and dominates the overall circulation. Analysis of kinetic energy indicates shear as the dominant production term when symmetric instability is active, which then switches to buoyancy production during baroclinic wave growth. Comparisons between the LES results and a version of the K-profile mixing parameterization that accounts for non-local mixing suggest that resolving geostrophic and inertial shear is sufficient for representing the vertical turbulent transport by symmetric and mixed mode instabilities.