Langmuir Turbulence and Cold Filaments: Do they Couple?

Upper ocean mixing is impacted in unknown ways by submesoscale motions, surface gravity waves, and boundary-layer Langmuir turbulence -- processes that span scales ranging from 1 m (or less) to 10 km (or more). To investigate the couplings between these processes and their impact on ocean mixing we have carried out process studies using large eddy simulations with nearly $10^10$ gridpoints for varying combinations of surface winds and waves coupled to an idealized cold dense filament. In a horizontally heterogeneous boundary layer, surface waves induce both mean and fluctuating Stokes-drift vortex forces that modify a linear, hydrostatic turbulent thermal wind (TTW) approximation for momentum, and we use TTW to interpret the LES. Process studies are made with surface winds and waves oriented in across-filament and down-filament directions. Frontogenesis is induced by Lagrangian secondary circulations in the boundary layer, and it is shown to be strongly impacted by surface waves, in particular the propagation direction relative to the filament axis. Down-filament winds and waves significantly reduce the peak level of frontogenesis by fragmenting the filament into primary and secondary downwelling sites in a broad frontal zone. At peak frontogenesis, opposing down-filament jets tend to overlie each other in a vertical x-z plane. Cold filament frontogenesis is also sensitive to oblique orientations of the surface waves relative to the filament axis. The turbulent vertical velocity patterns, indicative of coherent Langmuir cells, change markedly across the LES horizontal domain for both across-filament and down-filament winds under the action of submesoscale currents.