Presenter: Delphine Hypolite (UCLA)

Description:
We explore surface gravity wave effects on currents (WEC) in an active submesoscale current regime in a set of realistic coastal simulations in California. We use a new method that takes into account the full surface gravity wave spectrum and produces larger Stokes drift than the monochromatic peak-wave approximation.  We investigate two high wave events lasting several days -one from a remotely generated swell and another associated with local wind-generated waves - and perform a systematic comparison between solutions with and without WEC at three submesoscale-resolving horizontal grid resolutions (dx=270 m, 100 m and 30m).  WEC results in the enhancement of open-ocean surface density and velocity gradients when the averaged significant wave height is relatively large (>4.2m). For smaller waves, WEC is a minor effect overall.  For the remote swell, WEC maintains submesoscale structures and accentuates the cyclonic vorticity and horizontal convergence skewness of submesoscale fronts and filaments. The mechanism behind this is the vertical shear of Stokes drift becoming an important extra source of horizontal enstrophy, which in turn increases a vertical enstrophy source and reinforces surface frontogenesis. Wind-forced waves also enhance surface gradients, up to the point where they generate a small-submesoscale roll-cell pattern with high vorticity and divergence that extends vertically through the entire mixed layer.  The emergence of these roll-cells results in a buoyancy gradient sink near the surface that causes a modest reduction in the typically large submesoscale density gradients. At our highest resolution, these Langmuir roll-cells, developing over pre-existing submesoscale features, present spatial inhomogeneity and modify the forward energy cascade of the submesoscale regime.