Langmuir Turbulence and Symmetric Instabilities in Submesoscale Fronts

The ocean surface boundary layer is rife with submesoscale fronts formed by strong winds and diabatic forcing. These fronts are subject to instabilities that may restratify or mix the the surface boundary layer. The same wind forcing creates waves which induce a time averaged current (Stokes drift) that changes the momentum balances through Stokes Coriolis and Stokes shear forces. These additional forces can change the vertical shear in the boundary layer, and force Langmuir circulations. Large eddy simulations of submesoscale fronts with Stokes drift exhibit several interesting turbulent features, some of which are well described by linear instability theory. For example, the onset of symmetric instabilities is not determined by a Richardson number based on Eulerian, Lagrangian, or Stokes shears alone, but rather by the Ertel potential vorticity. Regardless of the shear, when the Ertel potential vorticity take the opposite sign to the Coriolis frequency, symmetric instabilities can form, and extract energy from the mean Eulerian shear. The additional shear of the geostrophic flow in the front may slightly intensify Langmuir circulations, however, the primary effect of the front on Langmuir circulations is due to changing vertical stratification. The Stokes drift induces an anti-Stokes, Eulerian flow which contributes to the shear in the Ekman layer which tips the fronts over making both stable and unstable fronts. These changes in vertical stratification significantly change the strength of the Langmuir circulations. In an unstable front, the near-surface flow is initially dominated by a mixed shear-convective instability, which then transitions to long Langmuir circulations. Unexpectedly, this transition occurs shortly after a reduction in the Stokes shear production.