On the role of secondary instabilities for symmetrically unstable fronts in the surface mixed layer

Symmetric instability (SI) plays an important role in the energy transfer from geostrophically balanced fronts to turbulent kinetic energy in the oceanic surface mixed layer.  SI can occur when the potential vorticity (PV) takes the opposite sign of the Coriolis parameter. Due to the “impermeability” of PV, the averaged PV  in a volume bounded by two outcropping isopycnals in the mixed layer can only be changed by PV fluxes through the surface or from the stratified interior.  Much attention is paid to PV fluxes at the surface caused by momentum and buoyancy fluxes. Here, we concentrate on the PV flux from the stratified interior which plays an important role in stabilising a symmetrically unstable front. Large Eddy simulations are used to study the impact of secondary Kelvin-Helmholtz instabilities  on the PV fluxes and the associated restratification of the mixed layer. Sensitivities to grid resolution show that a resolution of ~1m is needed to resolve the full influence of Kelvin-Helmholtz instabilities on the evolution of SI. Ideas for parameterising their effects will be discussed.