On the development and stabilisation of symmetrically unstable fronts in the surface mixed layer

Destabilising atmospheric forcing can create regions where potential vorticity (PV) takes the opposite sign of the Coriolis parameter, leading to the onset of symmetric instability (SI)—a hybrid convective-inertial perturbation. SI facilitates energy transfers from geostrophically balanced fronts to turbulent kinetic energy in the oceanic surface mixed layer (SML). Using linear theory and high-resolution Large Eddy Simulations (LES), SI’s role in the PV budget and subsequent restratification of the water column is explored. Spin-down experiments with and without a stratified thermocline below the SML reveal that, in the absence of destabilizing atmospheric forcing, PV fluxes from the ocean interior play a minor role in restratification. Instead, cross-frontal Reynolds stress divergences, driven by SI, generate a secondary circulation that efficiently stratifies the SML through a modified turbulent thermal wind response. SI-induced vertical momentum fluxes also drive frontogenesis, forming sharp non-geostrophic fronts at the SML boundaries. These fronts act as hotspots for vertical PV fluxes, where secondary Kelvin-Helmholtz instabilities (KHI) emerge. The complex interplay between SI and KHI, shaped by turbulent energy dissipation, significantly influences the efficiency of restratification and energy redistribution, with important implications for submesoscale dynamics and parameterisations in climate models.