How well can we model turbulence and mixing in ocean fronts?

Ocean surface-layer fronts are hotspots of energy turnover, mixing, and water mass transformation in the ocean, and their reliable representation in numerical ocean models is an essential requirement. However, turbulence models used to represent the effects of (unresolved) small-scale instabilities and turbulence in numerical ocean models are based on a boundary layer approximation that assumes that only vertical shear, stratification, and atmospheric forcing at the surface are relevant for the computation of the vertical turbulent fluxes. It is at the moment unclear to what extent this type of models is able to represent the effects of ocean surface-layer fronts, which are defined by the existence of dynamically relevant horizontal density gradients. While fronts with typical widths a few kilometers are sufficiently large-scale to be resolved in regional ocean models, the numerous small-scale hydrodynamic instabilities (e.g., symmetric and inertial instabilities) observed inside ocean fronts are typically “sub-grid”. Based on high-resolution turbulence data from field campaigns and large-eddy simulations (LES) of idealized fronts, I will discuss the capabilities of ocean turbulence models to represent the effect of such “sub-grid” processes in idealized frontal configurations. The discussion will focus especially on the models’ ability to reliably represent the vertical turbulent fluxes, the frontal energetics, and the secondary circulation inside fronts.