Estimates of Lateral Mixing by Turbulence in Ocean Fronts

Wind, wave, and buoyancy forcing are the primary production mechanisms for turbulence in the ocean surface mixed layer. Fronts add a second source of energy through buoyancy production from Ekman transport and shear production through the geostrophic shear. Experiments with large-eddy simulation models suggest that for strong fronts ( dT/dx > 1 ^oC/10 km), symmetric instability can develop in response to surface Ekman buoyancy transport leading to rapid mixing via secondary Kelvin-Helmholtz instability. Weaker fronts (dT/dx < 1 ^oC/10 km) are more likely to develop instabilities of the wind-forced Ekman current or mixed instabilities combining shear production from both the geostrophic and wind-forced currents. In general, lateral mixing from turbulence causes weakening of the front, which limits the effectiveness of baroclinic-type instabilities. In this work, we use large-eddy simulation to examine how enhanced turbulence in finite width frontal zones affects the frontal strength and arrests instability processes by weakening Ekman buoyancy flux and geostrophic shear. Results are compared with a basic scaling model based on wind speed and horizontal temperature gradient.