Rapid changes in mixed layer stratification driven by submesoscale instabilities and winds

Abstract

Submesoscale eddies generated by baroclinic instability of upper ocean fronts lead to rapid restratification of the mixed layer on a time scale of days. This restratification can be opposed by a down‐front wind stress (acting in the direction of the geostrophic velocity) that drives a surface Ekman flow from the dense side to the light side of the front to arrest the slumping of isopycnals. A scaling diagnostic is suggested to determine whether the effect of eddies or wind dominates under different conditions. Using a numerical model, we investigate the juxtaposition of submesoscale eddies and down‐front winds acting on the mixed layer. By estimating the eddy‐induced overturning stream function in the mixed layer, we separate the along‐ and cross‐isopycnal fluxes of buoyancy associated with submesoscale mixed layer eddies and demonstrate the need for parameterization of the advective, along‐isopycnal flux. Though the cross‐front transport of buoyancy induced by the down‐front component of the wind opposes restratification by mixed layer eddies, it becomes diminished as the eddies and growth of the frontal instability disrupt alignment between the wind and frontal axis.