Geostrophic Turbulence in the Ocean Mixed Layer

The ocean surface mixed layer is regularly mixed by small-scale, turbulent convective and wind driven instabilities. Between these events the layer restratifies as a result of surface heating and  dynamical processes that slump horizontal density gradients.  Subsequent mixing events depend critically on the amount of  restratification that has occurred. Processes such as deep  convection, mode water formation, phytoplankton biology, and air-sea  chemical and heat exchange are thus sensitive to restratification.  The paradigm of dynamical restratification is the slumping of lateral buoyancy gradients through surface frontogenesis in regions of confluent flow.
In this presentation we show that in the real ocean  the bulk of restratification is associated with three dimensional  instabilities that develop along fronts and not with the two- dimensional slumping associated with frontogenesis. We present scaling laws based both on quasi-geostrophic theory and on the full  primitive equations for the rate of restratification by frontal  instabilities. The scalings laws are applied to the real ocean to show that frontal instabilities are a leading order effect in the mixed layer heat budget and in the exchange of nutrients between the surface ocean and the thermocline.