Earth, Atmospheric and Planetary Sciences, MIT
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.