Restratification by Mixed Layer Eddies

The ocean surface mixed layer is regularly remixed by convective and wind events. Between these events, surface heating and dynamical processes that slump horizontal density gradients restratify the layer. 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 the restratification. However, virtually all of the physical processes involved in dynamical restratification are missing or underestimated in current climate models. I will discuss some of these dynamical processes, and focus in particular on restratification by mixed layer eddies. Mixed layer eddies form from ageostrophic baroclinic instabilities of fronts in the mixed layer, and they populate the submesoscale (length scales from 1-10 km and time scales near 1 day). I will present a recently developed parameterization for the restratification of the mixed layer by these submesocale eddies, and demonstrate its efficacy and climate impact inferred from models and observations.