Submesoscale processes and mixing

In this chapter, we discuss the links between oceanic submesoscale (100 m–10 km) processes and mixing. Submesoscale currents occur on lateral scales of 100 m–10 km in the ocean and are associated with density fronts and filaments, vortices and topographic wakes at the surface and in the ocean's interior. In most cases, submesoscale processes do not directly contribute to mixing, however they have an important role in cascading energy and tracer variance from the largely adiabatic mesoscale down to the scales at which diapycnal mixing can occur. Submesoscale currents redistribute water properties, including buoyancy, momentum, heat, freshwater, and biogeochemical tracers. While submesoscale instabilities enhance vertical exchange, they drive an efficient restratification of the upper ocean. They can also have a strong impact in the bottom boundary layer, where they generate turbulent mixing and export mixed waters out of the bottom boundary layer. We first present an overview of submesoscale dynamics, starting with frontogenesis and several important instability mechanisms that generate submesoscale currents in the ocean: mixed-layer baroclinic, gravitational, symmetric and inertial/centrifugal instabilities. Particular attention is paid to the complex role of turbulent mixing in the formation, evolution and decay of submesoscale features. We then discuss the impact of the different types of submesoscale processes on the redistribution of density and passive tracers, including impacts for some biogeochemical tracers and buoyant material.