q 1998 American Meteorological Society Gravitational, Symmetric, and Baroclinic Instability of the Ocean Mixed Layer

A hierarchy of hydrodynamical instabilities controlling the transfer of buoyancy through the oceanic mixed layer is reviewed. If a resting ocean of horizontally uniform stratification is subject to spatially uniform buoyancy loss at the sea surface, then gravitational instability ensues in which buoyancy is drawn from depth by upright convection. But if spatial inhomogeneities in the ambient stratification or the forcing are present (as always exist in nature), then horizontal density gradients will be induced and, within a rotation period, horizontal currents in thermal-wind balance with those gradients will be set up within the mixed layer. There are two important consequences on the convective process: 1) Upright convection will become modified by the presence of the thermal wind shear; fluid parcels are exchanged not along vertical paths but, rather, along slanting paths in symmetric instability. Theoretical considerations suggest that this slantwise convection sets the potential vorticity of the mixed layer fluid to zero but, in general, will leave it stably stratified in the vertical. 2) The convective process ultimately gives way to a baroclinic instability of the horizontal mixed layer density gradients. The resulting baroclinic waves are important agents of buoyancy transport through the mixed layer and can be so efficient that the convective process all but ceases