The Relative Roles of Shear and Gravitationally Driven Turbulence in Driving Ocean Mixing

Diapycnal mixing models used in interpreting ocean microstructure measurements and in closure schemes in circulation models are essentially based around the concept that the background shear is the mechanism causing instability and hence mixing. Mixing can also occur when, by some mechanism, heavy fluid is locally lifted above light fluid, resulting in a local gravitationally driven instability and mixing. Pure shear driven mixing has a maximum flux Richardson number of Rif = 0.5, while pure gravitational instability has a maximum flux Richardson number of Rif = 0.73. This high efficiency of gravitational instability only occurs, however, at small buoyancy Reynolds number, and events are likely rare in the ocean. The two mechanisms also have quite different expressions for the diapcynal diffusivity K, although the two mechanisms can act in concert. These results are discussed in the context of field observations, DNS simulations and laboratory observations.