Differential Diffusion

Abstract

Differential diffusion is a turbulent process characterized by a more effective vertical mixing of temperature (T) than salinity (S) in ocean regions where mean gradients of both contribute to a stable density gradient. Because the molecular diffusivity of T is roughly 100 times that of S, T leaks out of a water particle displaced upward more rapidly than does S: a resulting buoyancy force acting on the salt-heavy remnant moves it downward, partially reversing initially upward flux and leaving the net upward transfer of S smaller than that of T. Differential diffusion in this sense has been documented in laboratory experiments and direct numerical simulations, using ranges of stratification and turbulent intensities that include those typical of the ocean. In all cases, the ratio of the turbulent diffusivity of S relative to T, d≡KS/KT, approaches 1 for strong turbulence but falls significantly below 1 for turbulence typical of episodic mixing in the stratified ocean interior. Various pieces of indirect observational evidence suggest that differential diffusion is active in at least some ocean regions. Because ocean general circulation models are sensitive to parametrization of small-scale mixing, differential diffusion at microscales may affect ocean structure and variability at much larger spatial and temporal scales.