Squeeze dispersion: enhancement of diapycnal mixing by diapycnal strain*

We describe a mechanism called “squeeze dispersion” whereby fluctuating strain enhances the diffusive transport of active and passive scalars. Squeeze dispersion implies that fluctuating strain always enhances diffusive transport when scalar diffusivity is constant, and lends outsized importance to correlations between strain and diffusivity in determining net diffusive transport when diffusivity varies, as is the case for oceanic diapycnal turbulent diffusivity. We illustrate squeeze dispersion with an example problem, and derive a formula for the effective diffusivity of scalars diffusing in the presence of fluctuating strain. We then estimate effective diffusivities from turbulence measurements in abyssal flow through Samoan Passage, finding that squeeze dispersion enhances diapycnal transport by factors of 2 to 3 across some deep isopycnals due to correlations between strain and mixing over bathymetric constrictions.