Diapycnal Upwelling Driven by Tidally Induced Mixing over Steep Topography

Abstract

Abstract Diapycnal upwelling along sloping topography has been shown to be an important component of the abyssal overturning circulation. Theoretical studies of mixing-driven upwelling have mostly relied on a time-averaged description of mixing acting on a mean stratification which ignores the intermittency of mixing. Typically, these studies prescribed a time-invariant turbulent diffusivity profile motivated by scenarios where tidal currents encounter gentle topography with small-scale corrugations, leading to subsequent propagation and breaking of internal waves. Here, a different scenario is considered where a tidal current interacts with smooth but steep topography, a case often encountered near continental margins and troughs. The performed nonhydrostatic simulations resolve both the strong oscillatory shear that develops along the steep critical topography and the associated mixing events. Strong diapycnal mixing is observed during the upslope phase of the tidal flow when both the near-boundary stratification and shear are enhanced. During the downslope phase, strong overturning events do develop, but they are associated with weak stratification and less efficient diapycnal mixing. These results highlight that the temporal evolution of both shear and stratification play a key role in setting when diapycnal mixing and water mass transformation occur along steep topography. In contrast, over gentle topography, tidal shears do not become sufficiently large to generate strong local mixing for typical oceanographic parameters.