Diapycnal mixing and turbulence dissipation over the Inner Shelf.

During a collaborative experiment over the inner shelf off Point Sal (California) in 2017 more than 70 newly-developed turbulence sensors were deployed. Distributed over more than 30 moorings ranging from 20 to 100 m water depth, these instruments continuously measured turbulence for a two-month period, covering a range of surface and tidal forcing. This data set provide an unprecedented opportunity to study diapycnal mixing and energy dissipation over the inner shelf. Using a framework initially proposed by Winters & D’Asaro 1996 for complete model data and bravely applied here to measurements, we evaluate effective diapycnal mixing rates over a large region of the inner shelf over the two-month time period. While direct wind forcing is potentially a significant source for diapycnal mixing in the near-surface layer, the main energy source for mixing appears to be the shoaling internal tide. We find a clear neap-spring cycle in diapycnal mixing rates. Additionally, the turbulence diffusivity is strongly modulated by the background stratification on the shelf. We contrast different lateral regions in terms of mixing. In the vertical we find that a major fraction of mixing falls in the stratified region just above the well-mixed bottom boundary layer, whereas the direct contribution of the bottom boundary layer (or at least depths < 2 mab) to mixing is smaller.