Cross-shelf transport of tracers induced by a submarine canyon

The exchange of water and solutes between the coastal area and the open ocean is of great importance to biogeochemical fluxes, nutrient budgets and their response to climate change and human activities. On a regional scale, submarine canyons are known to enhance physical processes such as shelf-slope mass exchange and mixing. There is good understanding of the flow around upwelling submarine canyons; however, the flux of tracers is less understood. The objective of this study is to characterize the combined effect that upwelling, advection and enhanced mixing within the canyon have on the exchange of tracers between the shelf and open ocean. For that purpose, a scaling analysis for mixing and advection of tracers within an idealized canyon during upwelling, and a numerical exploration of the corresponding parameter space were performed.

This work presents results from the scaling scheme and a space-varying diffusivity sensitivity analyses using the community model MITgcm; it also suggests a physical mechanism through which such exchange is possible. Using nutrient vertical profiles from Barkley Canyon, British Columbia as a case study, we find that for all tracers horizontal advection dominates over isopycnal diffusivity, and vertical advection and diapycnal mixing are both relevant processes. Numerical results show that the total amount of tracer on the shelf increases when the prescribed mixing rate within the canyon is locally enhanced and moderate outside the canyon, compared to the case of moderate mixing rates everywhere in the domain. Elevated mixing inside the canyon results in enhanced cross-shelf tracer transport because the water exchanged has a higher tracer concentration compared to the case with homogeneous diffusivity. Taken together, our results will demonstrate that submarine canyons considerably enhance the amount of tracer coming onto the shelf and drastically change the pathways through which these tracers flow.