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 biologically relevant tracers such as oxygen and nitrate is less understood. Tracer and nutrient transports are usually inferred from water transports. Although this assumption is a reasonable approximation, it does not take into account the contribution of mixing to the distribution of tracers. The objective of this work is to characterize the combined effect that enhanced mixing within the canyon and the specific initial profile of a tracer have on the spatial and temporal distribution of the tracer during an upwelling event. For that purpose, numerical experiments simulating an upwelling event near an idealized canyon with locally enhanced vertical diffusivity were performed using realistic nutrient and tracer profiles from Barkley Canyon, BC taken during the Pathways Cruise 2013.
This work presents results from numerical experiments using the community model MITgcm when varying the initial concentration profiles for six different tracers; it also suggests a physical mechanism through which each final distribution is reached. We find that for all tracers, the depth of strong gradients is key to the exchange process, allowing more tracer transport when located deeper than the shelf break. Added to this, enhanced mixing within the canyon drives a higher diffusive transport, while the lower background diffusivity allows relatively unmixed ocean water onto the shelf, near the bottom. Thus, the regime that leads to the lowest, deep oxygen concentration on the shelf, is not the same as that which leads to the highest shelf nitrate inventory. Taken together, our work shows that the tracer pathways developed by the canyon dynamics, locally enhanced mixing and initial tracer profile have significant implications for the final tracer distribution on the shelf.