Transport through submarine canyons

Exchanges of water, nutrients, and oxygen between coastal and open ocean are key components of on-shelf nutrient budgets and biogeochemical cycles. Submarine canyons are underwater topographic features that incise the continental shelf and enhance physical processes such as cross-shelf mass exchanges and mixing. There is a good understanding of the flow around upwelling submarine canyons; however, the flux of biologically relevant tracers and the collective impact of canyons is less well understood. This dissertation investigates the collective impact of submarine canyons on cross-shelf exchange of tracers and water and their distribution on the shelf, taking into account the impact of locally-enhanced mixing within the canyon and the initial geometry of the tracer profile. I performed numerical experiments, using the Massachusetts Institute of Technology general circulation model (MITGCM), simulating an upwelling event near an idealized canyon. To investigate the role of mixing I added a passive tracer with an initially linear profile, varying the spatial distribution of vertical eddy diffusivity and its magnitude. To investigate the impact of the initial tracer profile I added 10 passive tracers with initial profiles representing nutrients, carbon and dissolved gasses. I found that locally enhanced vertical diffusivity has a positive effect on the tracer that is advected by the upwelling flow and can significantly increase canyon-upwelled tracer flux; tracer flux also depends on the initial vertical tracer gradient within the canyon, the depth of upwelling and the upwelling flux. I identified a pool of low oxygen and high nutrient, methane, dissolved inorganic carbon and total alkalinity concentrations on the shelf bottom, downstream of the canyon. The horizontal extent of the pool depends on the canyon-induced advective fluxes feeding the pool and the initial background distribution of tracers on the shelf. The interaction between two identical canyons during an upwelling event was investigated using a laboratory model on a rotating tank. I found that canyons are primarily independent for the parameter regime explored but may interact through the arrival of the upstream canyon’s pool to the downstream canyon’s head.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat