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UBC Theses and Dissertations

Circulation and upwelling in Mackenzie Canyon, a dynamically wide submarine canyon in the Beaufort Sea Machuca, Idalia Alicia

Abstract

Mackenzie Canyon, in the southeastern Beaufort Sea, is a site for strong upwelling compared to the adjacent continental shelf and slope and potentially supplies the shelf with significant levels of nitrate. Research regarding the circulation and upwelling mechanisms in submarine canyons has previously been limited to dynamically narrow canyons, and most studies have used numerical models with idealized bathymetry. The main goal of the study presented in this thesis is to describe the circulation and upwelling in Mackenzie Canyon, which is classified as a dynamically wide canyon. This study also identifies key flow features that act as significant modifiers of upwelling, examines differences between idealized and realistic model simulations, and estimates the canyon-induced upwelling of nitrate. To address these goals, the circulation and upwelling associated with an upwelling event induced by an impulsive wind forcing in Mackenzie Canyon was simulated using a nested-grid modelling system configuration based on the Nucleus for European Modelling of the Ocean framework. Numerical simulations were conducted using realistic and idealized bathymetry and three cases of wind stress forcing. The model performance was evaluated using observational data from Mackenzie Canyon during an upwelling event. This study finds that near-geostrophic flows are topographically steered around the Mackenzie Canyon walls. Strong cyclonic vorticity is generated on the upstream corner of the canyon mouth and evolves into a closed, cyclonic eddy, which becomes a site for strong upwelling. A coastal trapped wave (CTW) is induced on the downstream side of the canyon and propagates upstream. It is characterized as a shelf wave using a model that searches for the free wave solutions of CTWs along straight coastlines. An upwelling signal in the canyon exits the canyon and propagates along the slope with the CTW. Unlike narrow canyons, upwelling in Mackenzie Canyon is stronger on the upstream side than on the downstream side, likely as a consequence of the upstream propagation of the CTW. The nitrate flux across the nitracline depth supplied by upwelling in Mackenzie Canyon during the initial 36 hours of an upwelling event is estimated to be twice the seasonal draw-down in the Beaufort Sea.

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