An idealized study of flow across submarine canyons: Analytical and numerical approaches to canyon dynamics with applications to the LoVe ocean region

• Main Author: Sjur, Anna Lina Petruseviciute
• Format: Master Thesis
• Language: English
• Published: 2021
• Subjects: Lofoten; Vesterålen
• Online Access: http://hdl.handle.net/10852/88825; http://urn.nb.no/URN:NBN:no-91442

Motivated by the possible connection between the many submarine canyons and the high bioproductivity in the Lofoten-Vesterålen (LoVe) ocean region, this study investigates the flow over an idealized continental slope with a submarine canyon, focusing on cross-slope transport. An important feature is the addition of a well-developed eddy-field, and also the inclusion of periods with reversed wind. In addition, the suitability of quasi-geostrophic theory for predicting flow over a canyon is studied. Both a stratified and a single-layer analytical, quasi-geostrophic model for flow over a slope with topography are developed, based on atmospheric Mountain-wave theory, and numerical model simulations of along-shore wind driven flows are conducted. Numerical model simulations show that there is an asymmetrical response to flow direction in early flow stages. Under retrograde conditions (flow in the same direction as the topographic wave propagation), there is increased cross-slope transport when a canyon is included, while under prograde conditions (flow in the opposite direction of the topographic wave propagation), the canyon has little effect on the transport. The analytical model predicts this asymmetry to be due to arrested topographic waves resonating with the canyon under retrograde flow. Comparing wavelengths of the arrested topographic waves between the theoretical quasi-geostrophic models and numerical model simulations, we find good agreement, especially in unstratified systems. There is also qualitatively good agreement between flow patterns for prograde flow between the analytical and numerical models. The slope currents in the LoVe ocean region are highly unstable, motivating us to further explore how increased cross-slope transport, and thus increased nutrient concentration in the shelf region, can occur under these flow conditions. Two possible mechanisms are further explored; 1) the effect of high eddy-activity, both under prograde and retrograde conditions, and 2) periods of reversed wind under mean prograde conditions. We find the same asymmetry under high eddy-activity as for early flow stages, with increased transport in canyon-runs for retrograde flow, and little difference between canyon and no-canyon runs for prograde flow. However, when periods of reversed winds are included in the prograde runs, we see heightened transport in canyon runs in the period after the wind reversal, compared to no-canyon runs. Since observations show periods of winds opposing the flow direction in LoVe, this mechanism may be part of the reason why we see such high bioproductivity in the region.