Effects of a Dynamically Wide Submarine Canyon on Coastal Currents During an Upwelling Event PO14B-2189 Idalia Machuca and Susan Allen Background Mackenzie Canyon Numerical Model
Submarine canyons are steep-walled topographical Extensive research has been conducted into the effects of A regional configuration of the features along continental margins. Upwelling events are narrow submarine canyons (widths less than double the Rossby NEMO (Nucleus for European Flow episodes of enhanced deep-ocean exchange with the radius of deformation) on local circulation patterns and upwelling. Modelling of the Ocean) model Downstream Upstream shelf, which result from the disruption of the geostrophic In this work, we explore the characteristics of flows around a was developed to simulate the balance constraining alongshore currents to follow dynamically wide canyon. circulation around Mackenzie isobaths. These events are induced by alongshore flows Mackenzie Canyon is located Canyon during an upwelling moving in the direction opposite to that of Kelvin wave on the western edge of the event. The model uses the propagation. [1] Mackenzie Shelf in the Beaufort realistic bathymetry of the region Sea. It is 100 km long, 60 km and an idealized version in wide, and 475 deep. In contrast, separate simulations. Flows in Astoria (Oregon) and Barkley the domain are forced by winds (British Columbia) Canyons, in an east to west direction to which are common subjects for imitate realistic, upwelling- research, are 15 km and 13 km favourable conditions. wide, respectively. Preliminary Results Day Day Day These figures show slices of the idealized and realistic model results at a depth of 100 m for days 1 2 4 1, 2, and 4 of the simulations. Circulation
The Rossby number (Ro=U/fR) gives an indication for the [2]tendency of flows to follow isobaths given the flow velocities and radius of curvature of the topography. Given the large
width of Mackenzie Canyon, Ro is comparatively small. As a result, flows entering the canyon exhibit a smaller separation from the bathymetry than that seen in narrow A westward-moving jet along the slope turns into the Water flows around the canyon along the topography, As the speed of the incoming flow starts to decrease, canyons, and the water is guided downstream canyon on the eastern side of the mouth. and the speed of the incoming flow increases. the flow entering the canyon separates from the along the canyon walls. bathymetry. Coastal Trapped Wave The wave is generated as a result of the flow dynamics introduced by the canyon in the simulations. As the wave exits the canyon and propagates along the continental slope, the resulting pressure gradient produces a geostrophic current westward which amplifies the velocities of the incoming currents on the upstream side of the canyon. This is evident The results for vertical velocities show the generation The signal propagates eastward along the canyon The wave travels around the canyon, exits on the by the asymmetry in speeds along the slope. of a wave disturbance on the western canyon wall. walls indicating that it is a coastal trapped wave. eastern side, and travels along the slope. Upwelling Upwelling in canyons is driven by the unbalanced cross-shore pressure gradient resulting from the physical constraints of the topography. Upwelling is typically observed over the downstream wall of narrow canyons. However, in the case of Mackenzie Canyon, a significant level of upwelling occurs at the upstream wall of the canyon and along the Upwelling is first observed on the western side of the Water is upwelled from greater depths, and the The region of upwelling in the ideal case extends upstream slope. This behaviour indicates that canyon in both idealized and realistic models. strongest upwelling is concentrated near the head of towards the mouth along the eastern wall. In the real the wave acts as an additional driver for the canyon. case, it remains centralized near the canyon head. upwelling in this region. Form Drag Observational Studies References
[1] Allen, S.E., and Hickey B.M. “Dynamics of Advection-Driven Upwelling over a Shelf Break [5] [3] Submarine Canyon.” Journal of Geophysical Research, 2010. [2] Allen, S.E. “Restrictions on Deep Flow Across the Shelf-Break and the Role of Submarine Canyons in Facilitating Such Flow.” Surveys In Geophysics, 2004. [3] Carmack, E.C., and Evgueni A.K. “Wind-Forced Upwelling and Internal Kelvin Wave Generation in Mackenzie Canyon, Beaufort Sea.” Journal of Geophysical Research: Oceans, 1998. [4] Mirshak, R. “Spin-up and the Effects of a Submarine Canyon: Applications to Upwelling in Astoria Canyon.” Journal of Geophysical Research, 2005. [5] Williams, W. J., et al. “Joint Effects of Wind and Ice Motion in Forcing Upwelling in Mackenzie Trough, Beaufort Sea.” Continental Shelf Research, 2006. Water flowing across the domain experiences a drag force imposed Field studies show [5]complex flow patterns similar to the model by the canyon topography. [4]The stresses normal to the topography results, with currents entering Mackenzie Canyon from the east and Contact [email protected] provide a sense of the evolution of upwelling in the canyon. Given moving westward along the bathymetry. There is also [3]evidence of a https://www.eoas.ubc.ca/~imachuca/ 2 the traditional definition for form drag, DForm= ρCDL1L2v , the drag first mode internal Kelvin wave propagating in a northeast direction www.linkedin.com/in/idalia-machuca coefficient in the simulations is estimated to be between 4 and 8. and of upwelling occuring on the eastern wall of the canyon. @idalia_machuca