Flow over a shelf-submarine canyon system : a numerical study

Abstract

The baroclinic response due to forcing by a narrow current of a coastal ocean overlying a shelf with a submarine canyon was examined using a 1.5-layer enstrophy-conserving numerical model. Three canyon shapes were employed: a slot, a slot with exponential slope, and a depression with Gaussian cross-sections. The canyon width relative to the first baroclinic deformation radius was varied to see if a narrow canyon influences the flow less than a wide one. The models were forced using a geostrophically-balanced jet, oriented northward, normal to the canyon's principal axis. Simulations ranging in length from 30 to 50 days were made. The response has phases which are best classified according to time-scales. The initial response occurs within 2 inertial periods. During this phase the jet reacts strongly to the canyon by deflecting in a direction consistent with the notions of conservation of vorticity and contour following flow. A double Kelvin wave (DKW) forms over each canyon wall. The upstream DKW propagates offshore and is lost. The downstream DKW travels toward the coast where its energy accumulates. Some of this energy escapes as Kelvin waves. During the second phase, characterized by a slow vortex-stretching time-scale, closed circulations form over the shelf and canyon. The extent of the patterns inside the canyon and the degree of their organization appears to be related to the canyon's width; wider canyons contain better organized flow. The shelf flow patterns are more a result of the proximity to the domain ends than the canyon's width. An interesting phenomenon is the generation of freely-translating eddies which form at the canyon-coastline junction. They form as fluid is moved from the canyon onto the shelf by the cross-step velocity of short wavelength DKW. These anticyclonic features leave the canyon within 6 days. Their radius is the local deformation radius. Lagrangian tracers verify that fluid in the anticyclone is from the canyon...