OFFSHORE TRANSPORT OF DENSE SHELF WATER IN THE PRESENCE OF A SUBMARINE-CANYON

The formation and offshore transport of dense water over a uniformly s loping shelf crosscut by a submarine canyon is examined using a three- dimensional primitive-equation numerical model. A constant negative bu oyancy flux is applied in a limited region adjacent to a straight coas t to represent brine rejection from ice production in an idealized coa stal polynya. A sharp density front forms at the edge of the forcing r egion, with surface and bottom intensified jets along the front. The f low around the head of the submarine canyon triggers a frontal instabi lity that initially grows only on one side of the canyon. The unstable waves on the other side of the canyon are blocked by a localized baro tropic flow that develops near the canyon head. Unstable waves also gr ow where the forcing region intersects the coast. The frontal waves gr ow rapidly (with O(1 day) c-folding timescales) and form eddies with h orizontal scales of O(15 km) which extract the densest water from the forcing region and carry it offshore, directly across isobaths. In thi s way the eddies limit the maximum water density that appears in the m odel despite continued negative buoyancy forcing. Some dense water des cends into the canyon, forming a bottom-trapped plume that transports the dense water offshore ahead of the eddies. The plume moves relative ly slowly (i.e., small Froude number), with little turbulent entrainme nt, so the advancement and structure of the plume nose can be describe d successfully as a simple gravity current with an advective-diffusive heat balance. Eddies may slump into the canyon from the side, alterin g both the density anomaly and speed of the canyon plume! suggesting t hat canyon plumes are likely to be highly variable in both space and t ime.