Topographically Generated, Subinertial Flows within A Finite Length Canyon

Abstract

The presence of a canyon cutting the continental shelf has been observed to enhance wind-driven upwelling. In particular, in the vicinity of Juan de Fuca Canyon at the mouth of the Juan de Fuca Strait an eddy containing deep water (from a depth of approximately 450 m) has been documented. Strong upcanyon flows have been observed within numerous canyons including Astoria Canyon, which cuts the shelf offshore of the mouth of the Columbia River. The author develop a linear theory for wind-driven flow over an infinitesimally thin but finite length canyon to illustrate the basic mechanism. Two regimes are considered, the initial growing velocity field and a later steady velocity field. The flow toward the shore is enhanced by O (10) by the presence of the canyon in a homogeneous fluid. The presence of stratification introduces smaller horizontal length scales, the baroclinic Rossby radius, and allows further enhancement of the upcanyon flow. Numerical simulations show that the linear theory is a reasonable approximation for canyons of finite but narrow width compared with the baroclinic Rossby radius. The effect of nonlinearity is to advert the flow pattern downstream, which gives results closer to what is observed. In contrast to the case for linear flow, strong cyclonic vorticity is generated over the canyon, as is observed over Astoria Canyon.