A numerical study of secondary flows over continental shelf edges

Abstract

The circulation over the shelf edge of a homogeneous, wind-driven coastal ocean is examined. A two-dimensional finite element model in the plane perpendicular to the coast is used allowing a detailed examination of the thin horizontal and vertical frictional boundary layers characteristic to the continental shelf. After spinning-up the flow from an initial state of rest, it is found that frictional effects cause the alongshore current to develop a horizontal shear over the continental slope and shelf break, as well as a recirculation cell seaward of the break. At the shelf edge, bottom Ekman layers over the shelf and the deep ocean are disrupted and, in contrast to very gently sloping topographies, no bottom layer (in the classical Ekman sense) develops over the continental slope. Instead a thicker upwelling layer, similar to the one present next to the coastal wall, develops next to the continental slope. In the cases considered herein, the results indicate that the dynamics of the circulation over realistic continental shelf edges are closely related to the dynamics of slopes modeled as a vertical wall. Further, during upwelling favorable winds, the onshore flow in the region seaward of the shelf break is drawn from depths on the order of the shelf break (∼200 m) and is, in turn, fed by the upwelling layer on the continental slope. Existing analytic studies and laboratory experiments have shown the occurrence of free vertical shear layers in rotating fluids in the presence of abrupt changes of the azimuthal velocity field. Similarities between the conditions at the shelf edge and these studies suggest that the secondary motions predicted by the model solution may be explained by analogy to Stewartson layer dynamics.