On the Possibilities of Coastal, Mid-Shelf, and Shelf Break Upwelling

Abstract

The steady-state circulation on a continental shelf under the combined influence of a wind stress, a surfacedensity distribution, and a longshore current over the shelf break is investigated in a linear, ?-plane modelthat allows a longshore pressure gradient. The problem is quasi-two-dimensional and lends itself readily toa standard boundary-layer analysis. For the range of parameters considered, the hydrostatic Lineykin layerallows a vertical mass transport into the surface Ekman layer to compensate for the one-sided divergencecreated by the wind stress at the coast and is, therefore, primarily responsible for coastal upwelling. Anequatorward longshore current over the shelf break, on the other hand, contributes to a shelf break upwelling due to the Sverdrup relation. There is, in this case, also a possibility for a poleward undercurrentover the continental shelf. When the equatorward longshore velocity at the shelf break bottom is sufficientlylarge, however, the poleward undercurrent may not exist at all, and the whole shelf water may move equatorward. The resulting onsho're transport in the bottom Ekman layer then causes upward motion in the Stewartson E½ layer, and allows for an appearance of coastal upwelling in the presence of upwelling at the continental shelf break. The interior density anomaly in the model is always diffusive and admits an upwellingcirculation beneath sharp surface contrasts with a shoreward gradient. While such contrasts in surfacedensity anomaly can, and do, occur at mid-shelf points, the intensity of upwelling generally remains thegreatest in the coastal boundary layer.