The spring transition in currents over the Oregon Continental Shelf

Abstract

Current meter moorings maintained over the Oregon continental shelf in 1973 and 1975 clearly show the difference between winter and spring oceanographic regimes and the rapid transition between the regimes. In winter the mean alongshore current is northward at all depths and strongest near shore; there is no mean vertical shear, no mean offshore density gradients, and mean sea level is high. In spring the mean alongshore current is weak near the bottom and strongly southward at the surface with a maximum over the mid‐shelf; the strong vertical shear is balanced by strong offshore density gradients, and mean sea level is low. In both 1973 and 1975 the transition between these regimes occurs within about a week during a strong southward wind event. In many respects the transition event does not differ significantly from previous southward wind events, but the lateral density gradients established during the transition event exceed those of earlier events. The transition appears to be the result of a large cumulative offshore Ekman transport caused by local wind stress rather than by propagation of effects generated elsewhere. The lateral density gradients and the vertical shear established during the transition event subsequently persist, even during northward wind events with moderate onshore Ekman transport.