Separation of a coastal upwelling jet at Cape Blanco, Oregon, USA

Abstract

The coastal upwelling region near Cape Blanco (43°N), Oregon, off the west coast of the United States, was studied using a towed conductivity-temperature-depth instrument on SeaSoar, a shipborne Acoustic Doppler Current Profiler, satellite sea surface temperature maps and satellite-tracked surface drifters during three cruises: August 1994, May and August 1995. Results demonstrate that the baroclinic coastal upwelling jet (and associated front), which was over the shelf poleward of Cape Blanco in all three cruises, separates from the continental shelf, providing an important mechanism for transporting material across the continental margin to the deep ocean. This flow-topography interaction mechanism is a universal phenomenon, and is likely to be important in other eastern boundary current regions of the world. The observations from the two August cruises show two different phenomena. In 1994, cyclogenesis was observed, during which the coastal jet was connected with a cyclonic eddy offshore before the connection was severed and the jet again flowed continuously around Cape Blanco but shifted eastwards. In 1995, the coastal jet meandered in the vicinity of Cape Blanco and then continued equatorward as an oceanic jet in deep water, but it was stronger and displaced farther seawards than the previous year. Drifters released early in the upwelling season (May 1995), when the strengthening longshore upwelling jet was only minimally perturbed by the Cape, were transported rapidly equatorwards and were swept through a large portion of the eastern boundary current region. Drifters released later in the upwelling season (August) were initially swept offshore and equatorwards near the Cape, but after interaction with the spatially complex mesoscale circulation, eventually returned to the continental margin with the seasonal reversal in winds and near-surface currents. These differing flow trajectories are likely to have a significant impact on the biology of eastern boundary currents.