A separating coastal upwelling jet at Cape Blanco, Oregon and its connection to the California Current System

Abstract

The coastal upwelling region near Cape Blanco, Oregon (43°N) off the west coast of the United States was studied using a towed CTD on SeaSoar, a shipboard acoustic Doppler current profiler (ADCP), satellite sea surface temperature maps and surface drifters during August 1995. The equatorward upwelling jet was inshore of the shelfbreak north of Cape Blanco, meandered gently offshore around the Cape, and then veered sharply offshore just to the south of the Cape. Analysis of vertical sections of density, velocity and “spiciness” confirmed the separation of an upwelling jet and front; the jet was followed from over the shelfbreak north of the Cape to 100 km offshore downstream of the Cape. The separating jet originates from the coastal upwelling jet leaving the shelf, but is augmented by an offshore branching of the poleward undercurrent. By combining information from ADCP streamfunction maps with analysis of hydrographic data, including distributions of the tracer-like quantity “spiciness”, a conceptual model of the three-dimensional circulation near the Cape emerges. A mid-shelf upwelling jet encounters a coastal promontory then turns offshore, stretching and deepening as it crosses the continental margin. The jet then turns back shoreward (cyclonically) where the deepened equatorward flow encounters the top of the poleward undercurrent flowing along the continental slope. This causes a portion of the undercurrent to turn offshore to join and strengthen the equatorward transport in the separated jet. Throughout the separation process the coastal upwelling front and jet are continuous, robust features. Separation of a coastal upwelling jet is an important mechanism for cross-shelf transport from the coast to the deep ocean. Cape Blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current, characteristic of the California Current System, is found offshore. Drifters released late in the upwelling season are swept offshore and equatorward near the Cape, interact with the spatially complex circulation associated with the separated jet, and then return to the continental margin with the seasonal reversal in winds and near-surface currents.