A Numerical Modeling Study of the Upwelling Source Waters along the Oregon Coast during 2005

Abstract

Regional ocean circulation along the Oregon coast is studied numerically for forcing fields derived from year 2005 and climatological-mean conditions. The primary object is to study directly the Lagrangian pathways by which fluid arrives in the Oregon upwelling zone. Roughly half of the upwelling fluid is found to arrive in the regional domain from alongshelf source points, primarily north but also south of the Oregon upwelling zone, while the other half ultimately arrives from points offshore and west of the zone. For both the year 2005 and the climatological simulations, different regimes of dominant alongshelf source waters are found, with Cape Blanco being a dividing point for northern versus southern sources, and with the water parcels coming primarily from depths below 100 m. For the offshore sources, most upwelling fluid originates from depths between 150 and 250 m during 2005, but from within the upper 150 m for the climatological simulations. In both cases there are specific regions along the shelf of enhanced vertical motion, which appear to be associated with topographic features such as submarine banks and canyons. A perhaps surprising result is the apparently small role played by the poleward undercurrent as a direct, immediate source of upwelling fluid; on the seasonal time scale considered here, most trajectories are found to move southward over the slope and shelf, with weaker northward motion only farther offshore, in the deep interior. In 2005 the water parcels cover distances longer than in the climatology, and the meridional exchange along their paths is more vigorous. These differences are likely associated in part with the presence of mesoscale vortical structures seaward of the shelf, which are largely absent in the climatological simulation, perhaps because their formation may depend on the large-amplitude, short-time-scale wind variations and reversals that occur in the year 2005 wind field but not in the multiyear mean climatological wind field.