Modeling upwelling circulation off the Oregon coast

Time-dependent, three-dimensional, upwelling circulation on the continental shelf off the Oregon coast is studied using a primitive equation numerical model. A limited area domain with a high-resolution curvilinear grid is utilized. The response of the coastal ocean to forcing by observed wind stress and heat flux during the summer 2001 time period of the Coastal Ocean Advances in Shelf Transport (COAST) field experiment is investigated. Model results are compared to COAST current and hydrographic measurements. The shelf velocity and density fields are generally characterized by the presence of a southward alongshore coastal jet with an upwelling density front on the shoreward side of the jet. The large variability in shelf topography associated with Heceta Bank exerts a major influence on the shelf velocity and density fields. Over the bank the alongshore coastal jet is displaced offshore, and colder upwelled water extends farther from the coast. Northward mean flow and upward motion are found inshore of the jet. Three-dimensional flow structures in response to variable shelf bottom topography are presented, and an analysis of time- and space-dependent alongshore momentum balances is applied to clarify the associated dynamics. In general, northward pressure gradients, set up over the bank during southward upwelling winds, accelerate currents on the inshore side of the jet northward when the winds relax. Analysis of term balances in the depth-averaged equation for potential temperature shows that during upwelling, across-shore advection makes the major contribution to cooling over most of the region, except inshore over Heceta Bank, where alongshore advection also plays a significant role.