A Modeling Study of the Three-Dimensional Continental Shelf Circulation off Oregon. Part II: Dynamical Analysis

Abstract

Sixty-day simulations of the subinertial continental shelf circulation off Oregon are performed for a hindcast study of summer 1999. In Part I, the model results are shown to compare favorably with in situ currents and hydrographic measurements obtained from an array of moored instruments and field surveys and high-frequency radar-derived surface currents. In this paper, the modeled three-dimensional, time-varying circulation and dynamical balances are analyzed, providing a detailed synoptic description of the continental shelf circulation off Oregon for summer 1999. The circulation is clearly wind driven and strongly influenced by alongshore variations in shelf topography. In the region of the coast where the alongshore topographic variations are small the upwelling circulation is consistent with standard conceptual models for two-dimensional across-shore circulation. In the regions where the alongshore topographic variations are greater, the upwelling circulation is highly threedimensional. Over Heceta Bank the upwelling circulation is complicated, with weaker direct coupling to the wind forcing over most of the shelf. It is demonstrated that the upwelled water that is found over the midshelf off Newport is upwelled to the north and is advected to the south. Additionally, upwelled water over Heceta Bank is drawn from a different location to the south. The dynamical balances over the inner shelf are divided into two regimes; in the coastal jet and inshore of the coastal jet. In the coastal jet the tendency of the alongshore depth-averaged velocity V t is large and is driven by the difference between the surface and bottom stresses during upwelling. Inshore of the coastal jet, V t is small and is driven by the difference between the surface stress and a negative alongshore pressure gradient during upwelling. When the wind stress becomes small after upwelling, V t is primarily balanced by the negative alongshore pressure gradient and a northward flow is generated. Subsequently, the alongshore pressure gradient decreases, the flow becomes geostrophic, and the northward flow persists until the next significant event. A region to the south of Newport over the inner shelf is identified as the region where the northward momentum is initially generated.