Downwelling Circulation on the Oregon Continental Shelf. Part I: Response to Idealized Forcing

Abstract

Time-dependent downwelling on the Oregon continental shelf is studied with a two-dimensional approximation, that is, spatial variations across shelf and with depth, using the Blumberg-Mellor, finite-difference, stratified, hydrostatic, primitive equation model. The time-dependent response of a coastal ocean at rest to constant, downwelling-favorable wind stress is examined. Topography and stratification representative of the Oregon continental shelf are used for the basic case experiment. The wind stress forces onshore flow in a turbulent surface boundary layer. The compensating flow below the surface layer advects the density field downward and offshore and accelerates an alongshore current in the form of a vertically and horizontally sheared coastal jet. The dominant feature of the response flow field is a downwelling front that moves slowly offshore, leaving behind an inshore region where the density is well mixed. The downwelling front in the density field is concentrated near the bottom, while the front in alongshore velocity extends over the full depth and is nearly vertical, separating weak alongshore velocities inshore from the coastal jet offshore. The front contains strong vertical motion from the surface to the bottom and some recirculation. Much of the offshore flow from the base of the front is characterized by time- and space-dependent fluctuations involving spatially periodic separation and reattachment of the bottom boundary layer and accompanying recirculation cells. This flow has positive potential vorticity and appears to be finite-amplitude slantwise convection resulting from a hydrostatic symmetric instability. Additional experiments show the dependence of the response flow field an the magnitude of the wind stress, the initial stratification, and the shelf topography. Experiments with the vertical turbulent kinematic viscosity and diffusivity parameterized by a different turbulence closure scheme, as a function of a local Rich-ardson number, or as constants show dependence of the response flow field on the choice of turbulence submodel. The occurrence of a well-mixed region inshore and the existence of time- and space-dependent fluctuations associated with slantwise convection in the near-bottom offshore flow appear to be robust features of the two-dimensional downwelling response.