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

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  • 1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon
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Abstract

Time-dependent upwelling 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, primitive equation model. The time-dependent response of a coastal ocean at rest to constant, upwelling 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 offshore flow in a turbulent surface boundary layer. The compensating onshore flow below the surface layer accelerates an alongshore current in the form of a vertically and horizontally sheared coastal jet. Dense water is advected onshore and upward into the surface layer. An upwelling front characterized by relatively large horizontal gradients in density and alongshore velocity v is formed near the surface at the inshore edge of the coastal jet. Large vertical gradients in v and large values of turbulent kinetic energy are also found in the frontal region. Additional experiments show the dependence of the response flow field on the initial stratification and the shelf topography. Experiments with the vertical turbulent kinematic viscosity and diffusivity parameterized as constants or as functions of a local Richardson number show substantial dependence of the response flow field on the choice of turbulence submodel.

Abstract

Time-dependent upwelling 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, primitive equation model. The time-dependent response of a coastal ocean at rest to constant, upwelling 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 offshore flow in a turbulent surface boundary layer. The compensating onshore flow below the surface layer accelerates an alongshore current in the form of a vertically and horizontally sheared coastal jet. Dense water is advected onshore and upward into the surface layer. An upwelling front characterized by relatively large horizontal gradients in density and alongshore velocity v is formed near the surface at the inshore edge of the coastal jet. Large vertical gradients in v and large values of turbulent kinetic energy are also found in the frontal region. Additional experiments show the dependence of the response flow field on the initial stratification and the shelf topography. Experiments with the vertical turbulent kinematic viscosity and diffusivity parameterized as constants or as functions of a local Richardson number show substantial dependence of the response flow field on the choice of turbulence submodel.

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