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  • Author or Editor: Gerald S. Janowitz x
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Leonard J. Pietrafesa
and
Gerald S. Janowitz

Abstract

The effects of surface buoyancy flux. atmospheric wind stress and bottom topography on the horizontal and vertical structure of the density and alongshore velocity fields over a continental shelf are investigated within the context of a two-dimensional steady-state model. Using an iterative procedure, similarity solutions are obtained which include the important nonlinear advective effects in the density diffusion equation. In the absence of a wind stress, a reasonable value for the surface buoyancy flux produces alongshore velocities on the order of 20 cm s−1 and an upwelling-like vertical plane circulation. The depth variation across the shelf significantly affects the vertical structure of the density and velocity fields. The introduction of upwelling favorable winds decreases the horizontal density gradient and its associated baroclinic current. A simple physical explanation for this effect, based on heat conservation, is presented.

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Shenn-Yu Chao
and
Gerald S. Janowitz

Abstract

A model for the effect of a localized topographic irregularity on a barotropic sheared current flowing along a continental margin with shallow water to the left of the current is developed. The topographic irregularity is assumed to be small and smooth compared to the background water depth and the background bottom slope, respectively. It is shown that the amplitude of the disturbance depends on the volume of the irregularity and its location on the margin. For a certain class of velocity and topographic profiles a closed form solution is obtained. The results show that the current is deflected seaward down- stream of the disturbance with the maximum deflection occurring one-fourth of a wavelength downstream of the irregularity. Closed eddies are formed in shallow water and sometimes in deep water. If the ratio of relative shear to the speed of the approaching current is large at the continental margin, a simple analytical solution is applicable. The model is applied to the Gulf Stream flowing off the Carolina Coast in the region north of Charleston, South Carolina, and the results of Gulf Stream deflection and wavelengths of the leewaves are in modest agreement with observations.

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John M. Klinck
,
Leonard J. Pietrafesa
, and
Gerald S. Janowitz

Abstract

A linear, two-dimensional model of a rotating, stratified fluid is constructed to investigate the circulation induced by a moving, localized line of surface stress. This model is used to analyze the effect of moving cold fronts on continental shelf circulation.

The nature of the induced circulation depends on the relative magnitude of the translation speed of the storm and the natural internal wave speed. If the surface stress moves slower than the internal wave speed. the disturbance is quasi-geostrophic and moves with the storm. If the storm moves faster than the internal wave speed, two sets of internal-inertial waves are produced. One set of waves is forced by the surface forcing and travels at the speed of the storm. Another set of waves is produced by reflection of the directly forced waves from the coastal wall.

We conclude that free surface deflection (slope) is responsible for the low-frequency. quasi-geostrophic currents due to passing cold fronts. The internal response is composed of tree inertia waves which radiate away from the coast, leaving no residual circulation.

Model results are compared to current meter data collected during the passage of a cold front over the South Atlantic Bight on 9 January 1978. The inertia frequency response observed at the mooring is reproduced by the model calculation.

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Shenn-Yu Chao
,
Leonard J. Pietrafesa
, and
Gerald S. Janowitz

Abstract

A model for the scattering of a continental shelf wave by a small, isolated and smooth topographic irregularity is developed. It is found that a wave of frequency ω, incident on a bump of a sufficiently small horizontal extent such that the solution for a delta-function bump will apply, will trigger all other allowable modes of the same frequency with the highest modes having the largest amplitudes. Further, the higher the mode of the incoming wave, the more strongly will it be scattered. Thus, for a continuous spectrum of continental shelf waves propagating over complicated and extended topography, one would expect a net cascading process toward the higher wavenumber end of the spectrum due solely to the effects of topography. It is noted, however, that if the solution is integrated over a bump of large horizontal extent, the behavior of the forward-scattered and backscattered waves could be entirely different from that of a delta function bump.

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