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  • Author or Editor: Leonard J. Pietrafesa 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
Leonard J. Pietrafesa

Abstract

The subtidal frequency response of sea level to atmospheric forcing along the coastal region between Cape Hatteras and Charleston is investigated for a four-month period: 1 September-31 December, 1974. It is found that low-frequency sea level fluctuations are preferentially forced by wind stress components which are aligned with the local topography. Also, a localized, one-dimensional model of sea surface response to a clockwise rotating wind for the Charleston coastal regime is developed. The phase spectrum of the alongshore wind component versus sea level as predicted by the model is shown to compare favorably to that derived from actual observations at Charleston, an open ocean coastal site. The model results and observations also suggest that wind-induced fluctuations of coastal sea level are trapped within 40 km of the coast by the combined effects of friction, Coriolis force and bottom topography. The outer shelf is dominated by fluctuations which are less related to wind stress and are attenuated rapidly in the shoreward direction. A reasonable estimate of bottom frictional coefficient, r = 0.05 cm s−1, is also established.

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Larry P. Atkinson
and
Leonard J. Pietrafesa

Abstract

Onslow Bay, North Carolina, is repeatedly flushed by intrusions of Gulf Stream water. An exponential dilution model based on intrusion models indicates 20–60 days are required for 50% dilution of Bay waters.

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Lian Xie
,
Xiaoming Liu
, and
Leonard J. Pietrafesa

Abstract

The effect of the isobathic curvature on the development and evolution of Gulf Stream frontal waves (meanders and eddies) in the vicinity of the Charleston Bump (a topographic rise on the upper slope off Charleston, South Carolina; referred to as CB hereinafter) is studied using the Hybrid-Coordinate Ocean Model (HYCOM). Baroclinic and barotropic energy transfers from the Gulf Stream to its meanders and eddies that appear as cold and warm anomalies are computed for four different cases. In case I, the curvature of the isobaths is artificially reduced and the CB is removed from the bathymetry. In this simulation, the simulated Gulf Stream meanders were barely noticeable in the study region. Energy transfer from the Gulf Stream to meanders and eddies was negligible. In case II, the curvature of the isobaths was the same as in case I, but a bump of the scale of the CB was added to the bathymetry. In this simulation, Gulf Stream meanders were amplified while passing over the CB. In case III, the CB was removed from the bathymetry as in case I, but the curvature of the isobaths was similar to the actual bathymetry, which was larger than that of cases I and II. In this simulation, large meanders were simulated, but the development of these meanders was not confined to the region of the CB. The total baroclinic and barotropic energy transfer rate in this case was an order of magnitude greater than in case II, suggesting that isobathic curvature was able to generate Gulf Stream meanders and eddies even without the presence of the CB. In case IV, actual bathymetry data, which contain both the CB and the isobathic curvature, were used. In this case, large-amplitude Gulf Stream meanders were simulated and there was also a tendency for the amplification of the meanders to be anchored downstream of the CB, consistent with observations. The results from this study suggest that the formation of the “Charleston Trough,” a Gulf Stream meander that appears as a low pressure or depressed water surface region downstream of the bump, is the result of the combined effect of the CB and the isobathic curvature in the region. The isobathic curvature plays a major role in enhancing the baroclinic and barotropic energy transfer rates, whereas the bump provided a localized mechanism to maximize the energy transfer rate downstream of the CB.

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Joseph J. Cione
,
Sethu Raman
, and
Leonard J. Pietrafesa

Abstract

Midlatitude cyclones develop off the Carolinas during winters and move north producing gale-force winds, ice, and heavy snow. It is believed that boundary-layer and air-sea interaction processes are very important during the development stages of these East Coast storms. The marine boundary layer (MBL) off the mid- Atlantic coastline is highly baroclinic due to the proximity of the Gulf Stream just offshore.

Typical horizontal distances between the Wilmington coastline and the western edge of the Gulf Stream vary between 90 and 250 km annually, and this distance can deviate by over 30 km within a single week. While similar weekly Gulf Stream position standard deviations also exist at Cape Hatteras, the average annual distance to the Gulf Stream frontal zone is much smaller off Cape Hatteras, normally ranging between 30 and 100 km.

This research investigates the low-level baroclinic conditions present prior to observed storm events. The examination of nine years of data on the Gulf Stream position and East Coast winter storms seems to indicate that the degree of low-level baroclinicity and modification existing prior to a cyclonic event may significantly affect the rate of cyclonic deepening off the mid-Atlantic coastline. Statistical analyses linking the observed surface-pressure decrease with both the Gulf Stream frontal location and the prestorm coastal baroclinic conditions are presented. These results quantitatively indicate that Gulf Stream-induced wintertime baroclinicity may significantly affect the regional intensification of East Coast winter cyclones.

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Kuo-Hsu Su
,
Ping-Tung Shaw
, and
Leonard J. Pietrafesa

Abstract

Distinct offshore and upward phase propagation at periods of 12 and 24 days was previously observed in the horizontal flow field on the outer shelf in the southern Mid-Atlantic Bight during the Shelf Edge Exchange Processes Experiment in spring 1988. A linear, forced-wave model is invoked to explain the observed phase propagation. It is found that the observed phase propagation can exist when currents are in resonance with a vorticity source on the continental slope. It is essential that the thermohaline field be characterized by an upper-ocean stratification an the slope for resonance to occur. The phase difference near resonance is weakly dependent on the bottom frictional parameter and the alongshore length scale of forcing. Resonance is due to onshore propagation of topographic waves. It is suggested that low-frequency oscillations on the outer shelf in the Mid-Atlantic Bight in winter an associated with sources on the upper slope.

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John A. Dutton
,
Leonard J. Pietrafesa
, and
John T. Snow
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Charles R. McClain
,
Norden E. Huang
, and
Leonard J. Pietrafesa

Abstract

The problem of a small-amplitude wave propagating over a flat porous bed is reanalyzed subject to the bottom boundary conditionwhere u represents the horizontal velocity in the fluid,ũ s represents the horizontal velocity within the bed as predicted by Darcey's law, K is the permeability and the subscript 0 denotes evaluation at the bottom (z=0). The term α is a constant whose value depends on the porosity of the bed at the interface and must be determined experimentally. The boundary condition is of the form of a “radiation-type” condition commonly encountered in heat conduction problems.

The important physical quantities (velocity, velocity potential, streamfunction, shear stress and energy dissipation) have been derived and are presented, subject to natural conditions. The bottom boundary layer is represented by the linearized Navier-Stokes equations under the usual boundary layer approximation. It is found that the boundary layer velocity distribution and shear stress can be greatly altered from impermeable bed predictions. Theoretical results for energy dissipation and shear stress are compared to existing data and are found to agree very well. The predictions of classical small-amplitude wave theory are not appreciably modified away from the boundary.

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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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