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Jackson O. Blanton

Abstract

River and estuarine discharges mix to form a frontal zone along a 400 km length of the coast of the south-eastern United States. The frontal width extends from the coast outward 10–20 km. Two ships, anchored across the frontal zone during autumn, obtained simultaneous hourly profiles of currents and density for, five consecutive tidal cycles. The frontal zone contained a baroclinic coastal current flowing southward. The flow was strongly convergent; more water entered on the coastal side than exited on the seaward side. Much of the inflow was apparently turned southward. Thus, the frontal zone acted as a dynamic barrier that inhibited the advection of mixed river discharge farther offshore.

A thermal wind relationship suitably predicted vertical shear through the frontal zone. Bottom friction seemed to play only a secondary role. Data suggest that the baroclinic coastal current was modified by the presence of a barotropic current seaward of the frontal zone. The force responsible for the barotropic current must be an alongshore pressure gradient acting northward due either to wind set-up against the Florida coast or to the fall in steric sea level along the western edge of the Gulf Stream.

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Franklin B. Schwing and Jackson O. Blanton

Abstract

The use of land based wind data in nearshore oceanographic work is common, but these winds do not accurately reflect coastal oceanic winds. Ocean winds are often underestimated by a factor of 2 and directional differences are also observed. Wind time series from land and sea regimes in the South Atlantic Bight (SAB) were applied to a reduced form of the momentum equation to estimate the alongshore current. Currents were closely approximated by ocean wind stress, but were consistently underestimated by land data. Further statistical analyses verified this discrepancy in speed and also indicated significant differences between ocean and speed-adjusted land winds. The bottom frictional coefficient required to balance alongshore momentum was unrealistically small when land based wind data were used as input.

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Franklin B. Schwing, Lie-Yauw Oey, and Jackson O. Blanton

Abstract

Because of relatively strong tidal currants and little notification, the response of continental shelf water to wired off South Carolina is predominantly frictional and barotropic. Currents measured at the 10, 30 and 40 m isobaths were highly coherent with wind and coastal sea level (CSL). Alongshelf current at 10 m responded rapidly to wind oscillations and led CSL by 6–12 hours at periods greater than 2 days. Coastal sea level led, or was in phase with, alongshelf currents at the 30 and 40 m isobaths.

A linear frictional barotropic model that assumes alongshelf homogeneity is used to explain the observed phase relationships. Cross-shelf variation in bottom friction due to the change in water depth and a fluctuating alongshelf pressure gradient at the shelf break which lags slightly behind the wind account for most of the observed features. Nearshore flow is dominated by frictional forces while inertial terms are important on the outer shelf. The boundary separating these two regions is the point at which alongshelf current is in phase with CSL.

Model results are summarized in a response diagram defining five nondimensional quantities: CSL amplitude and phase, width of the nearshore frictional strip, bottom friction coefficient, and alongshelf pressure gradient amplitude at the shelf break. Observations from South Carolina and other broad shelf regions are correlated in the diagram. Observations and model results provide a clear understanding of the frictionally controlled, wind-driven barotropic dynamics in shallow continental shelf regions.

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Lie-Yauw Oey, Larry P. Atkinson, and Jackson O. Blanton

Abstract

In winter, cooling of the South Atlantic Bight continental shelf water results in higher density in the middle shelf region relative to the shelf-break region where the western flank of the Gulf Stream flows. Shoreward, estuarine-like intrusion of the upper Gulf Stream water in the presence of such a positive onshore density gradient is then possible through advective processes triggered either by the meander of the Stream or onshore Ekman transports by southward wind stresses. Repeated cross-shelf hydrographic transects were conducted from 10 January through 30 January 1986 to more closely study this intrusion process. These observations show many features predicted by a previous numerical model study. A semi-empirical model is proposed here wherein the state of stratification of water on the outer continental shelf region just inshore of the shelf break is used as an indicator of the intrusion process. Model analysis suggests correlating the observed time rate of change of potential energy of the water column with wind-induced cross-shelf Ekman transport. The correlation fit is good for at least half of the dataset, suggesting that wind-induced intrusion was significant during the observations. The analysis also suggests that it is possible to distinguish intrusion processes which are wind induced from those which are induced by Gulf Stream meanders.

Both observations and the previous numerical model study show transient shelf-break upwelling following a southward wind impulse. A simplified model suggests that the upwelling is a result of a cyclonic vortex, bounded at the shelf break, produced by interaction of wind stress and sloping bottom topography. Transient upwellings introduce Gulf Stream water from below the mixed layer to the sea surface, where it is transported onshore to the continental shelf by intrusion processes. This provides a mechanism by which nutrient-rich, deeper Gulf Stream water can replenish the water mass of the adjacent continental shelf.

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Pijush K. Kundu, Jackson O. Blanton, and Mona M. Janopaul

Abstract

An analysis is made of HF radar measurements of surface currents in shallow water near the Georgia coast, and also of vertical profile measurements of current and density. The dominant structure is found to be a clockwise ellipse of semidiurnal periodicity, the ellipses becoming smaller, narrower and turning clockwise with depth. There is a definite phase lead of the bottom currents with respect to the upper currents, and some evidence of veering of the currents in the non-Ekman sense. In order to explain the observed vertical variations, a simple analytical expression is developed for periodic rotary currents in a barotropic ocean of constant eddy viscosity and depth h, when the free-surface elliptic motion is known. The solution depends on the ratios ω/f and h/h Ekman, and also on the sense of turning of the free surface ellipse. The model is able to explain several features of the observed vertical variations as frictional effects.

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Dana K. Savidge, Jackson O. Blanton, Thomas N. Lee, and Robert H. Evans

Abstract

A 4.3-month-long dataset from moorings on the continental shelf off South Carolina during 1986 showed unusual midrecord shifts in bottom pressure, temperature, stratification, and alongshelf currents. The Gulf Stream moved farther offshore during the second half of the time period. This offshore position appears to have facilitated Gulf Stream influence on shelf waters all across the continental shelf.

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