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Eric Firing and Robert C. Beardsley

Abstract

An experimental method for producing an isolated eddy in a laboratory tank is described, along with the simple viscous theory of the behavior of the eddy in an ordinary cylindrical tank without the β-effect. The linear inviscid theory incorporating the β-effect is then developed as an initial value problem, and the solution is found as a summation of normal Rossby wave modes of the basin. This theoretical solution is compared with results from laboratory experiments and with numerical simulations obtained for the “sliced-cylinder” laboratory model. It is found that nonlinear effects lead to a cyclonic circulation in the northern half of the tank and an anticyclonic circulation in the southern half. Two simple models are developed to account for these induced circulations.

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Bradford Butman and Robert C. Beardsley

Abstract

Nearly continuous current measurements at 45 and 75 m were made from May 1975 to March 1979 at 40°51′N, 67°24′W on the southern flank of Georges Bank in water 85 m deep. Measurements at 15 and 84 m were made less often. The mean flow at 45 and 75 m was southwestward at approximately 8.5 and 3.7 em s−1, respectively. At 45 m the monthly along-bank flow ranged from 2 to 17 cm s−1, and the average seasonal change was approximately 6 cm s−1; strongest southwestward flow was in September and weakest flow was in March. Most of this seasonal change was driven by the seasonal change in the cross-bank density field. At 75 m there was no significant seasonal change in the monthly mean along-bank flow. In winter, only about 21 percent of the along-bank flow at 45 m can be explained by tidal rectification, the density field, and wind stress. In contrasts, in late summer almost all of the flow at 45 m can be explained by these three driving mechanisms. The monthly averaged cross-bank flow was very weak, confidence limits were too large to determine any statistically significant vertical shear in the seasonal mean cross-bank flow. The current observations on the southern flank and additional measurements made at other locations around the perimeter of Georges Bank suggest that, although a monthly mean subsurface clockwise circulation around the bank exists throughout the year, the flow was strongest in late summer and early fall and that recirculation around Georges Bank may be most likely in late summer. The flow was weakest and most variable in winter.

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Changsheng Chen and Robert C. Beardsley

Abstract

Tidal rectification over a two-dimensional finite-amplitude symmetrical bank is studied using the Blumberg and Mellor primitive equation coastal ocean circulation model (ECOM-si). In the homogeneous case, the nonlinear interaction of tidal currents with the variable bottom topography generates an along-isobath residual circulation over the bank, which tends to increase as either the slope or height of the bank is increased. In the stratified case, internal waves at tidal and higher frequencies are generated over the sloping sides of the bank. Tidal mixing occurs in the bottom boundary layer, leading to horizontal tidal mixing fronts. The resulting stratified tidal rectification associated with the tidal mixing front, the generation of internal tides, and the modification of internal friction due to stratification leads to a subsurface intensification of the along-isobath residual current at the front and at the top of the bottom mixed layer over the slope, and a cross-bank double cell circulation pattern centered at the front near the shelf break. Model results for tidal mixing are in reasonable agreement with a simple energy argument in which the thickness of the tidal mixed layer is proportional to the magnitude of the tidal current and inversely to stratification.

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Atsuhiko Isobe and Robert C. Beardsley

Abstract

The interannual variation in cold-air outbreak activity over the Japan Sea is investigated using Japan Meteorological Agency buoy 21002 and Quick Scatterometer (QuikSCAT) wind data, Japan Oceanographic Data Center sea surface temperature (SST) data, NCEP–NCAR reanalysis surface wind and sea level pressure (SLP) data, and the winter Arctic Oscillation (AO) index of Thompson and Wallace. Cold-air outbreaks occur during the “winter” November–March period, and wind data for this season for the 19-winter period 1981–2000 were analyzed. Wavelet spectra averaged between 5- and 15-day periods were used to evaluate the intensity of cold-air outbreaks quantitatively. The winter mean wavelet spectra exhibited a clear interannual variation and a significant positive correlation with the AO index, indicating that intensive cold-air outbreaks frequently occur during relatively warm winters caused by a quasi-decadal AO. Based on the SST and SLP data, the low atmospheric surface pressure disturbances tend to develop over the warm East China Sea in warm winters in the positive AO phase. As these low SLP disturbances advance toward the northern Japan islands during the positive AO phase, they intensify more, leading to stronger cold-air outbreaks over the Japan Sea and increased sea surface cooling over the northern Japan Sea.

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David C. Chapman and Robert C. Beardsley

Abstract

Based on a limited set of available oxygen isotope measurements, it is hypothesized that the mean now in the Middle Atlantic Bight is part of a 5000 km-long buoyancy-driven, coastal current which originates along the southern coast of Greenland. This idea is consistent with most features of the known circulation of the region.

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Peter R. Daifuku and Robert C. Beardsley

Abstract

A description is given of the K1 tide over the northeast continental shelf off North America from Nova Scotia to Cape Hatteras. Analyzed pressure data obtained from W. Brown and J. Irish (University of New Hampshire) have been used to draw up the K1 cotidal map and existing current data have been analyzed to give the associated velocity map. Offshore, there is a sweep of the tide from north to south, in general agreement with what is known of the oceanic K1 tide in the North Atlantic. On the shelf, there is a trapping of phase lines to the coast, creating, in particular, a virtual amphidrome south of Cape Cod. Maximum amplitudes of around 15 cm are found in the Gulf of Maine, lowest around 7 cm south of Cape Cod. The K1 currents are generally barotropic and current ellipses are aligned with the local topography. Maximum currents of about 10 cm s−1 are found south of Cape Cod.

A simple model for the K1 pressure field is developed using the free and forced inviscid barotropic waves on a two-dimensional shelf. The theoretical solutions are fitted to the K1 pressure data using a least-squares method. The model results confirm that the K1 tide is composed of both a Kelvin wave and a shelf wave, with the Kelvin wave dominating the pressure field, and the shelf wave dominating the current field. The two free waves account for 99% of the variance of the difference of the observed pressures and the calculated forced wave, but unfortunately some of the observed features are not accurately reproduced. Possible model improvements should include the addition of bottom friction and longshore topographic variations (especially the changes in shelf geometry associated with the Gulf of Maine).

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Changsheng Chen, Hedong Liu, and Robert C. Beardsley

Abstract

An unstructured grid, finite-volume, three-dimensional (3D) primitive equation ocean model has been developed for the study of coastal oceanic and estuarine circulation. The model consists of momentum, continuity, temperature, salinity, and density equations and is closed physically and mathematically using the Mellor and Yamada level-2.5 turbulent closure submodel. The irregular bottom slope is represented using a σ-coordinate transformation, and the horizontal grids comprise unstructured triangular cells. The finite-volume method (FVM) used in this model combines the advantages of a finite-element method (FEM) for geometric flexibility and a finite-difference method (FDM) for simple discrete computation. Currents, temperature, and salinity in the model are computed in the integral form of the equations, which provides a better representation of the conservative laws for mass, momentum, and heat in the coastal region with complex geometry. The model was applied to the Bohai Sea, a semienclosed coastal ocean, and the Satilla River, a Georgia estuary characterized by numerous tidal creeks and inlets. Compared with the results obtained from the finite-difference model (ECOM-si), the new model produces a better simulation of tidal elevations and residual currents, especially around islands and tidal creeks. Given the same initial distribution of temperature in the Bohai Sea, the FVCOM and ECOM-si models show similar distributions of temperature and stratified tidal rectified flow in the interior region away from the coast and islands, but FVCOM appears to provide a better simulation of temperature and currents around the islands, barriers, and inlets with complex topography.

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Wendell S. Brown and Robert C. Beardsley

Abstract

The mean circulation on the northeast continental shelf in the region of the Gulf of Mexico is discussed in terms of a simple box model, based on volume transports and mean salinities estimated from existing data. The results of this calculation indicate that warm salty water from the continental slope must mix with colder, fresher water at intermediate depths within the Gulf. Field measurements obtained as part of a study of the winter circulation in an offshore region in the western Gulf of Maine suggest that winter storms may be responsible for most of this vertical mixing. Ten 1-day hydrographic cruises wore conducted between the passage of seasonal storms from November 1974 to January 1975. A description of the early winter evolution of the density field was thus obtained concurrently with moored measurements of current, temperature and bottom pressure, and coastal measurements of sea level and atmospheric variables. The principal vertical mixing process observed during this period was an intermittent overturning of the near-surface water caused by surface cooling by offshore winds. The observed vertical homogeneity suggests that the fresher near-surface Gulf of Maine water and the more saline deep basin water are frequently mixed during the early winter in the western region to produce Gulf of Maine Intermediate Water.

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Changsheng Chen, Robert C. Beardsley, and Richard Limeburner

Abstract

Tidal rectification over an idealized two-dimensional cross section of Georges Bank, which is a large, shallow, elongated submarine bank in the Gulf of Maine, is studied using a primitive equation coastal ocean circulation model. In the homogeneous case, the model predicts a topographically controlled residual circulation over Georges Bank, flowing northeastward as a strong jet with a maximum speed of about 16 cm s−1 along the northern flank and southwestward as a relatively weak and broad flow with a maximum speed of about 3 cm s−1 on the southern flank. As stratification is added, tidal rectification and tidal mixing intensify the along- and cross-isobath residual currents and create tidal fronts. During summer, the tidal fronts are located at the 40-m isobath on the northern flank and at the 50–60-m isobath on the southern flank, while during winter, the position of the tidal front remains fixed on the northern flank; however, it moves to the shelf break on the southern flank. The summer and winter maxima of the along-bank current are about 32 cm s−1 and 26 cm s−1 on the northern flank and 8 cm s−1 and 6 cm s−1 on the southern flank, respectively. The model results are in reasonable agreement with observations. The summertime intensification of the residual flow is mainly due to nonlinear interaction between the stratified tidal currents over the northern flank with the steep bottom topography there and to the baroclinic density gradient created in part by tidal mixing over the southern flank where the bottom slope is smaller.

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Robert C. Beardsley and Dale B. Haidvogel

Abstract

A numerical model of the wind-driven transient ocean circulation in the Middle Atlantic Bight is described. The model incorporates realistic topography and covers the continental shelf between the coast and the 200 m isobath from Cape Hatteras to the southern tip of Nova Scotia. The traditional shallow-water dynamics are used, i.e., the vertically integrated and linearized equations for the flow of a homogeneous fluid driven by atmospheric pressure and wind stress fluctuations and damped by a quadratic bottom stress. The equations are integrated in time using a simple modification of Platzman's (1972) finite-difference scheme, with a 12.7 km grid spacing. At the coast, normal flow is required to vanish; at non-coastal boundaries, the equivalent surface elevation is held fixed.

Several classes of initial value experiments are used to study the free and forced modes of this model, and the damped flow driven by a spatially uniform and stationary wind stress and by an idealized travelling synoptic-scale wind-stress pattern. The numerical experiments indicate that several time scales are important in the regional adjustment process. These are an inertial time scale dependent on the regional long-wave propagation speed, a local frictional time scale dependent on the strength of the depth-averaged velocity field, and a longer time scale which reflects the adjustment process within the entire model. The transient response within the Middle Atlantic Bight proper from Cape Cod to Cape Hatteras to an alongshore wind stress is clearly dominated by friction and rotation. The effective spinup time scale for a 2 dyn cm−2 wind stress is about 10 h at New York. This is sufficiently short in comparison to the 4–10 day lime scales characterizing atmospheric transients that the storm-driven currents should be quasi-steady. Within the deeper Gulf of Maine basin, the effective spinup time scale is much longer and the normal modes of the basin excited by the wind forcing are only weakly damped in time.

A comparison of model and observational data on current and sea level variability indicates that the model response is more realistic within the Middle Atlantic Bight section of the model domain. Differences within the Gulf of Maine are due primarily to the specific boundary condition imposed on the upcoast (Scotian shelf) boundary.

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