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Xiaohui Xie, Ming Li, and William C. Boicourt

Abstract

The 2-month-long mooring data were collected in a straight midsection of Chesapeake Bay to document the lateral circulation driven by along-channel winds. Under upestuary winds, the lateral circulation featured a clockwise (looking into estuary) circulation in the surface layer, with lateral Ekman forcing as the dominant generation mechanism. Under downestuary winds, however, the lateral circulation displayed a structure dependent on the Wedderburn number W: a counterclockwise circulation at small W and two counterrotating vortices at large W. The surface lateral velocity was phase locked to the along-channel wind speed. Analysis of the streamwise vorticity equation showed that the strength and structure of the lateral circulation in this stratified estuary were largely determined by the competition between the tilting of planetary vorticity by along-channel currents and lateral baroclinic forcing due to sloping isopycnals. Under strong, downestuary winds, the lateral baroclinic forcing offset or reversed the tilting of planetary vorticity on the western half of the estuarine channel, resulting in two counterrotating lateral circulation cells. A bottom lateral flow was observed in the deep channel and appeared to be generated by lateral Ekman forcing on the along-channel currents.

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Shenn-Yu Chao and William C. Boicourt

Abstract

The onset of estuarine plumes is numerically studied here, using a three-dimensional, primitive-equation model. The model ocean consists of a narrow estuary that is connected to an otherwise enclosed ocean basin. The basin is initially filled with saline water. Subsequently, freshwater is pumped in near the surface and the saline water is withdrawn from below at the head of the estuary. To maximize the chance of development for a baroclinic flow field, a rigid-lid and a flat bottom are assumed, and the inflow–outflow profile has no barotropic component.

The plume expands in the direction of propagation of the coastally trapped waves after the freshwater release. The intrusion speed inside the estuary is consistently higher than that along the shelf. Energy is therefore accumulated near the estuary mouth, forming a bulge of anticyclonic surface flow. The far-field flow consists of a bore intrusion along the shelf. The transitional zone between the near-field and far-field flows is characterized by strong cyclonic surface flow and also strong downwelling. For reasonable amounts of vertical mixing and bottom drag, two-layer opposite flows are confined inside the bulge and the far-field bore intrusion is unidirectional. In the limit of small vertical mixing and vanishing bottom drag, the difference in intrusion speeds in and out of the estuary is reduced. The seaward expansion of the bulge decreases, and the undercurrent leaks out of the bulge and propagates behind the nose of the bore. The three-dimensional structures of the density-driven estuarine circulation and also of the bore intrusion along the shelf have been identified.

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Xiaohui Xie, Ming Li, Malcolm Scully, and William C. Boicourt

Abstract

Internal solitary waves are commonly observed in the coastal ocean where they are known to contribute to mass transport and turbulent mixing. While these waves are often generated by cross-isobath barotropic tidal currents, novel observations are presented suggesting that internal solitary waves result from along-isobath tidal flows over channel-shoal bathymetry. Mooring and ship-based velocity, temperature, and salinity data were collected over a cross-channel section in a stratified estuary. The data show that Ekman forcing on along-channel tidal currents drives lateral circulation, which interacts with the stratified water over the deep channel to generate a supercritical mode-2 internal lee wave. This lee wave propagates onto the shallow shoal and evolves into a group of internal solitary waves of elevation due to nonlinear steepening. These observations highlight the potential importance of three-dimensionality on the conversion of tidal flow to internal waves in the rotating ocean.

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Wen-Ssn Chuang, Dong-Ping Wang, and William C. Boicourt

Abstract

Low-frequency current variability on the continental shelf, 84 km off the mouth of the Chesapeake Bay, was examined from 4-month (mid-March to June 1975) current, sea level and meteorological records. Taking into account the seasonal change in wind stress and stratification, the record was divided into two 60-day periods. In both periods, the transient alongshore currents wore barotropic and coherent with sea level fluctuations.

During the first period (March and April 1975), winds were in the east–west direction, and the shelf water was homogeneous. At time scales longer than 4 days, sea level was a large-scale feature (coherent over the entire Mid-Atlantic Bight). At shorter time scales, sea level was driven by the local, alongshore wind. In contrast, the cross-shelf current, which was mainly barotropic, was driven by the alongshore wind at all time scales.

During the second period (May and June 1975), winds were in the north-south direction and the shelf water was stratified. Sea level was mainly driven by the local alongshore wind at all time scales. The cross-shelf current, which was baroclinic at time scales longer than 4 days, and barotropic at shorter time scales, was also driven by the alongshore wind.

The difference in response characteristics of the two periods indicate that circulation on the southern Mid-Atlantic Bight is strongly affected by local wind forcing, nonlocal effect, density stratification and the duration of alongshore wind.

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Hsien Wang Ou, Robert C. Beardsley, Dennis Mayer, William C. Boicourt, and Bradford Butman

Abstract

Subtidal current fluctuations in the Middle Atlantic Bight are examined from current-meter data collected in 1975 and 1976. Spectral analysis provides evidence for both locally wind-forced response and free waves that propagate downshelf3 which are not correlated with the local wind. A simple empirical model has been constructed to fit two linearly independent plane waves to the observed current spectra. Application of the model to the current data obtained at a pair of stations in the New York Bight during the period of 26 October 1975 to 4 April 1976 indicates that the two waves propagate in opposite directions along the coast, and with the additional evidence from rotary-coefficient calculations, it is suggested that they correspond to the forced and free waves speculated upon earlier. The noise level is a free parameter in the model and is determined by adjusting the phase speed of the forced wave to the translation speed of the observed wind field. This gives a 526 km day−1 phase speed for the free wave, and the forced wave, free wave and noise compose ∼41, 39 and 20% of the total variance.

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David M. Goodrich, William C. Boicourt, Peter Hamilton, and Donald W. Pritchard

Abstract

Multiyear continuous observations of velocity and salinity in the Chesapeake Bay indicate that wind-induced destratification occurs frequently from early fall through midspring over large areas of the estuary. Storm-driven breakdown of summer stratification was observed to occur near the autumnal equinox in two separate years. Surface cooling plays an important, though secondary, role in the fall destratification by reducing the vertical temperature gradient in the days prior to the mixing event. Large internal velocity shear precedes mixing events, suggesting a mechanism involving the generation of dynamic instability across the pycnocline. Destratification is shown to fundamentally alter the response of the velocity field to subsequent wind forcing; in stratified conditions, response is depth-dependent, while after mixing a depth-independent response is observed.

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Malcolm E. Scully, Alexander W. Fisher, Steven E. Suttles, Lawrence P. Sanford, and William C. Boicourt

Abstract

Measurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (<0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased anisotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness.

Conditions that favor convection usually also have strong Langmuir forcing, and these two processes probably both contribute to the surface mixed layer turbulence. Conditions in which traditional stress-driven turbulence is important are limited in this dataset. Unlike other shallow coastal systems where full water column Langmuir circulation has been observed, the salinity stratification in Chesapeake Bay is nearly always strong enough to prevent full-depth circulation from developing.

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