David A. Brooks

The Wake of Hurricane Allen in the Western Gulf of Mexico

Abstract In August 1980, Hurricane Allen passed over a moored array of instruments recording current, temperature and conductivity in the western Gulf of Mexico. An alongshore surge occurred during the storm passage, with the horizontal current speed reaching 91 cm s−1 in the thermocline (200 m) and diminishing to 15 cm s−1 32 m above the bottom (732 m). A wake of near-inertial frequency internal waves commenced after the storm passed the array. The alongshore current oscillation reached a maximum range of 50 cm s−1 within 3 days and decayed thereafter with a time scale of about 5 days. The current oscillations were clockwise-polarized and slightly elliptical, with a period of 22–23 h or about 85% of the local inertial period. Near-uniform upwelling of ±20 m occurred in the thermocline region (200–300 m) during the most intense part of the wake. Depth-leading phases of the horizontal current and temperature oscillations indicated downward radiation of wake energy. The wake oscillations were highly coheren...

Gulf Stream Fluctuations and Meanders over the Onslow Bay Upper Continental Slope

Abstract Gulf Stream fluctuations observed over the 200 and 400 m isobaths off Onslow Bay, North Carolina have a prominent weekly time scale. The principal fluctuations observed during the 4-month winter experiment are consistent with Websters (1961a) description of downstream propagating, skewed, lateral meanders of the Gulf Stream over the upper continental slope. The subtidal velocity fluctuations were highly coherent over the vertical extent (∼120 m) and over the horizontal extent (64 km) of our array. The implied downstream propagation speed was ∼30 km day−1 for the weekly period meanders. Concurrent satellite images of a sea surface temperature (SST) meander pattern indicate that subsurface temperature, salinity, velocity and relative-vorticity maxima occurred as meander crests (shoreward SST-front excursions) passed over the experiment site. The meandering currents were not coherent with nearby wind stress or coastal sea level fluctuations. Eddy-flux estimates indicate energy conversion from the f...

A Model Study of the Buoyancy-Driven Circulation in the Gulf of Maine

Abstract A quasi-linear prognostic numerical model has been used to study the three-dimensional baroclinic circulation in the Gulf of Maine region, with a focus on the buoyancy-driven circulation in the inner basins and near the coast where the influence of relatively fresh water from rivers and the Scotian shelf is important. The model uses a horizontal grid with 6-km resolution and a terrain-following vertical coordinate with ten levels to represent the bathymetry and stratification. Several model experiments explore the influences of fluxes from the principal rivers, tidal mixing, and shelf water inflow, beginning with a case with no initial horizontal density gradients and ending with cases initialized by temperature and salinity data from cruises in June of 1982 and 1983, augmented by climatological data. With composite initialization, the model develops a circulation containing realistic elements, including an anticlockwise gyre in the interior of the gulf and a prominent southwestward coastal curre...

Particle transport through a narrow tidal inlet due to tidal forcing and implications for larval transport

For estuarine-dependent species, especially those that spawn offshore and whose larvae must reach estuarine nursery areas, advective transport through tidal inlets may be a major factor influencing recruitment variability. We examined the role of tidal forcing on particle transport through a narrow, microtidal inlet along the Texas coast by using a three-dimensional hydrodynamic and particle transport model. Although tidal forcing is relatively small in the study area, tidal currents through the inlet effectively transport passive particles a distance of about 15 km landward of the inlet. The majority of the particles that enter the inlet are transported to regions that are not suitable for larval settlement. There is limited tidal dispersion of the particles into the bays due to shoreline geometry and bathymetry. Most of the particles that enter the inlet are expelled offshore in the ebb tidal jet resulting in estuarine-shelf exchange of particles. When acting alone, tidal forcing is not effective at retaining particles in a suitable estuarine habitat, suggesting that other physical or biological mechanisms are required to maintain larvae in an estuarine habitat or that there is substantial along-shelf transport of larvae.

Modeling Tidal Circulation and Exchange in Cobscook Bay, Maine

Abstract Cobscook and Passamaquoddy Bays and their connecting passages lie at the entrance to the Bay of Fundy, on the eastern boundary between the United States and Canada where the mean tidal range is about 6 m. Vigorous tidal currents maintain cold temperatures and efficient exchange with offshore waters year-round. Over the last several decades, a net-pen salmon aquaculture industry has developed in both bays. Recent outbreaks of fish diseases have led to heightened concerns about tidal coupling between net-pen sites and potential pathways for disease transmission. This paper summarizes a Cobscook Bay circulation study by Brooks et al. (1999) and presents some new results to improve understanding of the tidal circulation and potential exchange pathways linking the bays. A dipole pair of back-eddies forms in the central part of Cobscook Bay on each flood and plays an important role in the dispersion and retention of particles. Direct and indirect observations support the existence of the eddy pair and some associated flow details. Model flushing times are a day or two in the Outer Bay where most aquaculture lease sites are located but a week or longer in the inner arms of the Bay. The inner part of South Bay appears to be a repository for particulate matter. Model experiments including both bays and their connecting passages suggest that most water entering Cobscook Bay comes from Head Harbor Passage adjacent to Campobello Island, illustrating the importance of well-coordinated international plans for ecosystem management.

Free, Stable Continental Shelf Waves in a Sheared, Barotropic Boundary Current

Abstract The dispersion characteristics of stable, discrete, barotropic, continental shelf wave (CSW) modes propagating in a barotropic boundary current are strongly modified by the dynamical effects of nonuniform horizontal shear. For example, the CSWs propagate cum sole with no mean current, but their direction of propagation can be reversed by an opposing uniform mean current. In contrast, an opposing sheared mean current increases the tendency for cum sole propagation relative to an opposing uniform mean current, and produces a high-wavenumber cutoff, at least for modes higher than the first. If the sheared mean flow vanishes somewhere, the discrete CSW modes all propagate cum sole once again. For the mean current profiles considered, the high-frequency cutoff is lowered in the nonuniform shear case compared to the zero current case. In a simple geometry motivated by the Florida Current and Florida Straits, southward CSW propagation can occur, in opposition to the Current, primarily because the cyclo...

Subtidal Sea Level Fluctuations and Their Relation to Atmospheric Forcing along the North Carolina Coast

Abstract Coastal sea level fluctuations in Onslow Bay are selectively coupled with local atmospheric forcing variables. The coupling is strongest in period bands of 2.5–3.5 and ≳10 days, which span absolute zero group speed, barotropic continental shelf wave periods. The phase of the sea level disturbances propagated upstream from Beaufort to Wilmington, N. C., as expected for stable discrete shelf waves, consistent with earlier results by Mysak and Hamon (1969). The “barometric function”, when corrected for coherent wind stresses, indicated selective, super barometric atmospheric-pressure-to-sea-level coupling in the zero group speed period bands. Stochastic models of atmospheric cold front wind stress and wind stress curl fields were found to selectively force barotropic shelf wave responses near zero group speed periods and wavelengths. A strong Onslow Bay response to the model cold front occurred near the second harmonic forcing frequency, the wind stress curl contributed importantly to the shape and ...

Tides and tidal power in Passamaquoddy Bay: a numerical simulation

Abstract A three-dimensional sigma-coordinate numerical model was used to simulate the circulation in Passamaquoddy Bay, located in the eastern Gulf of Maine at the mouth of the Bay of Fundy where the mean tidal range is 5.6 m. The model was forced by tidal height variations at the oceanic boundary, fresh water runoff from rivers, and parameterized fluxes of heat and momentum at the sea surface and sea bottom. Simulations were run for cases approximating the natural system, and also for cases modified to include the gates, dams and powerhouse structures of a tidal power project that was previously under consideration for the region. In the natural system, the results show strong tidal currents (∼2m s −1 ) in the channels and passages, with weaker currents (∼0.2m s −1 ) and essentially zero residual flow in the bay, in good agreement with available observations. In the modified system, the volume of each tidal flood is reduced, but a significant tidal-residual flow passes from Passamaquoddy Bay through the powerhouse into the adjacent Cobscook Bay. During a severe and sudden winter cold event, the extent of surface freezing in the region is increased when the tidal-power structures are present, but this occurs mostly because of reduced heat flux from the bottom rather than from an increase of surface stratification in the presence of reduced vertical tidal mixing. When the cold event follows a long period of near-freezing conditions, so that the heat contributions from the bottom and from rivers are smaller, the tidal structures have a moderating influence in Passamaquoddy Bay and the St Croix river estuary, where surface freezing is slightly reduced compared to the unmodified system because the persistent tidal-residual flow brings warmer ocean water into the bay. However, the areal extent of freezing in the model is much more sensitive to surface wind speed than to the presence of the tidal structures, which suggests that some of the environmental impacts of the tidal-power project might be less severe than previously thought.

Coupling of the Middle and South Atlantic Bights by Forced Sea Level Oscillations

Abstract The eastern United States continental margin is naturally divided by Cape Hatteras into the Middle and South Atlantic Bights. The margin depth profile is relatively uniform throughout the Middle Atlantic Bight, but in the South Atlantic Bight it bifurcates into inner and outer slope regions. Coastal tide gage records indicate that long period (1–2 weeks) sea level oscillations propagate southward as continental shelf waves in both Bights, thereby providing a coupling mechanism between the Bights. However, short-period (several days) oscillations appear to be confined to the south Atlantic Bight, and may result from backscattering of long-wave energy by the variable topography and the Gulf Stream. The coastal sea level short-period phase data are not easily attributable to monochromatic propagating waves; rather, it appears that wave group properties may lead to a more consistent explanation of the phases. Cross-shelf and longshelf wind stress components were both strongly coupled to sea level flu...

Asymmetric oceanic response to a hurricane: Deep water observations during Hurricane Isaac

The eye of Hurricane Isaac passed through the center of an array of six deep water water-column current meter moorings deployed in the northern Gulf of Mexico. The trajectory of the hurricane provided for a unique opportunity to quantify differences in the full water-column oceanic response to a hurricane to the left and right of the hurricane trajectory. Prior to the storm passage, relative vorticity on the right side of the hurricane was strongly negative, while on the left, relative vorticity was positive. This resulted in an asymmetry in the near-inertial frequencies oceanic response at depth and horizontally. A shift in the response to a slightly larger inertial frequencies ∼1.11f was observed and verified by theory. Additionally, the storm passage coincided with an asymmetric change in relative vorticity in the upper 1000 m, which persisted for ∼15 inertial periods. Vertical propagation of inertial energy was estimated at 29 m/d, while horizontal propagation at this frequency was approximately 5.7 km/d. Wavelet analysis showed two distinct subinertial responses, one with a period of 2–5 days and another with a period of 5–12 days. Analysis of the subinertial bands reveals that the spatial and temporal scales are shorter and less persistent than the near-inertial variance. As the array is geographically located near the site of the Deep Water Horizon oil spill, the spatial and temporal scales of response have significant implications for the fate, transport, and distribution of hydrocarbons following a deep water spill event.