Glen Gawarkiewicz

A climatology of the shelfbreak front in the Middle Atlantic Bight

Description of the shelfbreak front in the Middle Atlantic Bight is hampered by the extreme variability of the front. In order to gain insight into both the seasonal variability and regional variations in the mean frontal structure and associated baroclinic jet, historical data are used to produce two-dimensional climatological fields of temperature and salinity for the region south of Nantucket shoals, along the south flank of Georges Bank, and off the coast of New Jersey. Associated cross-shelf fields of density and geostrophic velocity are also computed. The climatological temperature and salinity are consistent with previous descriptions of the frontal hydrography. The temperature contrast across the front varies seasonally between 2° and 6°C. The salinity contrast is 1.5–2, with little seasonal variation. The near-surface density gradients are strongest during the winter and weakest during the summer, when the seasonal thermocline is established. The cross-frontal density gradients are strongest near the foot of the front. Despite the inherent smearing of frontal gradients incurred by averaging over large temporal and spatial scales, the geostrophic velocity field south of Nantucket shows a strong (0.2–0.3 m s−1) baroclinic jet associated with the frontal density gradients. The core of the jet, having a width of 15–20 km, is located near the 150-m isobath. Transport calculations for the flow over the outer shelf and slope are in the range of 0.2–0.3 Sverdrups (Sv) to the west. This is comparable to the estimated transport (0.4 Sv) shoreward of the 100-m isobath.

A numerical study of dense water formation and transport on a shallow, sloping continental shelf

The circulation and transport of dense water generated by an idealized coastal polynya is studied using a three-dimensional primitive equation model. Starting with a homogeneous, quiescent ocean, a constant negative buoyancy flux is imposed at the surface over a half-elliptical region adjacent to the coastal boundary on a gently sloping continental shelf. The flow response can be divided into the following three phases: geostrophic adjustment, instability, and offshore eddy transport. During geostrophic adjustment the fluid within the forcing region becomes denser and the flow at the edge of the forcing region accelerates in response to the strong density gradient there. Eventually, the flow at the leading edge of the forcing region (relative to Kelvin wave propagation) becomes unstable and a train of counterrotating eddies develops. These eddies then form a complex three-dimensional flow field and rapidly transport dense water offshore, across isobaths. The density within the forcing region reaches a maximum which remains fairly constant after the eddies begin to transport the dense fluid offshore. The results are qualitatively insensitive to weakening of the negative buoyancy forcing and to changing the bottom slope. Eddy scales and velocities are consistent with observations in the Arctic. The results suggest that instability processes and eddy fluxes are important in transporting dense water off continental shelves and into marginal seas.

Observations of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study

Observations are presented of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study conducted between July 26 and August 5, 1996. Current and temperature measurements were made with an upward looking acoustic Doppler current profiler (ADCP) located on the 147 m isobath near the shelfbreak and three vertical thermistor moorings located upshelf. Data from the ADCP and two nearby thermistor chains show energetic internal tides propagating at roughly 0.9 m s 21 to the north-northwest, nearly perpendicular to the local topography with 10 -15 cm s 21 horizontal currents and 15-30 m vertical displacements. These waves evolve rapidly within a 5.8 km range into an undular internal tidal bore. Cross-isobath barotropic tidal currents, responsible for generating the internal tides are in the 5-12 cm s 21 range. The bore formation is highly variable. There is evidence of a correlation between internal tide steepening and a shelfbreak front jet orientation that is oppositely directed to the internal tide propagation. There is no correlation between steepening and the jets vertical shear. Statistics of the undular bores show rms travel time fluctuations from 0.8 to 1.7 hours and average tidal bore durations from 12 to 9 hours. The average undular bore speed is 0.9 m s 21 , with an rms fluctuation of 0.4 m s 21 . The number of high-frequency waves in the bore varies from 0 to 8 near the shelfbreak and increases to 30 waves 26.7 km upshelf. The observed distribution function of temporal spacing between high-frequency internal waves is spread between 4 and 20 min.

The Role of Stratification in the Formation and Maintenance of Shelf-Break Fronts

Abstract A mechanism is described for the formation or a front at the edge of a continental shelf in an initially linearly stratified fluid lacking horizontal density gradients. A primitive equation numerical model is used with a specified vertically uniform inflow imposed at the upstream boundary, and the flow is allowed to evolve alongshelf under the influence of bottom friction. As the flow progresses downstream, the shelf water moves steadily offshore due to the Ekman flux concentrated in the bottom boundary layer. This offshore flow transports light water under heavier water, which leads to convective overturning and ultimately a vertically well-mixed density field over the shelf. Large cross-shelf density gradients appear along the bottom at the shelf break where the vertically well-mixed shelf water abuts the linearly stratified water on the upper slope. At the shelf break, the bottom boundary layer detaches and continues offshore along upward sloping isopycnals. Neutrally buoyant particles in the ...

Offshore transport of dense shelf water in the presence of a submarine canyon

The formation and offshore transport of dense water over a uniformly sloping shelf crosscut by a submarine canyon is examined using a three-dimensional primitive-equation numerical model. A constant negative buoyancy flux is applied in a limited region adjacent to a straight coast to represent brine rejection from ice production in an idealized coastal polynya. A sharp density front forms at the edge of the forcing region, with surface and bottom intensified jets along the front. The flow around the head of the submarine canyon triggers a frontal instability that initially grows only on one side of the canyon. The unstable waves on the other side of the canyon are blocked by a localized barotropic flow that develops near the canyon head. Unstable waves also grow where the forcing region intersects the coast. The frontal waves grow rapidly (with O(1 day) e-folding timescales) and form eddies with horizontal scales of O(15 km) which extract the densest water from the forcing region and carry it offshore, directly across isobaths. In this way the eddies limit the maximum water density that appears in the model despite continued negative buoyancy forcing. Some dense water descends into the canyon, forming a bottom-trapped plume that transports the dense water offshore ahead of the eddies. The plume moves relatively slowly (i.e., small Froude number), with little turbulent entrainment, so the advancement and structure of the plume nose can be described successfully as a simple gravity current with an advective-diffusive heat balance. Eddies may slump into the canyon from the side, altering both the density anomaly and speed of the canyon plume, suggesting that canyon plumes are likely to be highly variable in both space and time.

Interaction of a Slope Eddy with the Shelfbreak Front in the Middle Atlantic Bight

Abstract Spring conditions at the shelf break in the Middle Atlantic Bight mark the transition period between the generally well-mixed shelf water in winter and the highly stratified shelf conditions during summer. A high-resolution hydrographic survey made during early May 1996 is used to describe the thermohaline and velocity structure of the shelfbreak front. The front was strongly affected by the presence of a slope eddy immediately offshore of the front. The eddy, which had a diameter of 25 km, was anticyclonic with onshore/offshore flows of 0.2 m s−1 on opposing sides. On the western side of the eddy, where the flow seaward of the front was predominantly onshore, the front was very steep and the frontal jet was particularly strong, with maximum near-surface velocities of 0.5 m s−1. On the eastern side of the eddy, the front was drawn offshore and was much less steep, with near-surface velocities of only 0.2 m s−1. A surprising feature was the presence of a second jet over the foot of the front, shor...

Shallow Convection and Buoyancy Equilibration in an Idealized Coastal Polynya

Abstract The recent theoretical approach of Visbeck, Marshall, and Jones is used to examine shallow convection and offshore transport of dense water from an idealized coastal polynya. A constant negative buoyancy flux is applied in a half-elliptical region adjacent to a coastal boundary, surrounded by a forcing decay region with uniform width W over which the imposed buoyancy flux decreases smoothly to zero. Initially, the density beneath the forcing increases linearly with time. A baroclinically unstable front forms at the edge of the forcing region. The width of the front is imposed by the width of the forcing decay region, provided this distance is larger than the baroclinic Rossby radius. Baroclinic eddies, whose velocities are inversely proportional to W, develop along the front and exchange dense water from the forcing region with ambient water, eventually reaching an equilibrium in which the lateral buoyancy flux by eddies balances the prescribed surface buoyancy flux. The time to reach equilibrium...

A large-amplitude meander of the shelfbreak front during summer south of New England : observations from the Shelfbreak PRIMER experiment

[1] In order to examine spatial and temporal variability of the shelfbreak front during peak stratification, repeated surveys using a towed undulating vehicle (SeaSoar) are used to describe the evolution of shelfbreak frontal structure during 26 July to 1 August 1996 south of New England. Spatial correlation (e-folding) scales for the upper 60 m of the water column were generally between 8 and 15 km for temperature, salinity, and velocity. Temporal correlation scales were about 1 day. The frontal variability was dominated by the passage of a westward propagating meander that had a wavelength of 40 km, a propagation speed of 0.11 m s -1 , and an amplitude of 15 km (30 km from crest to trough). Along-front geostrophic velocities (referenced to a shipboard acoustic Doppler current profilers) were as large as 0.45 m s -1 , although subject to significant along-front variations. The relative vorticity within the jet was large, with a maximum 0.6 of the local value of the Coriolis parameter. Seaward of the front, a small detached eddy consisting of shelf water was present with a diameter of approximately 15 km. Ageostrophic contributions to the velocity field are estimated to be as large as 0.3 m s -1 in regions of sharp curvature within the meander. These observations strongly suggest that during at least some time periods, shelfbreak exchange is nonlinear (large Rossby number) and dominated by features on a horizontal scale of order 10 km.

Water mass distribution and polar front structure in the western Barents Sea

The water mass distribution in the western Barents Sea, the thermohaline structure of the western Barents Sea Polar Front, and the local formation of a dense water mass are described on the basis of an analysis of historical hydrographic data. This study concentrated on the frontal region between Bjornoya and Hopen Island where Arctic water is found on the Spitsbergen Bank and Atlantic water in the Bear Island Trough and Hopen Trench. The distributions of Atlantic and Arctic waters in relation to topography were consistent with the hypothesis that the location of the polar front is fixed at about the 250 m isobath by the barotropic circulation of Atlantic water within the Bear Island Trough and Hopen Trench. In winter, vertical gradients of temperature and salinity were weak throughout the frontal region, consistent with a barotropic, topographically controlled front. In summer, vertical gradients remained weak below 100 m depth but increased in the upper layer as a result of the presence of fresh, warm surface water produced by melting ice. The topographic control of thermohaline properties at the surface was disrupted by the meltwater pool, and the meltwater contributed to water mass modification in the frontal region. The following seasonal cycle of water mass formation was hypothesized: Summer heating melts the sea ice on the Spitsbergen Bank and produces the surface meltwater pool. This meltwater not only increases vertical thermohaline gradients on the bank but also crosses the front and freshens the surface layer throughout the western Barents Sea. Subsequent winter cooling, which creates ice over the bank, also forms dense water in the Bear Island Trough and Hopen Trench by convective mixing of Atlantic water and the overlying meltwater.

Instability of a Shelfbreak Front

Abstract In an attempt to understand whether local instabilities can account for the observed frontal variability in the Middle Atlantic Bight, a linear stability analysis was conducted for a wide range of background density and velocity fields. Three-dimensional perturbations superposed on a continuously stratified shelfbreak front were investigated using the hydrostatic primitive equations. Model results indicate that the shelfbreak frontal jet is unstable over the wide parameter range dictated by the observed velocity and density structure. Model growth rates, on the order of one day, and wavelengths of ∼10–50 km compare favorably to observations, suggesting that local baroclinic/barotropic instabilities are a likely source for the strong temporal and spatial variability of the shelfbreak front in the Middle Atlantic Bight.