James H. Churchill

The effect of commercial trawling on sediment resuspension and transport over the Middle Atlantic Bight continental shelf

Abstract Numerous field observations have revealed that turbulence created in the wake of trawl doors can generate large and highly turbid clouds of suspended sediment. Time-averaged concentrations of sediment resuspended by trawls from various areas of the Middle Atlantic Bight continental shelf have been estimated using a simple mathematical model and National Marine Fisheries Service records of commercial trawling activity. Mean concentrations of sediment put into suspension by currents have also been computed using a modified form of the Glenn and Grant model. The results indicate that sediment resuspension by trawling can be a primary source of suspended sediment over the outer shelf, where storm-related bottom stresses are generally weak. The concentration estimates further suggest that sediment resuspended by trawls makes a sizeable contribution to the total suspended sediment load over the heavily trawled central shelf area of Nantucket Shoals during all times except winter and early spring. The level of trawling activity declines dramatically going seaward across the outer shelf. This decline coupled with cross-shore water motions in the area appears to result in a net offshore transport of sediment across the shelf edge. However, the estimated magnitude of this transport indicates that trawling does not produce significant short-term erosion of outer shelf sediments.

Transport of Middle Atlantic Bight shelf water to the Gulf Stream near Cape Hatteras

Flowing along the U.S. eastern seaboard from Nantucket Shoals to Cape Hatteras is a low-salinity (<34 practical salinity units) water mass known as Middle Atlantic Bight (MAB) shelf water. It has long been recognized that a large quantity of this water is entrained into the Gulf Stream near Cape Hatteras. We examine this process using moored instrument, hydrographic, and drifter track data acquired in the Cape Hatteras region over 1992–1994. These measurements reveal that the cross section and volume transport of MAB shelf water tend to shrink considerably going southward toward and past Cape Hatteras. In particular, our hydrographic data show shelf water cross section to be reduced by about a factor of 8 over a region stretching 80 km north of Diamond Shoals (between 35° 20′N and 36°10′N). There appear to be two fairly distinct zones of export in this region. Our data suggest that shelf water loss in the northern zone, over 35°40′N–36°10′N, occurs principally seaward of the shelf edge. Seaward movement of shelf water into Gulf Stream meanders and into the circulation of water discharged from the Gulf Stream are identified as important agents effecting this loss. Export of shelf water in the southern zone occurs over middle and outer shelf, as indicated by 5–10 cm s−1 mean off shelf velocities measured by moored current meters. Contributing to this export is a seaward flow of shelf water in a strong current at the edge of the front separating MAB shelf water from the more saline shelf water of the South Atlantic Bight. The offshore transport of shelf water at the edge of this front is principally observed when the front is migrating shoreward across the shelf. This intermittency in the seaward transport of shelf water at the front may be partly responsible for the variability of the presence and transport of shelf water seen at the edge of the Gulf Stream.

Water mass linkages between the Middle and South Atlantic bights

Abstract Time and frequency domain analyses are used to relate coastal meteorological data with 7 years of daily surface temperature and salinity collected at three coastal light stations; offshore of the mouth of Chesapeake Bay, Virginia, on Diamond Shoals, at Cape Hatteras, North Carolina and on Frying Pan Shoals, off Cape Fear, North Carolina. Salinity fluctuations at Diamond Shoals are highly correlated with alongshore wind stress, implying wind driven advection of the front between Virginia Coastal Water (VCW) and Carolina Coastal Water (CCW) across Diamond Shoals. The data collected at Diamond Shoals indicate that more than half the time there is significant encroachment of Mid Atlantic Bight water into the South Atlantic Bight around Cape Hatteras, contrary to the notion that VCW is entirely entrained into the Gulf Stream. In fact, VCW can appear as far south as Frying Pan Shoals, thereby extending across the entire North Carolina Capes inner to mid shelf. Temperature and salinity time series also indicate that water masses overlying Diamond Shoals respond quickly to cross-shelf winds. Cross-shelf wind stress is significantly correlated with surface water temperature at Diamond Shoals, for periods between 2 and 12 days. Changes in temperature can be brought about by wind-driven cross-shelf circulation and by wind-induced upwelling. Seasurface temperature satellite (AVHRR) imagery taken during the SEEP II confirm these concepts.

Near-Surface Measurements of Quasi-Lagrangian Velocities in Open Water

Abstract Near-surface water velocities have been measured in the coastal zone of Lake Huron and Cape Cod Bay by tracking drifters and drogues using acoustic travel time and compass sighting techniques. The near-surface current, defined as the velocity of near-surface drifters and drogues relative to a drogue set at 1.8 m, varied on the depth scale on the order of 1 m, and was directed nearly parallel to the wind and to predominant wave propagation velocity. Velocity profiles were logarithmic with depth to order 1 m depth, and realistic values of stress were calculated using a law of the wall formula and a Karmans constant of 0.4. Inferred roughness lengths were of the order of 30 cm. Anomalously high values of wind stress were inferred from velocity profiles observed during conditions of light wind and steady swell. These may be due to the similarity of Stokes drift distribution to turbulent shear flow profiles.

Sediment resuspension over the continental shelf east of the Delmarva Peninsula

Abstract Resuspension of sediment over the continental shelf east of the Delmarva Peninsula has been examined using records of light-beam attenuation, near-bottom current speed and surface-wave height spectra collected during 1988 and 1989. These data give evidence of a factor of three variation in the bottom stress threshold required for sediment resuspension at the outer shelf. This appears to be related to resuspension history as the largest thresholds are observed after lengthy periods without resuspension. Episodes of shelf-wide sediment resuspension are evidenced only during very intense atmospheric storms. A 7-month-long set of records from the 90 m isobath show storm-induced sediment resuspension on only three occasions. The failure of storms of modest intensity to effect resuspension at the outer shelf is largely due the decline of surface-wave currents with depth. High-frequency currents, presumably due to internal waves, are shown to be an important agent in initiating sediment motion at the shelf edge. On a number of occasions, supertidal currents pushed the near-bottom current speed measured near the seafloor at the 131 m isobath above the estimated level required for sediment resuspension. Numerous clouds of turbid water detected by the light-beam attenuation records could not be attributed to local sediment resuspension. A probability analysis indicates that some, but not all, of these could have resulted from sediment resuspension by bottom fishing.

Gulf Stream water on the shelf and upper slope north of Cape Hatteras

Abstract Although intrusions of water from the Gulf Stream have often been observed over the Carolina shelf, there has been no published report of Gulf Stream water near the continental margin north of Cape Hatteras. By examining sea surface temperature distributions and hydrographic data collected over an 11-yr period, we have found that water discharged from the Gulf Stream often appears over the shelf and upper slope north of Cape Hatteras. For example, in the band between 36 and 38°N, roughly 80–300 km north of the tip of Cape Hatteras, Gulf Stream water was detected 13–27% of the time at sites on the upper slope and 3–9% of the time at locations on the shelf. In most instances, sea surface temperature distributions suggest that the Gulf Stream water which appeared near the continental margin was not part of the Gulf Streams main current but was fluid which had been expelled from the Gulf Stream. The Gulf Stream water found on the shelf did not appear to significantly influence the local circulation or alongshelf density structure. Its subtidal flow was largely driven by the alongshelf wind stress; and in most instances its vertical density profile nearly matched that of adjacent shelf water situated alongshore. In contrast, the discharged Gulf Stream water observed over the upper slope was significantly less dense than abutting fluid of equivalent depth located along the slope to the northeast. The circulation associated with this density contrast often conveyed Middle Atlantic Bight shelf water seaward of the continental margin. The resulting rate of shelf water export is not well resolved by the available data. Some rough estimates put it at 0.1 Sv, comparable with the estimated mean alongshore transport of shelf water over the Middle Atlantic Bight shelf.

Mixing of shelf, slope and Gulf Stream water over the continental slope of the Middle Atlantic Bight

Abstract A combination of water masses was observed over the continental slope between Cape Hatteras and Hudson Canyon following the passage of a Gulf Stream meander. It was largely made up of fluid discharged from the Gulf Stream in the wake of the meander and also contained intrusions of Middle Atlantic Bight (MAB) shelf water and slope water. Its most prominent dynamical feature was an anticyclonic eddy that had an initial diameter of about 100 km but contained relatively weak currents, not greater than 30 cm s −1 in magnitude. Discharged Gulf Stream water made up the eddys core, and a band of entrained shelf water circulated at its margin. This band was connected with another entrained shelf water band that flowed along the northern margin of the Gulf Stream. Transport of the shelf water at the edge of the eddy was estimated to be 1.3 × 10 5 m 3 s −1 , which is comparable with the estimated alongshore transport of water over the MAB shelf. Vigorous mixing of both shelf water bands with surrounding discharged Gulf Stream water is indicated by the large differences of the latters T/S properties from those found within the Gulf Stream. These differences were used to estimate salt fluxes into the shelf water band at the edge of the anticyclonic eddy, with results in the range of 1.9–6.6 × 10 −6 g salt cm −2 s −2 . Density ratios and Richardson numbers determined from hydrographic data suggest that these salt fluxes were primarily the result of mixing by double diffusive processes, and that shear-induced turbulence combined with double diffusion to effect vertical mixing at the margins of the shelf water band next to the Gulf Stream.

Drifter observations of the Gulf of Maine Coastal Current

Two-hundred and twenty seven satellite-tracked drifters were deployed in the Gulf of Maine (GoM) from 1988 to 2007, primarily during spring and summer. The archive of tracks includes over 100,000 kilometers logged thus far. Statistics such as transit times, mean velocities, response to wind events, and preferred pathways are compiled for various areas of the coastal GoM. We compare Lagrangian flow with Eulerian estimates from near-by moorings and evaluate drifter trajectories using Ekman theory and 3-D ocean circulation models. Results indicate that the Gulf of Maine Coastal Current is a strong and persistent feature centered on the 94 ± 23 meter isobath, but that particles: a) deviate from the seasonal-mean core fairly regularly, and are often re-entrained; b) follow a slower (9 cm/s), less-constrained path in the western portion off the coast of Maine relative to the eastern (16 cm/s) section; and c) can be affected by wind events and small scale baroclinic structures. Residence times calculated for each ½ degree grid cell throughout the GoM depict some regions (Eastern Maine and Western Nova Scotia) as being relatively steady, flow-through systems, while others (Penobscot, Great South Channel) have more variable, branching pathways. Travel times for drifters that are retained within the coastal current along the entire western side of the Gulf of Maine are typically less than two months (55 days).

Near-bottom currents over the continental slope in the Mid-Atlantic Bight

From a set of 28 current meter records we have found that near-bottom currents faster than 0.2 m s−1 occur frequently over the outer continental shelf of the Mid-Atlantic Bight (bottom depth <210 m) but very rarely (<1%of the time) between bottom depths of 500 m and 2 km over the slope. The rarity of strong near-bottom flow over the middle and lower slope allows the accumulation of fine-grained sediment and organic carbon in this region. Fast near-bottom currents which do occur over the slope are invariably associated with topographic waves, although it is often superimposed inertial oscillations which increase current speed above the level of 0.2 m s−1. Episodes of intense inertial oscillations occur randomly and last typically for 10–20 days. Their energy source is unknown. Topographic wave energy exhibits a slight, but statistically significant, minimum over the mid-slope. These waves appear irregularly and vary both along isobaths and in time. The irregularity is presumably a consequence of random topographic wave generation by Gulf Stream instability. The current regime within sea-floor depressions in the slope (canyons and gullies) is distinctly different from that of the open slope; most notable is the near absence of topographic wave motion within depressions.

The character and motion of suspended particulate matter over the shelf edge and upper slope off Cape Cod

Abstract Time series of suspended particulate matter (SPM) concentration made by transmissometers deployed on the shelf edge and upper slope off New England are examined together with data from companion sensors. At the shelf-edge instrument cluster (on the 125-m isobath) the highest SPM concentrations were apparently due to sediment resuspended further onshore and advected to the transmissometer by offshore motion of the shelf-edge density front. This was due to a tendency for more frequent episodes of high bottom stress, and thus more sediment resuspention, progressing onshore from the outer shelf. The decrease of surface wave current amplitude with depth was primarily responsible for this cross-shelf gradient in bottom stress. On one occasion very turbid water was observed at the shelf edge when the bottom stress was low and the frontal motion was small. Data obtained from the U.S. National Marine Fisheries Service showed unusually intensive bottom trawling activity in the vicinity of the transmissometer during this event. This data further indicates that resuspension by trawlers may be an important source of suspended material on the shelf edge. The record of the transmissometer deployed over the upper slope indicates that transport of particulate material in suspension from the near bottom on the shelf into the slope region occurred primarily along density surfaces, and was not continuous but intermittent.