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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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Marinna Martini, Bradford Butman, and Michael J. Mickelson

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A field evaluation of two new dissolved-oxygen sensing technologies, the Aanderaa Instruments AS optode model 3830 and the Sea-Bird Electronics, Inc., model SBE43, was carried out at about 32-m water depth in western Massachusetts Bay. The optode is an optical sensor that measures fluorescence quenching by oxygen molecules, while the SBE43 is a Clark polarographic membrane sensor. Optodes were continuously deployed on bottom tripod frames by exchanging sensors every 4 months over a 19-month period. A Sea-Bird SBE43 was added during one 4-month deployment. These moored observations compared well with oxygen measurements from profiles collected during monthly shipboard surveys conducted by the Massachusetts Water Resources Authority. The mean correlation coefficient between the moored measurements and shipboard survey data was >0.9, the mean difference was 0.06 mL L−1, and the standard deviation of the difference was 0.15 mL L−1. The correlation coefficient between the optode and the SBE43 was >0.9 and the mean difference was 0.07 mL L−1. Optode measurements degraded when fouling was severe enough to block oxygen molecules from entering the sensing foil over a significant portion of the sensing window. Drift observed in two optodes beginning at about 225 and 390 days of deployment is attributed to degradation of the sensing foil. Flushing is necessary to equilibrate the Sea-Bird sensor. Power consumption by the SBE43 and required pump was 19.2 mWh per sample, and the optode consumed 0.9 mWh per sample, both within expected values based on manufacturers’ specifications.

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Marlene Noble, Bradford Butman, and Edward Williams

Abstract

Strong correlations were observed among subtidal longshelf currents from the Middle Atlantic Bight (MAB) to the Georges Bank region, a distance spanning 615 km. The longshelf current consisted predominantly of wind-forced motions and freely propagating events, which together accounted for 75%–90% of the longshelf current energy. Much stronger longshelf currents were observed in the MAB than on Georges Bank. The MAB/Georges Bank energy ratio for wind-forced currents on the 60 m isobath was 20. The ratio for freely propagating events was 3. The magnitudes of many of the terms in the vertically integrated wind-driven momentum equations were estimated from observations of current, pressure and surface stress, and from calculations of bottom stress. The cross-shelf momentum balance was geostrophic. Surface and bottom stress, the longshelf pressure gradient, and the Coriolis force on the cross-shelf flow were important terms in the longshelf momentum balance. An analytic model of wind-forced current, which incorporates the significant force balances, accounted for the observed longshelf variation of the wind-forced currents. Average bottom-drag and bottom-resistance coefficients estimated from current and bottom-stress records range from 4–8 (× 10−3) and 0.07–0.20 cm s−1, respectively.

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Marlene Noble, Bradford Butman, and Mark Wimbush

Abstract

Comparison of several years of current observations on the southern flank of Georges Bank with nearby wind data shows that the wind–current coupling is primarily between longshelf wind stress and longshelf current. The strongest wind–currnet coupling occurs in winter, when the water column is homogeneous. The weakest coupling is in late summer and early fall, when the water column is highly stratified. The coherence and transfer coefficient between longshelf wind and longshelf current is highest for periods between 4 and 12 days, decreasing both for longer periods (out to 56 days) and shorter periods (down to 2 days). Models of the wind–current coupling indicate that a highly damped resonance may exist on Georges Bank and that a smaller current response is expected when the water column is stratified. The observations also indicate that the wind-driven currents on Georges Bank are strongly controlled by friction. The near-surface current moves to the right of wind stress and there is a spring–neap modulation of the wind–current transfer coefficient caused by the modulation of the bottom stress associated with the spring–neap tidal cycle. The longshelf current is linearly related to wind stress and responds almost symmetrically to wind forcing.

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Bradford Butman, Marlene Noble, David C. Chapman, and Robert C. Beardsley

Abstract

Long-term current observations at 45 and 75 m at one location on the southern flank of Georges Bank in water 85 m deep were examined for evidence of tidal rectification. Loder has shown analytically that rectification of the strong semidiurnal tidal current can cause a mean along-bank flow, and thus may partially drive the observed clockwise circulation around Georges Bank. The amplitude of the tidally rectified along-bank flow is proportional to the squared amplitude of the cross-bank tidal current. A simply extension of Loder's model to include the weaker N2 and S2 tidal components suggests that fortnightly (354 h) and monthly (661 h) variations of the square of the cross-bank tidal current should cause a modulation of the subtidal along-bank flow. The predicted ratio (R) of the fortnightly and monthly modulation of the along-bank flow to the mean along-bank flow on the southern flank was a function of position and ranged from ∼0.1–0.5. The amplitude of modulation of the along-bank flow at 360 and 648 h, estimated from the (weak) coherence between the observed along-bank flow and the subtidal envelope of a simulated surface tide, was less than ∼1.1 and 0.9 cm s−1, respectively, at 45 m. The amplitude of the modulation which can be attributed to tidal rectification may be in error by the astronomically forced Mm and MSf tidal currents, which are undescribed in this region. However, the magnitude of the mean along-bank tidally rectified current determined from the observed modulation and R predicted by the analytical model was ∼2.0 cm s−1 at 45 m (36% of the observed mean current in winter) and less than 1.6 cm s−1 at 75 m (43% of the observed mean current). Although R may change in a more realistic model, this analysis suggests that only part of the seasonal-mean along-bank flow on the southern flank of Georges Bank may be caused by tidal rectification.

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Mark Grosenbaugh, Steven Anderson, Richard Trask, Jason Gobat, Walter Paul, Bradford Butman, and Robert Weller

Abstract

This paper describes the design and performance of a two-dimensional moored array for sampling horizontal variability in the upper ocean. The mooring was deployed in Massachusetts Bay in a water depth of 84 m for the purpose of measuring the horizontal structure of internal waves. The mooring was instrumented with three acoustic current meters (ACMs) spaced along a 170-m horizontal cable that was stretched between two subsurface buoys 20 m below the sea surface. Five 25-m-long vertical instrument strings were suspended from the horizontal cable. A bottom-mounted acoustic Doppler current profiler (ADCP) was deployed nearby to measure the current velocity throughout the water column. Pressure sensors mounted on the subsurface buoys and the vertical instrument strings were used to measure the vertical displacements of the array in response to the currents. Measurements from the ACMs and the ADCP were used to construct time-dependent, two-dimensional current fields. The current fields were used as input to a numerical model that calculated the deformation of the array with respect to the nominal zero-current configuration. Comparison of the calculated vertical offsets of the downstream subsurface buoy and downstream vertical instrument string with the pressure measurements were used to verify the numerical code. These results were then used to estimate total deformation of the array due to the passage of the internal waves. Based on the analysis of the three internal wave events with the highest measured vertical offsets, it is concluded that the geometry of the main structure (horizontal cable and anchor legs) was kept to within ±2.0 m, and the geometry of the vertical instrument strings was kept to within ±4.0 m except for one instance when the current velocity reached 0.88 m s−1.

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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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Robert W. Houghton, Ronald Schlitz, Robert C. Beardsley, Bradford Butman, and J. Lockwood Chamberlin

Abstract

Temperature data spanning the entire Middle Atlantic Bight (MAB) during 1979 are used to study the structure and evolution of the cold pool. The Nantucket Shoals and New England Shelf appear to be the source of the coldest water found in the MAB in late winter. During the spring and summer, water within the cold pool in the New York Bight north of Hudson Canyon remains colder than any shelf water either to the northeast or southwest. Thus the coldest cold-pool water persists there as a remnant of winter-cooled water rather than being replenished by a colder upstream source, and south of Hudson Canyon, cold-pool temperatures decrease in June and July as colder water from upstream is advected southwestward along the coast. Both temperature data and direct current measurements suggest that the mean alongshore current has a minimum between Nantucket Shoals and Hudson Canyon. The alongshore variation of shelf topography appears to be responsible for the spatial variation in both the alongshelf drift speed and the thermal structure of the cold pool.

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