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Wendell S. Brown

Abstract

The semidiurnal tidal currents associated with the near-resonant response of the Gulf of Maine-Bay of Fundy system are amplified over the relatively shallow depths of Georges Bank, thus leading to enhanced energy dissipation, vertical mixing and secondary flows on the Bank. Within the western Gulf of Maine the tidal sea level amplitudes are larger but currents are less energetic than those observed on Georges Bank, while on the New England shelf the tidal response is the least energetic of the three regions. In this paper we explore some of the details of the tidal dynamics in these three very different tidal regimes by estimating terms in the volume-integrated momentum equations using observations of current and bottom pressure. The computations are performed for the M2 semidiurnal tidal constituent, which is the dominant tide in all of the regions, and are presented in terms of an instantaneous “stress” balance.

Results show that in the across-isobath direction on George Bank the M2 inertial term is balanced principally by the sum of the Coriolis and pressure gradient terms plus a small residual term, while in the along-isobath direction the principal balance is between the inertial and Coriolis terms. Even in this region of relatively high currents the nonlinear terms are found to be small in both directions, thus justifying the use of monochromatic input data. The instantaneous dynamic balances and the clockwise rotary elliptical currents are quantitatively consistent with the signature of an across-isobath propagating, forced gravitational-gyroscopic progressive wave which is strongly influenced by bottom slope. In the western Gulf of Maine a sum of the inertial and Coriolis terms in both the along- and across-isobath directions is balanced by the relatively large pressure gradient terms—dynamic balances that are consistent with those of a rotary standing wave. The distribution of counterclockwise rotary elliptical currents suggest the presence of a reflected Kelvin wave in the western Gulf. On the less energetic New England shelf the across-isobath inertial term is balanced by a sum of the Coriolis and pressure gradient terms as found on Georges Bank. However in the along-isobath direction, unlike Georges Bank, the same dynamical balance is found because of the importance of coastline irregularities in producing significant along-isobath tidal pressure gradients. The tidal response of the New England shelf combines the dynamical characteristics of those on Georges Bank and on the New Jersey shelf to the southwest and is less easily described in terms of the simple forced-wave models that are reasonably successful in the adjacent regions.

The Georges Bank and Gulf of Maine observed tides are compared with the Greenberg fine-grid numerical results with generally good overall result. Some small systematic difference which are found, may be due to the way friction is specified in the numerical model. Other results concerning the vertical structure and frictional character of Georges Bank tidal flow, which are presented here, suggest that the continued study of the way tidal energy dissipation is computed is warranted.

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Wendell S. Brown
and
Richard P. Trask

Abstract

A Method for inferring an area-averaged bottom stress and energy dissipation rate in a tidal estuarine channel is presented. The one-dimensional continuity and momentum relations are developed using simplifying assumptions appropriate for a well-mixed shallow and narrow estuary. The finite-difference form of these relations is derived for a section of the Great Bay Estuary, New Hampshire, an estuary which has been shown to have a relatively large energy dissipation rate. A set of current, bottom-pressure and sea-level measurements from the Estuary is used to estimate time series of all important first- and second-order terms in the momentum equation. Except near slack water, we find that the instantaneous first-order balance must be between the surface-slope-induced pressure gradient and bottom-stress forces. Important second-order contributions to the balance come from the inertial and convective acceleration terms. Time series of bottom stress are inferred by summing the estimated terms. For this study site the 14-day rms bottom stress is 45.1 ± 4,5 dyn cm−2 with a corresponding rms and mean dissipation rate of 3526 ± 420 and 2478 ± 297 ergs cm−2 s−1, respectively. The role of the first-order tidal motion and non-linearities in the mean second-order force balance is discussed.

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Wendell S. Brown
and
James D. Irish

Abstract

The annual evolution of the geostrophic flow structure in the Gulf of Maine was studied with a combined set of moored pressure time-series measurements and five hydrographic surveys from August 1986 through September 1987. A series of quasi-synoptic dynamic height maps depicts a gulf flow structure whose typical spatial scales decrease from order 100 km during the winter to about half that in the summer, when the evolution of surface, intermediate, and deep water masses is most rapid and complex. The unusually large amount of freshwater in the gulf during 1987 was partially responsible for the establishment of a north–south across-gulf front during the summer. Year-long time series of bottom pressure and internal pressure (derived from temperature and conductivity measurements in Georges and Jordan basins) have been differenced with coastal synthetic subsurface pressures (SSP) to yield a history of the basin-scale geostrophic flow variability. The basin-scale geostrophic transport was dominated by cyclonic flow (>0.5 × 106 m3 s−1) through the gulf during autumn 1986. During early January 1987, the flow around Jordan Basin became anticyclonic as relatively fresh Scotian shelf water flowed into the eastern gulf. Though temporarily disrupted during April and May, the Jordan Basin anticyclone persisted through July. Inflows of slope water to Georges Basin helped to establish a robust (0.5 × 106 m3 s−1) cyclonic flow around Georges Basin in June and July. Though somewhat weakened (∼0.3 × 106 m3 s−1), the cyclonic gyre migrated with the flow of slope and Maine bottom water to Jordan Basin in August. The delay in the establishment of the cyclonic gyre in Jordan Basin in 1987 appears to have been related in part to the effects of the anomalously large amount of freshwater in the gulf during 1987. A conceptual model of the annual evolution of gulf-scale flow, based on a hypothesized interplay of pressure gradient forcing produced by variable inflows (outflows), thermohaline forcing, and, to a lesser extent, wind forcing, is presented.

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Wendell S. Brown
and
Robert C. Beardsley

Abstract

The mean circulation on the northeast continental shelf in the region of the Gulf of Mexico is discussed in terms of a simple box model, based on volume transports and mean salinities estimated from existing data. The results of this calculation indicate that warm salty water from the continental slope must mix with colder, fresher water at intermediate depths within the Gulf. Field measurements obtained as part of a study of the winter circulation in an offshore region in the western Gulf of Maine suggest that winter storms may be responsible for most of this vertical mixing. Ten 1-day hydrographic cruises wore conducted between the passage of seasonal storms from November 1974 to January 1975. A description of the early winter evolution of the density field was thus obtained concurrently with moored measurements of current, temperature and bottom pressure, and coastal measurements of sea level and atmospheric variables. The principal vertical mixing process observed during this period was an intermittent overturning of the near-surface water caused by surface cooling by offshore winds. The observed vertical homogeneity suggests that the fresher near-surface Gulf of Maine water and the more saline deep basin water are frequently mixed during the early winter in the western region to produce Gulf of Maine Intermediate Water.

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James D. Irish
,
Wendell S. Brown
, and
Thomas L. Howell

Abstract

Geophysical signals are often intermittent, having statistics which vary with time. Optimal sampling of these signals requires a so-called “conditional sampling” scheme, a technique which changes the sampling program to match the time scales of the processes of interest. To optimize the limited tape storage capacity of remote oceanographic instruments, a conditional sampling scheme has been implemented using the computational power of microprocessor-controlled instruments, and several deployments have been made with various configurations of the conditional sampling algorithm. This algorithm monitors short-term changes in the energy of an incoming signal within a designated high-frequency band (by digital filtering techniques) and compares the resulting intensity with the longer term statistics of the signal. If the energy exceeds an intensity defined as critical according to some criteria, then an “event” is declared and the data are recorded at a higher than normal rate for the duration of the event. When the statistics of the expected signals are not well known, and criteria cannot be predetermined with confidence, an “adaptive” technique is required whereby the instrument makes an in situ determination of the critical intensity level for each signal based on the statistics of that signal.

Several deployments of the conditional sampling instruments have been made which demonstrate the operation of the technique. In Massachusetts Bay, a burst of high-frequency internal wave energy was identified and recorded by the adaptive critical algorithm applied to a moored temperature sensor array. On the northern California shelf, salinity was calculated in situ from moored temperature and conductivity sensors, and the resulting salinity time series conditionally sampled to identify salinity events as separate from temperature or pressure events.

Conditional sampling techniques may not be optimum for exploratory work. However, where the processes and expected signals are intermittent and have a specific signature, then the use of a conditional sampling technique can make more efficient use of the limited storage capacity of remote instrumentation.

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Wendell S. Brown
,
Neal R. Pettigrew
, and
James D. Irish

Abstract

The Nantucket Shoals Flux Experiment (NSFE) was a collaborative effort to measure the alongshelf transport of mass, heat, salt and nutrients from March 1979 through April 1980 with a dense army consisting of moored current, temperature and bottom pressure instruments in an across-shelf and upper-slope transect south of Nantucket Island. The pressure component of that experiment is described here.

Bottom pressure recorders were deployed at stations N1 (46 m), N2 (66 m), N4 (105 m), and N5 (196 m) during two half-year periods. spring–summer 1979 (SUMMER) and fall–winter 1979/80 (WINTER). A synthetic subsurface pressure (SSP) record was formed from atmospheric pressure and sea level observations at Nantucket Island. The low-pass filtered (periods > 36 h) or subtidal pressures were used for the subsequent analysis. It was found that Nantucket SSP and BP are very nearly equivalent for fluctuation periods less than about 50 days if steric changes in sea level, due to density changes above the seasonal pycnocline, were removed from the SSP record. However, if coastal SSP and bottom pressures are to be contrasted on seasonal time scales, then an internal pressure (or equivalent dynamic height) correction to bottom pressure is required. An empirical orthogonal function (EOF) analysis of the pressure field shows that most of the pressure variance for periods between 2 and 40 days (WINTER, 91.9%; SUMMER, 83.5%) is contained in a mode that (a) is characterized by a decreasing amplitude and small phase-lag increase at successive seaward locations and (b) is coherent with local alongshelf (75°T) wind stress. Pressure differences between stations were used to compute across-shelf pressure gradients, whose fluctuation distribution exhibits a conspicuous intensification over the outer shelf during the WINTER in contrast to the relatively uniform SUMMER distribution.

The excellent comparison found between “geostrophic currents,” which were inferred from bottom pressure differences, and suitably averaged “observed currents” represents the first direct confirmation of the quasi-geostrophy of alongshelf flow. Subtidal fluctuations of geostrophic currents are highly coherent with observed currents and lag them by periods that are consistent with typical geostrophic adjustment times for shelf flow fields. Frequency-domain EOF results indicate that most of the geostrophic current (pressure gradient) variance is contained in the primary mode, which is characterized by an outer-shelf amplitude maximum and an approximate two-day across-shelf phase lag. A variety of statistical results shows that most of the geostrophic current (pressure gradient) variance in the 2–14-day band is highly coherent with and lass alongshelf wind stress by 0–2 days in WINTER. SUMMER geostrophic currents (pressure gradients) show less coherence and phase lag with the considerably less energetic winds but larger response per unit stress than in WINTER. Such a response is consistent with a locally wind-forced shelf flow in which friction is important.

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Ilson C. A. da Silveira
,
Glenn R. Flierl
, and
Wendell S. Brown

Abstract

In this work, Pratt and Stern’s quasigeostrophic, 1½-layer, infinite jet model is connected to a western boundary by a system of two converging boundary currents. The model has a piecewise constant potential vorticity structure and the departing jet has a zonal cusplike profile in the ocean interior. The relative strengths of the coastal jets can be varied and the coastline can be tilted relative to north. The coastline tilt and the coastal current asymmetry cause an alongshore momentum imbalance that creates a spatially damped, quasi-stationary wave pattern. The presence of the boundary favors the long waves in the model, which behave fairly linearly in all study cases. The effects of the coastline tilt and the coastal current asymmetry are varied to reinforce or cancel each other. In the former case, a retroflection type of boundary current separation, like the one observed in most Southern Hemisphere western boundary currents, is obtained. In the latter case, a much smoother separation results, as when the Gulf Stream leaves the North American coast. In order to comply with the piecewise constant potential vorticity constraint, the β effect is included in the model only very crudely. The “beta” term in the potential vorticity relationship is totally compensated for by a steady flow pattern similar to the edge between two Fofonoff gyres. It is found that when β is nonzero, the wavelengths are somewhat shorter than those of f-plane cases.

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