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Steven J. Lentz

Abstract

Analyses of current time series longer than 200 days from 33 sites over the Middle Atlantic Bight continental shelf reveal a consistent mean circulation pattern. The mean depth-averaged flow is equatorward, alongshelf, and increases with increasing water depth from 3 cm s−1 at the 15-m isobath to 10 cm s−1 at the 100-m isobath. The mean cross-shelf circulation exhibits a consistent cross-shelf and vertical structure. The near-surface flow is typically offshore (positive, range −3 to 6 cm s−1). The interior flow is onshore and remarkably constant (−0.2 to −1.4 cm s−1). The near-bottom flow increases linearly with increasing water depth from −1 cm s−1 (onshore) in shallow water to 4 cm s−1 (offshore) at the 250-m isobath over the slope, with the direction reversal near the 50-m isobath.

A steady, two-dimensional model (no along-isobath variations in the flow) reproduces the main features of the observed circulation pattern. The depth-averaged alongshelf flow is primarily driven by an alongshelf pressure gradient (sea surface slope of 3.7 × 10−8 increasing to the north) and an opposing mean wind stress that also drives the near-surface offshore flow. The alongshelf pressure gradient accounts for both the increase in the alongshelf flow with water depth and the geostrophic balance onshore flow in the interior. The increase in the near-bottom offshore flow with water depth is due to the change in the relative magnitude of the contributions from the geostrophic onshore flow that dominates in shallow water and the offshore flow driven by the bottom stress that dominates in deeper water.

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Steven J. Lentz

Abstract

Fits of an annual harmonic to depth-average along-shelf current time series longer than 200 days from 27 sites over the Middle Atlantic Bight (MAB) continental shelf have amplitudes of a few centimeters per second. These seasonal variations are forced by seasonal variations in the wind stress and the cross-shelf density gradient.

The component of wind stress that drives the along-shelf flow over most of the MAB mid- and outer shelf is oriented northeast–southwest, perpendicular to the major axis of the seasonal variation in the wind stress. Consequently, there is not a significant seasonal variation in the wind-driven along-shelf flow, except over the southern MAB shelf and the inner shelf of New England where the wind stress components forcing the along-shelf flow are north–south and east–west, respectively.

The seasonal variation in the residual along-shelf flow, after removing the wind-driven component, has an amplitude of a few centimeters per second with maximum southwestward flow in spring onshore of the 60-m isobath and autumn offshore of the 60-m isobath. The spring maximum onshore of the 60-m isobath is consistent with the maximum river discharges in spring enhancing cross-shelf salinity gradients. The autumn maximum offshore of the 60-m isobath and a steady phase increase with water depth offshore of Cape Cod are both consistent with the seasonal variation in the cross-shelf temperature gradient associated with the development and destruction of a near-bottom pool of cold water over the mid and outer shelf (“cold pool”) due to seasonal variations in surface heat flux and wind stress.

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Steven J. Lentz

Abstract

Subtidal current dynamics at a northern California inner-shelf site are analysed using moored current observations in 30 m of water, in conjunction with wind and bottom pressure measurements acquired during the summer of 1981 as part of the first Coastal Ocean Dynamics Experiment. The subtidal flow is driven locally by both an alongshelf wind stress and an alongshelf pressure gradient, which tend to be similar in magnitude but opposite in direction. Model depth-average alongshelf currents are about twice as large as observed. Analyses suggest that this discrepancy is due to a larger drag on the inner-shelf currents than suggested by bottom tripod measurements at the site, due to the presence of large rock outcrops over the inner shelf in this region. A notable characteristic of the observations is the weakness of the alongshelf flow over the inner shelf; alongshelf current standard deviations are a factor of 4 smaller than at midshelf. Analyses suggest this is due to a decrease in the wind stress toward the coast, shallower water resulting in a weaker body force due to the alongshelf pressure gradient and the increased drag on the flow over the inner shelf noted above.

The moored current observations reveal a simple cross-shelf circulation pattern. The near-surface flow was typically offshore in response to equatorward winds with an onshore flow in the lower water column driven by the opposing alongshelf pressure gradient. The depth-average cross-shelf velocity was consistently zero to the accuracy of the observations, suggesting a two-dimensional circulation. A simple two-dimensional eddy viscosity model reproduced the basic features of the observed flow, including the vertical structure, orientation, and temporal variability. The model results showed persistent, substantial vertical stress divergence throughout the water column supporting the notion that the moored observations were at an inner-shelf site where the surface and bottom boundary layers merge.

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Steven J. Lentz

Abstract

The mean heat and salt balances over the Middle Atlantic Bight continental shelf are investigated by testing the hypothesis that surface fluxes of heat or freshwater are balanced by along-isobath fluxes resulting from the mean, depth-averaged, along-isobath flow acting on the mean, depth-averaged, along-isobath temperature or salinity gradient. This hypothesized balance is equivalent in a Lagrangian frame to a column of water, for example, warming because of surface heating as it is advected southward along isobath by the mean flow.

Mean depth-averaged temperatures increase from north to south along isobath at a rate of 2°C (1000 km)−1 at midshelf, which is consistent with the hypothesized balance and mean surface heat flux estimates from the 50-yr NCEP Reanalysis. However, mean surface heat flux estimates from the higher-resolution 20-yr Objectively Analyzed Air–Sea Fluxes (OAFlux) reanalysis are too small to balance the along-isobath heat flux divergence implying a cross-shelf heat flux convergence. It is unclear which surface heat flux estimate, NCEP or OAFlux, is more accurate. The cross-shelf heat flux convergence resulting from the mean cross-shelf circulation is too small to balance the along-isobath heat flux divergence.

Mean depth-averaged salinities increase from north to south along isobath at a rate of 1 (psu) (1000 km)−1 at midshelf. Mean precipitation and evaporation rates nearly balance so that the net freshwater flux is too small by more than an order of magnitude to account for the observed along-isobath increase in salinity. The cross-shelf salt flux divergence resulting from the mean cross-shelf circulation has the wrong sign to balance the divergence in the along-isobath salt flux. These results imply there must be an onshore “eddy” salt flux resulting from the time-dependent current and salinity variability.

The along-isobath temperature and salinity gradients compensate for each other so that the mean, depth-averaged, along-isobath density gradient is approximately zero. This suggests that there may be a feedback between the along-isobath density gradient and the onshore salt and heat fluxes that maintains the density gradient near zero.

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Steven J. Lentz

Abstract

The accuracy of subinertial sea level, as measured by tide gauges in the Southern California Bight, is investigated. Sea level differences formed from tide gauges separated by less than 50 km are examined. The observed differences give an upper bound an errors in the sea level data, provided errors at each station are uncorrelated with each other and with the true sea level signal. Sea level measurements at the San Diego and La Jolla tide gauges are also compared with simultaneous bottom pressure measurements made on the shelf 25 km north of San Diego. Both comparisons sunset that rms errors in tide-gauge measurements of sea level are order 1.5 cm. Approximately half the observed error variance has time scales of 1 month and is due, in part, to inaccuracies in determining reference levels.

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Steven J. Lentz

Abstract

Observations from the Oregon, northwest Africa, Peru, and northern California shelves are used to examine the characteristics of the surface boundary layer in coastal regions during the upwelling season. The observations from these four regions yield a consistent picture of the structure of the surface boundary layer. Both CTD and moored observations reveal the presence of surface mixed layers that are typically 0–20 m thick with variability at diurnal and subtidal (periods longer than 36 hours) frequencies. The subtidal surface mixed-layer depth variability scales as u */(N I f)½, where u * = (τ S 0)½ is the shear velocity, N I is the buoyancy frequency below the surface mixed layer, and f is the Coriolis frequency. Surprisingly, this relationship indicates that the subtidal variability of surface mixed-layer depth does not depend strongly on either the surface heat flux or advection of heat, both of which are large in coastal upwelling regions.

Within the surface mixed layer the cross-shelf current is vertically uniform. Below the surface mixed layer there is a transition layer characterized by a subsurface maximum in the vertical shear of the cross-shelf velocity. The wind-driven, cross-shelf transport in the surface boundary layer agrees well in magnitude and variability with the expected Ekman transport τ S 0 f), with a substantial fraction (50%–25%) of the transport occurring in the transition layer below the surface mixed layer. The alongshelf current contains a strong, near-surface, vertical shear. When this vertical shear is in the surface mixed layer (unstratified flow) it is linearly related to the shear velocity u * and is consistent with a near-surface wind-driven log-layer, though the magnitude of the shear is larger than expected. Below the surface mixed layer, during moderate to strong wind stresses, the vertical shear is related to the stratification such that the gradient Richardson number (estimated over vertical scales of a few meters) is near critical (Ri∼0.25). Well below the surface mixed layer, in the interior, the flow tends to be more stable (Ri≥0.25), though near-critical Richardson numbers do occur.

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Steven J. Lentz

Abstract

The sensitivity of the inner-shelf circulation to the form of the vertical mixing is examined using a steady, linear, two-dimensional, eddy viscosity model. For both alongshelf wind stress and pressure gradient forcing, the alongshelf circulation over the inner shelf is insensitive to the form of the eddy viscosity profile. However, the cross-shelf circulation is sensitive to the form of the eddy viscosity profile. In particular, the location and width of the cross-shelf divergence in the Ekman transport over the inner shelf, and hence the corresponding upwelling or downwelling, depend on the form of the eddy viscosity profile.

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Steven J. Lentz

Abstract

Wind-driven, cross-shelf circulation is studied using current observations spanning the 90 km wide North Carolina shelf. Most of the shelf is less than 40 m deep. Current measurements were made at five sites within 16 km of the coast from August through October or early December 1994 and at mid- and outer-shelf sites from February 1992 through February 1994. In both studies the water column was stratified in summer and often unstratified during fall and winter. The presence or absence of stratification had a profound influence on the wind-driven, cross-shelf circulation over this shallow shelf.

When the water column was stratified, the wind-driven cross-shelf circulation was consistent with a two-dimensional upwelling/downwelling response. Over the mid and outer shelf, near-surface and near-bottom cross-shelf transports had similar magnitudes but opposite directions and were approximately equal to the Ekman transports associated with the alongshelf wind stress and bottom stress, respectively. Wind-driven cross-shelf transports decreased toward the coast over a cross-shelf scale of ≈10 km, suggesting that upwelling and downwelling were confined near the coast during stratified conditions. Stratification may be maintained in the region of transport divergence near the coast by a balance between vertical mixing and buoyancy forcing.

When the water column was unstratified the wind-driven cross-shelf circulation at all mooring sites was substantially reduced relative to the stratified response for moderate to strong wind stresses (|τ s | > 0.1 N m−2); consistent with an Ekman depth greater than the water depth. The dependence of the cross-shelf transport on wind stress and water depth is roughly consistent with an unstratified, two-dimensional model where the eddy-viscosity profile depends on the stress and distance from the boundaries. Both observations and model results suggest that during unstratified conditions much of the divergence or convergence in the wind-driven cross-shelf transport, and hence the associated upwelling and downwelling, occurs near the shelfbreak on this shallow shelf.

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Steven J. Lentz

Abstract

The characteristics and dynamics of depth-average along-shelf currents at monthly and longer time scales are examined using 17 years of observations from the Martha’s Vineyard Coastal Observatory on the southern New England inner shelf. Monthly averages of the depth-averaged along-shelf current are almost always westward, with the largest interannual variability in winter. There is a consistent annual cycle with westward currents of 5 cm s−1 in summer decreasing to 1–2 cm s−1 in winter. Both the annual cycle and interannual variability in the depth-average along-shelf current are predominantly driven by the along-shelf wind stress. In the absence of wind forcing, there is a westward flow of ∼5 cm s−1 throughout the year. At monthly time scales, the depth-average along-shelf momentum balance is primarily between the wind stress, surface gravity wave–enhanced bottom stress, and an opposing pressure gradient that sets up along the southern New England shelf in response to the wind. Surface gravity wave enhancement of bottom stress is substantial over the inner shelf and is essential to accurately estimating the bottom stress variation across the inner shelf.

Significance Statement

Seventeen years of observations from the Martha’s Vineyard Coastal Observatory on the inner continental shelf of southern New England reveal that the depth-average along-shelf current is almost always westward and stronger in summer than in winter. Both the annual cycle and variations around the annual cycle are primarily driven by the along-shelf wind stress. The wind stress is opposed by a pressure gradient that sets up along the southern New England shelf and a surface gravity wave–enhanced bottom stress. The surface gravity wave enhancement of bottom stress is substantial in less than 30 m of water and is essential in determining the variation of the along-shelf current across the inner shelf.

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Weifeng Zhang
and
Steven J. Lentz

Abstract

The dynamics controlling the along-valley (cross shelf) flow in idealized shallow shelf valleys with small to moderate Burger number are investigated, and analytical scales of the along-valley flows are derived. This paper follows Part I, which shows that along-shelf winds in the opposite direction to coastal-trapped wave propagation (upwelling regime) force a strong up-valley flow caused by the formation of a lee wave. In contrast, along-shelf winds in the other direction (downwelling regime) do not generate a lee wave and consequently force a relatively weak net down-valley flow. The valley flows in both regimes are cyclostrophic with O(1) Rossby number. A major difference between the two regimes is the along-shelf length scales of the along-valley flows L x . In the upwelling regime L x depends on the valley width W c and the wavelength λ lw of the coastal-trapped lee wave arrested by the along-shelf flow U s . In the downwelling regime L x depends on the inertial length scale |U s |/f and W c . The along-valley velocity scale in the upwelling regime, given by
eq1
is based on potential vorticity (PV) conservation and lee-wave dynamics (H s and H c are the shelf and valley depth scales, respectively, and f is the Coriolis parameter). The velocity scale in the downwelling regime, given by
eq2
is based on PV conservation. The velocity scales are validated by the numerical sensitivity simulations and can be useful for observational studies of along-valley transports. The work provides a framework for investigating cross-shelf transport induced by irregular shelf bathymetry and calls for future studies of this type under realistic environmental conditions and over a broader parameter space.
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