Lentz Steven J.

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Lentz

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Steven J.

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0000-0001-7498-0281

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Acceleration of a stratified current over a sloping bottom, driven by an alongshelf pressure gradient

(American Meteorological Society, 2005-08) Chapman, David C. ; Lentz, Steven J.

An idealized theoretical model is developed for the acceleration of a two-dimensional, stratified current over a uniformly sloping bottom, driven by an imposed alongshelf pressure gradient and taking into account the effects of buoyancy advection in the bottom boundary layer. Both downwelling and upwelling pressure gradients are considered. For a specified pressure gradient, the model response depends primarily on the Burger number S = Nα/f, where N is the initial buoyancy frequency, α is the bottom slope, and f is the Coriolis parameter. Without stratification (S = 0), buoyancy advection is absent, and the alongshelf flow accelerates until bottom stress balances the imposed pressure gradient. The e-folding time scale to reach this steady state is the friction time, h/r, where h is the water depth and r is a linear bottom friction coefficient. With stratification (S ≠ 0), buoyancy advection in the bottom boundary layer produces vertical shear, which prevents the bottom stress from becoming large enough to balance the imposed pressure gradient for many friction time scales. Thus, the alongshelf flow continues to accelerate, potentially producing large velocities. The acceleration increases rapidly with increasing S, such that even relatively weak stratification (S > 0.2) has a major impact. These results are supported by numerical model calculations.
Observations of tidal variability on the New England shelf

(American Geophysical Union, 2004-06-04) Shearman, R. Kipp ; Lentz, Steven J.

Observations from the Coastal Mixing and Optics experiment moored array, deployed from August 1996 through June 1997, are used to describe barotropic and baroclinic tidal variability over the New England shelf. The dominant M 2 tidal elevations decrease toward the northeast to a minimum over the Nantucket shoals (about 34 cm), and barotropic tidal current amplitudes increase strongly toward the northeast to a maximum over the shoals (about 35 cm s−1). Estimates of the depth-averaged M 2 momentum balance indicate that tidal dynamics are linear, and along-shelf pressure gradients are as large as cross-shelf pressure gradients. In addition, tidal current ellipses are weakly polarized, confirming that the dynamics are more complex than simple plane waves. The vertical structure of the M 2 currents decreases in amplitude and phase (phase lead near bottom) over the bottom 20 m. The M 2 momentum deficit near the bottom approximately matches direct covariance estimates of stress, confirming the effects of stress on current structure in the tidally driven bottom boundary layer. Baroclinic current variability at tidal frequencies is small (2 cm s−1 amplitude), with a predominantly mode 1 vertical structure. High-frequency (approaching the buoyancy frequency) internal solitons are observed following the pycnocline. The internal solitons switch from waves of depression to waves of elevation when the depth of maximum stratification is deeper than half the water column depth. Both low-mode baroclinic tidal and high-frequency internal wave energy decrease linearly with bottom depth across the shelf.
The influence of wind forcing on the Chesapeake Bay buoyant coastal current

(American Meteorological Society, 2006-07) Lentz, Steven J. ; Largier, John

Observations of the buoyant coastal current that flows southward from Chesapeake Bay are used to describe how the thickness, width, and propagation speed vary in response to changes in the along-shelf wind stress. Three basic regimes were observed depending on the strength of the wind. For weak wind stresses (from −0.02 to 0.02 Pa), the buoyant coastal current was relatively thin, the front slope was not steep, and the width was variable (1–20 km). For moderate downwelling (southward) wind stresses (0.02–0.07 Pa), wind-driven cross-shelf advection steepened the front, causing the plume to narrow and thicken. For stronger downwelling wind stresses (greater than 0.07 Pa), vertical mixing dominated, bulk Richardson numbers were approximately 0.25, isopycnals were nearly vertical, and the plume front widened but the plume width did not change. Plume thickness and width were normalized by the theoretical plume scales in the absence of wind forcing. Normalized plume thickness increased linearly from 1 to 2 as downwelling wind stresses increased from 0 to 0.2 Pa. Normalized plume widths were approximately 1 for downwelling wind stresses from 0.02 to 0.2 Pa. The observed along-shelf propagation speed of the plume was roughly equal to the sum of the theoretical propagation speed and the wind-driven along-shelf flow.
The 1995 Georges Bank Stratification Study and moored array measurements

(Woods Hole Oceanographic Institution, 2001-08) Alessi, Carol A. ; Beardsley, Robert C. ; Caruso, Michael J. ; Churchill, James H. ; Irish, James D. ; Lentz, Steven J. ; Limeburner, Richard ; Werner, R. ; Weller, Robert A. ; Williams, Albert J. ; Williams, William J. ; Manning, James P. ; Smith, P.

The 1995 Geoges Bank Stratification Study (GBSS) was the first intensive process study conducted as part of the U.S. GLOBEC Northwest Atlantic/Georges Bank field program. The GBSS was designed to investigate the physical processes which control the seasonal development of stratification along the southern flank of Georges Bank during spring and summer. Past work suggested that during this period, larval cod and haddock tended to aggregate to the thermocline on the southern flank where higher concentrations of their copepod prey were found. A moored array was deployed as part of GBSS to observe the onset and evolution of sesonal stratification over the southern flank with sufficient vertical and horizontal resolution that key physical processes could be identified and quantified. Moored current, temperature, and conductivity (salinity) measurements were made at three sites along the southern flank, one on the crest, and one on the northeast peak of the bank. Moored surface meteorological measurements were also made at one southern flank site to determine the surface wind stress and heat and moisture fluxes. The oceanographic and meteorological data collected with the GBSS array during January-August 1995 are presented in this report. Meteorological data collected on National Data Buoy Center environmental buoys 44011 (Georges Bank), 44008 (Nantucket Shoals), and 44005 (Gulf of Maine) are included in this report for completeness and comparison with the GBSS southern flank meteorological measurements.
2007 program of studies : boundary layers

(Woods Hole Oceanographic Institution, 2008-06) Cenedese, Claudia ; Whitehead, John A. ; Pedlosky, Joseph ; Lentz, Steven J.

The topic of the Principal Lectures for the forty-ninth year of the program was “Boundary Layers”. The subject centers around those problems in which the boundary conditions lead to a large gradient near the boundary. Nine of this year’s principal lectures were given by Joe Pedlosky and the tenth was given by Steve Lentz. The fluid mechanics of boundary layers was reviewed, first starting from its classical roots and then extending the concepts to the sides, bottoms, and tops of the oceans. During week four, a mini-symposium on “Ocean Bottom and Surface Boundary Layers” gathered a number of oceanographers and meteorologists together to report recent advances. And, finally, Kerry Emanuel of MIT delivered the Sears Public Lecture to a packed hall in Clark 507. The title was “Divine Wind: The History and Sciences of Hurricanes.”
Buoyant gravity currents along a sloping bottom in a rotating fluid

(Cambridge University Press, 2002-08-22) Lentz, Steven J. ; Helfrich, Karl R.

The dynamics of buoyant gravity currents in a rotating reference frame is a classical problem relevant to geophysical applications such as river water entering the ocean. However, existing scaling theories are limited to currents propagating along a vertical wall, a situation almost never realized in the ocean. A scaling theory is proposed for the structure (width and depth), nose speed and flow field characteristics of buoyant gravity currents over a sloping bottom as functions of the gravity current transport Q, density anomaly g[prime prime or minute], Coriolis frequency f, and bottom slope [alpha]. The nose propagation speed is cp [similar] cw/ (1 + cw/c[alpha]) and the width of the buoyant gravity current is Wp [similar] cw/ f(1 + cw/c[alpha]), where cw = (2Qg[prime prime or minute] f)1/4 is the nose propagation speed in the vertical wall limit (steep bottom slope) and c[alpha] = [alpha]g/f is the nose propagation speed in the slope-controlled limit (small bottom slope). The key non-dimensional parameter is cw/c[alpha], which indicates whether the bottom slope is steep enough to be considered a vertical wall (cw/c[alpha] [rightward arrow] 0) or approaches the slope-controlled limit (cw/c[alpha] [rightward arrow] [infty infinity]). The scaling theory compares well against a new set of laboratory experiments which span steep to gentle bottom slopes (cw/c[alpha] = 0.11–13.1). Additionally, previous laboratory and numerical model results are reanalysed and shown to support the proposed scaling theory.
Comparison of observed and model-computed low frequency circulation and hydrography on the New England Shelf

(American Geophysical Union, 2008-09-09) Cowles, Geoffrey W. ; Lentz, Steven J. ; Chen, Changsheng ; Xu, Qichun ; Beardsley, Robert C.

The finite volume coastal ocean model (FVCOM) is configured to study the interannual variability of circulation in the Gulf of Maine (GoM) and Georges Bank. The FVCOM-GoM system incorporates realistic time-dependent surface forcing derived from a high-resolution mesoscale meteorological model (MM5) and assimilation of observed quantities including sea surface temperature and salinity and temperature fields on the open boundary. An evaluation of FVCOM-GoM model skill on the New England shelf is made by comparison of computed fields and data collected during the Coastal Mixing and Optics (CMO) Program (August 1996–June 1997). Model mean currents for the full CMO period compare well in both magnitude and direction in fall and winter but overpredict the westward flow in spring. The direction and ellipticity of the subtidal variability correspond but computed magnitudes are around 20% below observed, partially due to underprediction of the variability by MM5. Response of subtidal currents to wind-forcing shows the model captures the directional dependence, as well as seasonal variability of the lag. Hydrographic results show that FVCOM-GoM resolves the spatial and temporal evolution of the temperature and salinity fields. The model-computed surface salinity field compares well, except in May when there is no indication of the fresh surface layer from the Connecticut River discharge noted in the observations. Analysis of model-computed results indicates that the plume was unable to extend to the mooring location due to the presence of a westward mean model-computed flow during that time that was stronger than observed. Overall FVCOM-GoM captures well the dynamics of the mean and subtidal flow on the New England shelf.
Observations of cross-shelf flow driven by cross-shelf winds on the inner continental shelf

(American Meteorological Society, 2008-11) Fewings, Melanie R. ; Lentz, Steven J. ; Fredericks, Janet J.

Six-yr-long time series of winds, waves, and water velocity from a cabled coastal observatory in 12 m of water reveal the separate dependence of the cross-shelf velocity profile on cross-shelf and along-shelf winds, waves, and tides. During small waves, cross-shelf wind is the dominant mechanism driving the cross-shelf circulation after tides and tidal residual motions are removed. The along-shelf wind does not drive a substantial cross-shelf circulation. During offshore winds, the cross-shelf circulation is offshore in the upper water column and onshore in the lower water column, with roughly equal and opposite volume transports in the surface and bottom layers. During onshore winds, the circulation is nearly the reverse. The observed profiles and cross-shelf transport in the surface layer during winter agree with a simple two-dimensional unstratified model of cross-shelf wind stress forcing. The cross-shelf velocity profile is more vertically sheared and the surface layer transport is stronger in summer than in winter for a given offshore wind stress. During large waves, the cross-shelf circulation is no longer roughly symmetric in the wind direction. For onshore winds, the cross-shelf velocity profile is nearly vertically uniform, because the wind- and wave-driven shears cancel; for offshore winds, the profile is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The Lagrangian velocity profile in winter is similar to the part of the Eulerian velocity profile due to cross-shelf wind stress alone, because the contribution of Stokes drift to the Lagrangian velocity approximately cancels the contribution of waves to the Eulerian velocity.
Seasonal variations in the circulation over the Middle Atlantic Bight continental shelf

(American Meteorological Society, 2008-07) Lentz, Steven J.

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.
Wave-driven inner-shelf motions on the Oregon coast

(American Meteorological Society, 2009-11) Kirincich, Anthony R. ; Lentz, Steven J. ; Barth, John A.

Recent work by S. Lentz et al. documents offshore transport in the inner shelf due to a wave-driven return flow associated with the Hasselmann wave stress (the Stokes–Coriolis force). This analysis is extended using observations from the central Oregon coast to identify the wave-driven return flow present and quantify the potential bias of wind-driven across-shelf exchange by unresolved wave-driven circulation. Using acoustic Doppler current profiler (ADCP) measurements at six stations, each in water depths of 13–15 m, observed depth-averaged, across-shelf velocities were generally correlated with theoretical estimates of the proposed return flow. During times of minimal wind forcing, across-shelf velocity profiles were vertically sheared, with stronger velocities near the top of the measured portion of the water column, and increased in magnitude with increasing significant wave height, consistent with circulation due to the Hasselmann wave stress. Yet velocity magnitudes and vertical shears were stronger than that predicted by linear wave theory, and more similar to the stratified “summer” velocity profiles described by S. Lentz et al. Additionally, substantial temporal and spatial variability of the wave-driven return flow was found, potentially due to changing wind and wave conditions as well as local bathymetric variability. Despite the wave-driven circulation found, subtracting estimates of the return flow from the observed across-shelf velocity had no significant effect on estimates of the across-shelf exchange due to along-shelf wind forcing at these water depths along the Oregon coast during summer.
Observations and a model of undertow over the inner continental shelf

(American Meteorological Society, 2008-11) Lentz, Steven J. ; Fewings, Melanie R. ; Howd, Peter A. ; Fredericks, Janet J. ; Hathaway, Kent

Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach can result in a compensating Eulerian offshore flow in the surf zone referred to as undertow. Observed offshore flows indicate that wave-driven undertow extends well offshore of the surf zone, over the inner shelves of Martha’s Vineyard, Massachusetts, and North Carolina. Theoretical estimates of the wave-driven offshore transport from linear wave theory and observed wave characteristics account for 50% or more of the observed offshore transport variance in water depths between 5 and 12 m, and reproduce the observed dependence on wave height and water depth. During weak winds, wave-driven cross-shelf velocity profiles over the inner shelf have maximum offshore flow (1–6 cm s−1) and vertical shear near the surface and weak flow and shear in the lower half of the water column. The observed offshore flow profiles do not resemble the parabolic profiles with maximum flow at middepth observed within the surf zone. Instead, the vertical structure is similar to the Stokes drift velocity profile but with the opposite direction. This vertical structure is consistent with a dynamical balance between the Coriolis force associated with the offshore flow and an along-shelf “Hasselmann wave stress” due to the influence of the earth’s rotation on surface gravity waves. The close agreement between the observed and modeled profiles provides compelling evidence for the importance of the Hasselmann wave stress in forcing oceanic flows. Summer profiles are more vertically sheared than either winter profiles or model profiles, for reasons that remain unclear.
King Abdullah University of Science and Technology (KAUST) mooring deployment cruise and fieldwork report, fall 2008 R/V Oceanus voyage 449-5, October 9, 2008–October 14, 2008

(Woods Hole Oceanographic Institution, 2009-07) Farrar, J. Thomas ; Lentz, Steven J. ; Churchill, James H. ; Bouchard, Paul R. ; Smith, Jason C. ; Kemp, John N. ; Lord, Jeffrey ; Allsup, Geoffrey P. ; Hosom, David S.

King Abdullah University of Science and Technology (KAUST) is being built near Thuwal, Saudi Arabia with the goal of becoming a world-class, graduate-level research university. As a step toward this goal, KAUST has partnered with the Woods Hole Oceanographic Institution (WHOI) to undertake various studies of the oceanography of the Red Sea in order to establish a research program in ocean sciences by the time the university opens its doors in the fall of 2009. Two of the KAUST-WHOI research projects involve deployment of surface moorings and associated instrumentation to measure physical properties of the Red Sea, such as temperature, salinity, and currents, at four locations off the coast of Saudi Arabia. The goal of these measurements is to better understand the evolution and dynamics of the circulation and air-sea interaction in the Red Sea. Two surface moorings and two bottom tripods (PI, Steven Lentz) were deployed at 50-55-m depth near 21°57'N, 38°46'E over the continental shelf close to the Saudi coast. An additional surface mooring/bottom tripod pair was deployed near 21°58'N, 38°50'E at the outer fringe of a reef system directly onshore of the shelf mooring/tripod pairs (PI, Lentz). The coastal moorings carry instruments to estimate temperature, salinity, and fluorescence; and the nearby bottom tripods support instruments to measure bottom pressure and the vertical profile of the currents. Additional instruments, principally bottom temperature sensors, were deployed over the reef system onshore of the shelf moorings. One air-sea interaction mooring (PI, J. Thomas Farrar) was deployed at 693-m depth near 22°10'N, 38°30'E. The air-sea interaction mooring carries instruments for measuring temperature, salinity, (water) velocity, winds, air temperature, humidity, barometric pressure, incident sunlight, infrared radiation, precipitation, and surface waves. A coastal meteorological tower was also installed on the KAUST campus in Thuwal (PI, Farrar). These measurements are of value because there are few time series of oceanographic and meteorological properties of the Red Sea that can be used to characterize the circulation, test numerical models of the Red Sea circulation, or formulate theoretical models of the physics of the Red Sea circulation. These measurements will permit a characterization of the Red Sea circulation with high temporal resolution at the mooring locations, and accurate in-situ estimates of the air-sea exchange of heat, freshwater, and momentum. In October 2008, a cruise was made aboard the R/V Oceanus to deploy the shelf and air-sea interaction moorings, and other fieldwork (e.g., tower instrumentation and deployment of reef instrumentation) was conducted after the cruise. Some additional data were collected during the cruise with shipboard instrumentation. This report documents the cruise and the data collected during the fall 2008 fieldwork.
Observations and a model of the mean circulation over the Middle Atlantic Bight continental shelf

(American Meteorological Society, 2008-06) Lentz, Steven J.

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.

Lentz Steven J.

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