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John C Warner - One of the best experts on this subject based on the ideXlab platform.
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alongshore momentum balance analysis on a cuspate foreland
Journal of Geophysical Research, 2013Co-Authors: Nirnimesh Kumar, George Voulgaris, Jeffrey H List, John C WarnerAbstract:[1] Nearshore measurements of waves and currents off Cape Hatteras, North Carolina, U.S.A, are used to investigate depth-averaged subtidal circulation and alongshore momentum balances in the surf and inner shelf region around a cuspate foreland. Data were collected on both sides of the cape representing shorefaces with contrasting shoreline orientation (north-south vs. northwest-southeast) subjected to the same wind forcing. In the nearshore, the subtidal flow is aligned with the local coastline orientation while at the cape point the flow is along the existing submerged shoal, suggesting that cape associated shoals may act as an extension of the coastline. Alongshore momentum balance analysis incorporating wave-current interaction by including vortex and Stokes-Coriolis forces reveals that in deep waters surface and Bottom Stress are almost in balance. In shallower waters, the balance is complex as nonlinear advection and vortex force become important. Furthermore, linearized momentum balance analysis suggests that the vortex force can be of the same order as wind and wave forcing. Farther southwest of Cape Hatteras point, wind and wave forcing alone fail to fully explain subtidal flow variability and it is shown that alongshore pressure gradient as a response to the wind forcing can close the momentum balance. Adjacent tide gauge data suggest that the magnitude of pressure gradient depends on the relative orientation of local coastline to the wind vector, and in a depth-averaged sense the pressure gradient generation due to change in coastline orientation even at km length scale is analogous to the effect of alongshore variable winds on a straight coastline.
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implementation of the vortex force formalism in the coupled ocean atmosphere wave sediment transport coawst modeling system for inner shelf and surf zone applications
Ocean Modelling, 2012Co-Authors: Nirnimesh Kumar, George Voulgaris, John C Warner, Maitane OlabarrietaAbstract:Abstract The coupled ocean-atmosphere-wave-sediment transport modeling system (COAWST) enables simulations that integrate oceanic, atmospheric, wave and morphological processes in the coastal ocean. Within the modeling system, the three-dimensional ocean circulation module (ROMS) is coupled with the wave generation and propagation model (SWAN) to allow full integration of the effect of waves on circulation and vice versa. The existing wave-current coupling component utilizes a depth dependent radiation Stress approach. In here we present a new approach that uses the vortex force formalism. The formulation adopted and the various parameterizations used in the model as well as their numerical implementation are presented in detail. The performance of the new system is examined through the presentation of four test cases. These include obliquely incident waves on a synthetic planar beach and a natural barred beach (DUCK’ 94); normal incident waves on a nearshore barred morphology with rip channels; and wave-induced mean flows outside the surf zone at the Martha’s Vineyard Coastal Observatory (MVCO). Model results from the planar beach case show good agreement with depth-averaged analytical solutions and with theoretical flow structures. Simulation results for the DUCK’ 94 experiment agree closely with measured profiles of cross-shore and longshore velocity data from Garcez Faria et al., 1998 , Garcez Faria et al., 2000 . Diagnostic simulations showed that the nonlinear processes of wave roller generation and wave-induced mixing are important for the accurate simulation of surf zone flows. It is further recommended that a more realistic approach for determining the contribution of wave rollers and breaking induced turbulent mixing can be formulated using non-dimensional parameters which are functions of local wave parameters and the beach slope. Dominant terms in the cross-shore momentum balance are found to be the quasi-static pressure gradient and breaking acceleration. In the alongshore direction, Bottom Stress, breaking acceleration, horizontal advection and horizontal vortex forces dominate the momentum balance. The simulation results for the bar/rip channel morphology case clearly show the ability of the modeling system to reproduce horizontal and vertical circulation patterns similar to those found in laboratory studies and to numerical simulations using the radiation Stress representation. The vortex force term is found to be more important at locations where strong flow vorticity interacts with the wave-induced Stokes flow field. Outside the surf zone, the three-dimensional model simulations of wave-induced flows for non-breaking waves closely agree with flow observations from MVCO, with the vertical structure of the simulated flow varying as a function of the vertical viscosity as demonstrated by Lentz et al. (2008) .
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development of a coupled ocean atmosphere wave sediment transport coawst modeling system
Ocean Modelling, 2010Co-Authors: John C Warner, Brandy Armstrong, Ruoying He, Joseph B ZambonAbstract:Understanding the processes responsible for coastal change is important for managing our coastal resources, both natural and economic. The current scientific understanding of coastal sediment transport and geology suggests that examining coastal processes at regional scales can lead to significant insight into how the coastal zone evolves. To better identify the significant processes affecting our coastlines and how those processes create coastal change we developed a Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) Modeling System, which is comprised of the Model Coupling Toolkit to exchange data fields between the ocean model ROMS, the atmosphere model WRF, the wave model SWAN, and the sediment capabilities of the Community Sediment Transport Model. This formulation builds upon previous developments by coupling the atmospheric model to the ocean and wave models, providing one-way grid refinement in the ocean model, one-way grid refinement in the wave model, and coupling on refined levels. Herein we describe the modeling components and the data fields exchanged. The modeling system is used to identify model sensitivity by exchanging prognostic variable fields between different model components during an application to simulate Hurricane Isabel during September 2003. Results identify that hurricane intensity is extremely sensitive to sea surface temperature. Intensity is reduced when coupled to the ocean model although the coupling provides a more realistic simulation of the sea surface temperature. Coupling of the ocean to the atmosphere also results in decreased boundary layer Stress and coupling of the waves to the atmosphere results in increased Bottom Stress. Wave results are sensitive to both ocean and atmospheric coupling due to wave–current interactions with the ocean and wave growth from the atmosphere wind Stress. Sediment resuspension at regional scale during the hurricane is controlled by shelf width and wave propagation during hurricane approach.
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performance of four turbulence closure models implemented using a generic length scale method
Ocean Modelling, 2005Co-Authors: John C Warner, Christopher R Sherwood, Hernan G Arango, Richard P SignellAbstract:A two-equation turbulence model (one equation for turbulence kinetic energy and a second for a generic turbulence length-scale quantity) proposed by Umlauf and Burchard [J. Marine Research 61 (2003) 235] is implemented in a three-dimensional oceanographic model (Regional Oceanographic Modeling System; ROMS v2.0). These two equations, along with several stability functions, can represent many popular turbulence closures, including the k–kl (Mellor–Yamada Level 2.5), k–e, and k–ω schemes. The implementation adds flexibility to the model by providing an unprecedented range of turbulence closure selections in a single 3D oceanographic model and allows comparison and evaluation of turbulence models in an otherwise identical numerical environment. This also allows evaluation of the effect of turbulence models on other processes such as suspended–sediment distribution or ecological processes. Performance of the turbulence models and sediment–transport schemes is investigated with three test cases for (1) steady barotropic flow in a rectangular channel, (2) wind-induced surface mixed-layer deepening in a stratified fluid, and (3) oscillatory stratified pressure-gradient driven flow (estuarine circulation) in a rectangular channel. Results from k–e, k–ω, and gen (a new closure proposed by Umlauf and Burchard [J. Marine Research 61 (2003) 235]) are very similar for these cases, but the k–kl closure results depend on a wall-proximity function that must be chosen to suit the flow. Greater variations appear in simulations of suspended–sediment concentrations than in salinity simulations because the transport of suspended–sediment amplifies minor variations in the methods. The amplification is caused by the added physics of a vertical settling rate, Bottom Stress dependent resuspension, and diffusive transport of sediment in regions of well mixed salt and temperature. Despite the amplified sensitivity of sediment to turbulence models in the estuary test case, the four closures investigated here all generated estuarine turbidity maxima that were similar in their shape, location, and concentrations.
Steve Elgar - One of the best experts on this subject based on the ideXlab platform.
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Parameterization of a Two-Phase Sheet Flow Model and Application to Nearshore Morphology
2016Co-Authors: Steve Elgar, Tianjian Hsu, Daniel M HanesAbstract:The overall objective is to develop and test with laboratory and field observations a model that predicts sediment transport and morphological change in the nearshore for a range of wave conditions and sediment characteristics. The specific objectives of this project were to 1. parameterize the wave-induced Bottom Stress and sediment transport rate using a two-phase sheet flow model, 2. couple the sediment transport model with a time-domain Boussinesq hydrodynamic model to predict beach profile evolution, and 3. improve the two-phase sheet flow model by comparing its predictions with laboratory and field observations of sediment transport. Work Completed 20051110 027 A small-scale two-phase model [Hsu et al. 2004] that concurrently calculates bedload and suspended load transport processes was used to parameterize sediment transport. An earlier study [Hsu and Hanes 2004] considered simple wave shapes and suggested that most transport may be in-phase with Bottom Stress and hence the transport rate can be parameterized by a power law. In the beginning of this project, realistic wave forcing time series measured during the Duck94 [Gallagher et al. 1998] and SwashX [Raubenheimer 2002] field experiments were utilized to drive the two-phase model and a diluted suspended load model [Hsu and Liu 2004]. The validity of the power law approach was confirmed for typical sand grain sizes (d>-0.2mm) and wave periods (T>5sec). The limitation of the power law approach due to the effect of breaking wave turbulence also was studied using field data. The power law must be accompanied with a prediction of Bottom Stress to obtain the transport rate. Similar to that predicted by a discrete element model [Drake and Calantoni 2001], model results tested here with realistic wave forcing also suggest that under strong pitched-forward sea-swell waves, th
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modeled alongshore circulation and force balances onshore of a submarine canyon
Journal of Geophysical Research, 2015Co-Authors: Britt Raubenheimer, Jeff E Hansen, Jeffrey H List, Steve ElgarAbstract:Alongshore force balances, including the role of nonlinear advection, in the shoaling and surf zones onshore of a submarine canyon are investigated using a numerical modeling system (Delft3D/SWAN). The model is calibrated with waves and alongshore flows recorded over a period of 1.5 months at 26 sites along the 1.0, 2.5, and 5.0 m depth contours spanning about 2 km of coast. Field observation-based estimates of the alongshore pressure and radiation-Stress gradients are reproduced well by the model. Model simulations suggest that the alongshore momentum balance is between the sum of the pressure and radiation-Stress gradients and the sum of the nonlinear advective terms and Bottom Stress, with the remaining terms (e.g., wind Stress and turbulent mixing) being negligible. The simulations also indicate that unexplained residuals in previous field-based estimates of the momentum balance may be owing to the neglect of the nonlinear advective terms, which are similar in magnitude to the sum of the forcing (pressure and radiations Stress gradients) and to the Bottom Stress.
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Effects of wave rollers and Bottom Stress on wave setup
Journal of Geophysical Research, 2007Co-Authors: Alex Apotsos, Britt Raubenheimer, Steve Elgar, Robert T. Guza, Jerry A. SmithAbstract:[1] Setup, the increase in the mean water level associated with breaking waves, observed between the shoreline and about 6-m water depth on an ocean beach is predicted well by a model that includes the effects of wave rollers and the Bottom Stress owing to the mean flow. Over the 90-day observational period, the measured and modeled setups are correlated (squared correlation above 0.59) and agree within about 30%. Although rollers may affect setup significantly on beaches with large-amplitude (several meters high) sandbars and may be important in predicting the details of the cross-shore profile of setup, for the data discussed here, rollers have only a small effect on the amount of setup. Conversely, Bottom Stress (calculated using eddy viscosity and undertow formulations based on the surface dissipation, and assuming that the eddy viscosity is uniform throughout the water column) significantly affects setup predictions. Neglecting Bottom Stress results in underprediction of the observed setup in all water depths, with maximum underprediction near the shoreline where the observed setup is largest.
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wave induced sediment transport and onshore sandbar migration
Coastal Engineering, 2006Co-Authors: Steve Elgar, Robert T. GuzaAbstract:The 25-m onshore migration of a nearshore sandbar observed over a 5-day period near Duck, NC, is simulated with a simplified, computationally efficient, wave-resolving single-phase model. The modeled sediment transport is assumed to occur close to the seabed and to be in phase with the Bottom Stress. Neglected intergranular Stresses and fluid–granular interactions, likely important in concentrated flow, are compensated for with an elevated (relative to that appropriate for a clear fluid) model roughness height that gives the best fit to the observed bar migration. Model results suggest that when mean-current-induced transport is small, wave-induced transport leads to the observed onshore bar migration. Based on the results from the simplified phase-resolving model, a wave-averaged, energetics-type model (e.g., only moments of the near-Bottom velocity field are required) with different friction factors for oscillatory and mean flows is developed that also predicts the observed bar migration. Although the assumptions underlying the models differ, the similarity of model results precludes determination of the dominant mechanisms of sediment transport during onshore bar migration.
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turbulence measurements in the surf zone
Journal of Physical Oceanography, 2001Co-Authors: John H Trowbridge, Steve ElgarAbstract:Abstract Velocity measurements within 1 m of the Bottom in approximately 4.5-m water depth on a sand beach provide estimates of turbulent Reynolds shear Stress, using a dual-sensor technique that removes contamination by surface waves, and inertial-range estimates of dissipation. When combined with wave measurements along a cross-shore transect and nearby wind measurements, the dataset provides direct estimates of the terms in simplified equations for alongshore momentum and turbulence energetics and permits examination of semiempirical relationships between Bottom Stress and near-Bottom velocity. The records are dominated by three events when the measurement site was in the outer part of the surf zone. Near-Bottom turbulent shear Stress is well correlated with (squared correlation coefficient r2 = 0.63), but smaller than (regression coefficient b = 0.51 ± 0.03 at 95% confidence), wind Stress minus cross-shore gradient of wave-induced radiation Stress, indicating that estimates of one or more of these ter...
Joseph B Zambon - One of the best experts on this subject based on the ideXlab platform.
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development of a coupled ocean atmosphere wave sediment transport coawst modeling system
Ocean Modelling, 2010Co-Authors: John C Warner, Brandy Armstrong, Ruoying He, Joseph B ZambonAbstract:Understanding the processes responsible for coastal change is important for managing our coastal resources, both natural and economic. The current scientific understanding of coastal sediment transport and geology suggests that examining coastal processes at regional scales can lead to significant insight into how the coastal zone evolves. To better identify the significant processes affecting our coastlines and how those processes create coastal change we developed a Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) Modeling System, which is comprised of the Model Coupling Toolkit to exchange data fields between the ocean model ROMS, the atmosphere model WRF, the wave model SWAN, and the sediment capabilities of the Community Sediment Transport Model. This formulation builds upon previous developments by coupling the atmospheric model to the ocean and wave models, providing one-way grid refinement in the ocean model, one-way grid refinement in the wave model, and coupling on refined levels. Herein we describe the modeling components and the data fields exchanged. The modeling system is used to identify model sensitivity by exchanging prognostic variable fields between different model components during an application to simulate Hurricane Isabel during September 2003. Results identify that hurricane intensity is extremely sensitive to sea surface temperature. Intensity is reduced when coupled to the ocean model although the coupling provides a more realistic simulation of the sea surface temperature. Coupling of the ocean to the atmosphere also results in decreased boundary layer Stress and coupling of the waves to the atmosphere results in increased Bottom Stress. Wave results are sensitive to both ocean and atmospheric coupling due to wave–current interactions with the ocean and wave growth from the atmosphere wind Stress. Sediment resuspension at regional scale during the hurricane is controlled by shelf width and wave propagation during hurricane approach.
James N Moum - One of the best experts on this subject based on the ideXlab platform.
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structure and composition of a strongly stratified tidally pulsed river plume
Journal of Geophysical Research, 2009Co-Authors: Jonathan D Nash, Levi Kilcher, James N MoumAbstract:[1] The initial composition of a river plume depends on the cumulative turbulent entrainment within the estuary and how this dilutes the supplied freshwater. Here we examine the relative roles of turbulence and freshwater input using observations from the Columbia River estuary and plume during two periods with contrasting river flow. Within the estuary, intense turbulence observed on flood and ebb stages is controlled by the Bottom Stress and scales with tidally dominated near-Bottom velocity as utidal3. Shear associated with the estuarine circulation is found to have a much weaker influence on turbulence dissipation rates. On the basis of these observations, we suggest that properties of the Columbia River tidal plume should be controlled by the ratio of horizontal advection to turbulent mixing within the estuary. This ratio depends on the magnitude of freshwater river input (characterized by its volumetric flow rate Qf) as compared to turbulent fluxes due to tidal mixing. This is summarized in terms of the estuary Richardson number RiE, a nondimensional ratio between Qf and utidal3. From 17 tidally resolving offshore surveys during spring/neap tides and low/high river flows, we find that the plume's median salinity, thickness, and turbulent mixing are each predicted through RiE. It is hoped that these simple formulations will provide guidance in assessing critical properties of river plumes and their influence on coastal circulation.
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turbulence in the benthic boundary layer
Reference Module in Earth Systems and Environmental Sciences#R##N#Encyclopedia of Ocean Sciences (Second Edition), 2008Co-Authors: R Lueck, L St Laurrent, James N MoumAbstract:The oceanic Bottom boundary layer is a thin region of strong shear and friction with a characteristic scale of the Ekman height, H = u*/f, that extends some 10–50 m above the Bottom. A logarithmic velocity profile usually exists in the lower few percent of the Ekman height and the slope of this profile can be used to infer the Bottom Stress. It has only recently become feasible to make detailed vertical profiles of velocity and these reveal that the profiles frequently cannot be described by a single logarithmic layer. One interpretation has led to the identification of two logarithmic regions. The inner layer is controlled by the very local characteristics of the Bottom and its slope gives the Stress experienced by particulates on the Bottom. The outer layer reveals the large addition of form drag due to long horizontal-scale Bottom features and this drag provides the boundary condition for the flow well above the Bottom. That is, circulation models should use a drag coefficient consistent with the friction velocity derived from the outer layer. The outer layer may be important to sedimentation after the onset of suspension. An alternate interpretation that requires a single velocity scale rather than two also yields a significantly different drag, via (5), on the flow away from the Bottom. In the example shown in Figure 6b, the twofold difference in u* leads to a fourfold difference in drag. It is an important objective of current research to understand the consequence of Bottom boundary layer turbulence
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response of the Bottom boundary layer over a sloping shelf to variations in alongshore wind
Journal of Geophysical Research, 2005Co-Authors: A Perlin, James N Moum, Jody M KlymakAbstract:[1] Rapidly repeated transects of currents, density, and turbulence through the Bottom boundary layer across a relatively uniform stretch of the continental shelf off Oregon reveal the response to a sequence of strong upwelling followed by relaxation and thence a resumption of upwelling. Several definitions of boundary layer thickness are employed to describe the evolution of the Bottom boundary layer. Well-mixed and turbulent layers were typically confined to 10 m from the Bottom. However, boundary layer thicknesses were greatest during relaxation from upwelling (when mixed layer and turbulent layer thicknesses exceeded 20 m), and turbulence in the Bottom boundary layer was most intense at this time. Dense, near-Bottom fluid was observed to move upslope with upwelling and back down the slope with relaxation from upwelling. By tracking the intersection of near-Bottom isopycnals with the Bottom over successive transects, we estimate the cross-shore speed of fluid in the Bottom boundary layer. Cross-shore speed agrees well with dynamical estimates of cross-shore velocity in the Bottom Ekman layer derived from Bottom Stress measurements. This leads to a confirmation of the Ekman balance of alongshore momentum in the Bottom boundary layer across the full width of the shelf. Good correlation exists between alongshore velocity at the top of the Bottom boundary layer and cross-shore velocity of dense fluid in the Bottom boundary layer. Application of a derived proxy for Bottom Stress to moored velocity observations indicates Ekman balance of alongshore momentum at a midshelf location (81 m depth) for a 3 month period in spring/summer 2001.
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internal solitary waves of elevation advancing on a shoaling shelf
Geophysical Research Letters, 2003Co-Authors: Jody M Klymak, James N MoumAbstract:[1] A sequence of three internal solitary waves of elevation were observed propagating shoreward along a near-Bottom density interface over Oregon's continental shelf. These waves are highly turbulent and coincide with enhanced optical backscatter, consistent with increased suspended sediments in the Bottom boundary layer. Non-linear solitary wave solutions are employed to estimate wave speeds and energy. The waves are rank ordered in amplitude, phase speed, and energy, and inversely ordered in width. Wave kinetic energy is roughly twice the potential energy. The observed turbulence is not sufficiently large to dissipate the waves' energy before the waves reach the shore. Because of high wave velocities at the sea bed, Bottom Stress is inferred to be an important source of wave energy loss, unlike near-surface solitary waves. The wave solution suggests that the lead wave has a trapped core, implying enhanced cross-shelf transport of fluid and biology.
Steven J. Lentz - One of the best experts on this subject based on the ideXlab platform.
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buoyancy arrest and Bottom ekman transport part ii oscillating flow
Journal of Physical Oceanography, 2010Co-Authors: Steven J. LentzAbstract:Abstract The effects of a sloping Bottom and stratification on a turbulent Bottom boundary layer are investigated for cases where the interior flow oscillates monochromatically with frequency ω. At higher frequencies, or small slope Burger numbers s = αN/f (where α is the Bottom slope, N is the interior buoyancy frequency, and f is the Coriolis parameter), the Bottom boundary layer is well mixed and the Bottom Stress is nearly what it would be over a flat Bottom. For lower frequencies, or larger slope Burger number, the Bottom boundary layer consists of a thick, weakly stratified outer layer and a thinner, more strongly stratified inner layer. Approximate expressions are derived for the different boundary layer thicknesses as functions of s and σ = ω/f. Further, buoyancy arrest causes the amplitude of the fluctuating Bottom Stress to decrease with decreasing σ (the s dependence, although important, is more complicated). For typical oceanic parameters, arrest is unimportant for fluctuation periods shorter ...
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observations and a model of the mean circulation over the middle atlantic bight continental shelf
Journal of Physical Oceanography, 2008Co-Authors: Steven J. LentzAbstract: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 3t o 6c m 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 1c m 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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the importance of nonlinear cross shelf momentum flux during wind driven coastal upwelling
Journal of Physical Oceanography, 2004Co-Authors: Steven J. Lentz, David C ChapmanAbstract:Abstract A simple theory is proposed for steady, two-dimensional, wind-driven coastal upwelling that relates the dynamics and the structure of the cross-shelf circulation to the stratification, bathymetry, and wind Stress. The new element is an estimate of the nonlinear cross-shelf momentum flux divergence due to the wind-driven cross-shelf circulation acting on the vertically sheared geostrophic alongshelf flow. The theory predicts that the magnitude of the cross-shelf momentum flux divergence relative to the wind Stress depends on the Burger number S = αN/f, where α is the Bottom slope, N is the buoyancy frequency, and f is the Coriolis parameter. For S ≪ 1 (weak stratification), the cross-shelf momentum flux divergence is small, the Bottom Stress balances the wind Stress, and the onshore return flow is primarily in the Bottom boundary layer. For S ≈ 1 or larger (strong stratification), the cross-shelf momentum flux divergence balances the wind Stress, the Bottom Stress is small, and the onshore return ...
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the influence of stratification on the wind driven cross shelf circulation over the north carolina shelf
Journal of Physical Oceanography, 2001Co-Authors: Steven J. LentzAbstract: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. Win...
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physical oceanography of the amazon shelf
Continental Shelf Research, 1996Co-Authors: Rockwell W Geyer, Steven J. Lentz, Robert C Beardsley, Julio Candela, Richard Limeburner, W E Johns, Belmiro Mendes De Castro, Ivan Dias SoaresAbstract:Abstract The Amazon shelf is subject to energetic forcing from a number of different sources, including near-resonant semi-diurnal tides, large buoyancy flux from the Amazon River discharge, wind Stress from the northeasterly tradewinds and strong along-shelf flow associated with the North Brazil Current. Although the large volume of river discharge produces a pronounced salinity anomaly, the water motions on the shelf are dominated by the other forcing factors. Tidal velocities of up to 200 cm s−1 are generally oriented in the cross-shelf direction. Tide-induced mixing influences the position and structure of the Bottom salinity front that separates the well-mixed nearshore region from the stratified plume. High concentrations of suspended sediment trapped along the frontal zone increase the stability of the tidal boundary layer and thus reduce the Bottom Stress. At subtidal frequencies, motion is primarily along-shelf toward the northwest, both in the plume and in the ambient, high-salinity water of the outer-shelf. The plume is generally 5–10 m thick, with a salinity of 20–30 psu. The along-shelf velocity within the plume varies as a function of the along-shelf wind Stress. This variability results in large temporal variations in plume structure and freshwater content on the shelf. The net northwestward motion of the Amazon plume and of the ambient shelf water appears to be the result of a large-scale pressure gradient associated with the North Brazil Current system.