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Hans Burchard - One of the best experts on this subject based on the ideXlab platform.

  • Estuarine Circulation versus tidal pumping sediment transport in a well mixed tidal inlet
    Journal of Geophysical Research, 2016
    Co-Authors: Johannes Becherer, Lars Umlauf, Gotz Floser, Hans Burchard
    Abstract:

    High-resolution water column observations have been carried out in the Wadden Sea to understand suspended particulate matter (SPM) transport in well-mixed tidal channels. These observations include more than 4000 consecutive CTD, microstructure shear and turbidity profiles from a free-falling microstructure probe, as well as velocity data from an ADCP and SPM samples for calibration. A horizontal density gradient was established by a landward temperature gradient built up during an extraordinarily warm and calm spring season. Tidal averaging along σ-layers (relative depth) provides the first direct observations of along-channel Estuarine Circulation in the Wadden Sea, with net inflow near the bottom and outflow near the surface. Increased westerly (up-estuary) winds during the second part of the campaign weakened and eventually even reversed Estuarine Circulation and yielded a net barotropic eastward transport. SPM concentrations showed a strong quarter-diurnal signal with maxima near full flood and full ebb and were generally lower during the calm period and increased during the windy period, mainly due to wave-related resuspension over nearby intertidal flats. The sediment flux analysis was based on a decomposition of the vertically integrated SPM flux into a barotropic advective component, an Estuarine Circulation component and a tidal pumping component. As a result, tidal pumping (due to ebb-dominance weakly seaward) dominated the SPM flux during calm conditions, whereas barotropic advection dominated the strong landward SPM flux during the windy period. Along-channel Estuarine Circulation is found to be of minor importance for the net SPM transport in such well-mixed systems.

  • impact of the depth to width ratio of periodically stratified tidal channels on the Estuarine Circulation
    Journal of Physical Oceanography, 2015
    Co-Authors: Elisabeth Schulz, H M Schuttelaars, Ulf Grawe, Hans Burchard
    Abstract:

    The dependency of the Estuarine Circulation on the depth-to-width ratio of a periodically, weakly stratified tidal estuary is systematically investigated here for the first time. Currents, salinity, and other properties are simulated by means of the General Estuarine Transport Model (GETM) in cross-sectional slice mode, applying a symmetric Gaussian-shaped depth profile. The width is varied over four orders of magnitude. The individual along-channel Circulation contributions from tidal straining, gravitation, advection, etc., are calculated and the impact of the depth-to-width ratio on their intensity is presented and elucidated. It is found that the Estuarine Circulation exhibits a distinct maximum in medium-wide channels (intermediate depth-towidth ratio depending on various parameters), which is caused by a maximum of the tidal straining contribution. This maximum is related to a strong tidal asymmetry of eddy viscosity and shear created by secondary strain-induced periodic stratification (2SIPS): in medium channels, transverse Circulation generated by lateral density gradients due to laterally differential longitudinal advection induces stable stratification at the end of the flood phase, which is further increased during ebb by longitudinal straining (SIPS). Thus, eddy viscosity is low and shear is strong in the entire ebb phase. During flood, SIPS decreases the stratification so that eddy viscosity is high and shear is weak. The Circulation resulting from this viscosity–shear correlation, the tidal straining Circulation, is oriented like the classical, gravitational Circulation, with riverine outflow at the surface and oceanic inflow close to the bottom. In medium channels, it is about 5 times as strong as in wide (quasi onedimensional) channels, in which 2SIPS is negligible.

  • lateral Circulation generates flood tide stratification and Estuarine exchange flow in a curved tidal inlet
    Journal of Physical Oceanography, 2015
    Co-Authors: Johannes Becherer, Mark T Stacey, Lars Umlauf, Hans Burchard
    Abstract:

    AbstractCross-channel transect measurements of microstructure and velocity in a well-mixed and curved tidal inlet in the German Wadden Sea show the occurrence of significant late flood stratification. This stratification is found to be a result of lateral straining. This study observes a strong single-cell lateral Circulation, which is strongly pronounced at late flood and absent during most of ebb. This tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient. During flood a positive feedback between the terms generates strong lateral Circulation, whereas during ebb a negative feedback leads to a suppression of the cross-channel exchange. A theoretical framework based on vorticity is developed, which allows lateral and longitudinal Circulation to be studied in a consistent way. With this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal Estuarine Circulation, here ide...

  • impact of Estuarine convergence on residual Circulation in tidally energetic estuaries and inlets
    Geophysical Research Letters, 2014
    Co-Authors: Hans Burchard, Elisabeth Schulz, H M Schuttelaars
    Abstract:

    Estuarine convergence (landward reduction of width and/or depth) is known to have the potential to significantly enhance Estuarine Circulation, a result theoretically derived under the assumption of constant eddy viscosity. Recent studies of longitudinally uniform energetic tidal channels indicate that tidal straining, a process driven by tidally varying eddy viscosity, is a major driver of Estuarine Circulation. The combined effect of Estuarine convergence and tidal straining is investigated, for the first time, in this paper. The present idealized numerical study shows that Estuarine convergence is reducing or even reversing tidal straining Circulation in such a way that Estuarine Circulation can be weakened. This is a counterintuitive hydrodynamic effect of Estuarine convergence, which may reduce (rather than increase) up-estuary particulate matter transport in estuaries and tidal inlets.

  • observational evidence for the general presence of Estuarine Circulation in the wadden sea
    Journal of Coastal Research, 2013
    Co-Authors: Gotz Floser, Rolf Riethmuller, Janine J Nauw, Hans Burchard
    Abstract:

    ABSTRACT Floser, G., Nauw, J., Burchard, H., Riethmuller, R., 2013. Observational evidence for the inward transport of suspended matter by Estuarine Circulation in the Wadden Sea. Observational evidence is presented that corroborates the hypothesis that Estuarine Circulation represents a generic process in the Wadden Sea, independent of (the amount of) river runoff. Long-term current velocity data from three locations in the Wadden Sea, taken on moored acoustical current meters, onboard anchored ships and across-channel ship-of-opportunity platforms show in all cases features concurring with the predictions of the theory: a tidally residual outflow of Wadden Sea waters in the upper part and a residual inflow of seawater in the lower part of the water column. This may be regarded as a further confirmation for a generic process to import suspended sediments from the German Bight / North Sea into the Wadden Sea against the prevailing concentration gradient.

H M Schuttelaars - One of the best experts on this subject based on the ideXlab platform.

  • impact of the depth to width ratio of periodically stratified tidal channels on the Estuarine Circulation
    Journal of Physical Oceanography, 2015
    Co-Authors: Elisabeth Schulz, H M Schuttelaars, Ulf Grawe, Hans Burchard
    Abstract:

    The dependency of the Estuarine Circulation on the depth-to-width ratio of a periodically, weakly stratified tidal estuary is systematically investigated here for the first time. Currents, salinity, and other properties are simulated by means of the General Estuarine Transport Model (GETM) in cross-sectional slice mode, applying a symmetric Gaussian-shaped depth profile. The width is varied over four orders of magnitude. The individual along-channel Circulation contributions from tidal straining, gravitation, advection, etc., are calculated and the impact of the depth-to-width ratio on their intensity is presented and elucidated. It is found that the Estuarine Circulation exhibits a distinct maximum in medium-wide channels (intermediate depth-towidth ratio depending on various parameters), which is caused by a maximum of the tidal straining contribution. This maximum is related to a strong tidal asymmetry of eddy viscosity and shear created by secondary strain-induced periodic stratification (2SIPS): in medium channels, transverse Circulation generated by lateral density gradients due to laterally differential longitudinal advection induces stable stratification at the end of the flood phase, which is further increased during ebb by longitudinal straining (SIPS). Thus, eddy viscosity is low and shear is strong in the entire ebb phase. During flood, SIPS decreases the stratification so that eddy viscosity is high and shear is weak. The Circulation resulting from this viscosity–shear correlation, the tidal straining Circulation, is oriented like the classical, gravitational Circulation, with riverine outflow at the surface and oceanic inflow close to the bottom. In medium channels, it is about 5 times as strong as in wide (quasi onedimensional) channels, in which 2SIPS is negligible.

  • impact of Estuarine convergence on residual Circulation in tidally energetic estuaries and inlets
    Geophysical Research Letters, 2014
    Co-Authors: Hans Burchard, Elisabeth Schulz, H M Schuttelaars
    Abstract:

    Estuarine convergence (landward reduction of width and/or depth) is known to have the potential to significantly enhance Estuarine Circulation, a result theoretically derived under the assumption of constant eddy viscosity. Recent studies of longitudinally uniform energetic tidal channels indicate that tidal straining, a process driven by tidally varying eddy viscosity, is a major driver of Estuarine Circulation. The combined effect of Estuarine convergence and tidal straining is investigated, for the first time, in this paper. The present idealized numerical study shows that Estuarine convergence is reducing or even reversing tidal straining Circulation in such a way that Estuarine Circulation can be weakened. This is a counterintuitive hydrodynamic effect of Estuarine convergence, which may reduce (rather than increase) up-estuary particulate matter transport in estuaries and tidal inlets.

  • analysis of tidal straining as driver for Estuarine Circulation in well mixed estuaries
    Journal of Physical Oceanography, 2012
    Co-Authors: Hans Burchard, H M Schuttelaars
    Abstract:

    Tidal straining, which can mathematically be described as the covariance between eddy viscosity and vertical shear of the along-channel velocity component, has been acknowledged as one of the major drivers for Estuarine Circulation in channelized tidally energetic estuaries. In this paper, the authors investigate the role of lateral Circulation for generating this covariance. Five numerical experiments are carried out, starting with a reference scenario including the full physics and four scenarios in which specific key physical processes are neglected. These processes are longitudinal internal pressure gradient forcing, lateral internal pressure gradient forcing, lateral advection, and the neglect of temporal variation of eddy viscosity. The results for the viscosity–shear covariance are correlated across different experiments to quantify the change due to neglect of these key processes. It is found that the lateral advection of vertical shear of the along-channel velocity component and its interaction with the tidally asymmetric eddy viscosity (which is also modified by the lateral Circulation) is the major driving force for Estuarine Circulation in well-mixed tidal estuaries.

  • drivers of residual Estuarine Circulation in tidally energetic estuaries straight and irrotational channels with parabolic cross section
    Journal of Physical Oceanography, 2011
    Co-Authors: Hans Burchard, Robert D Hetland, Elisabeth Schulz, H M Schuttelaars
    Abstract:

    The generation of residual Circulation in a tidally energetic estuary with constant longitudinal salinity gradient and parabolic cross section is examined by means of a two-dimensional cross-sectional numerical model, neglecting river runoff and Stokes drift. It is shown how the longitudinal and lateral residual Circulation can be decomposed into contributions from various processes such as tidal straining Circulation, gravitational Circulation, advectively driven Circulation, and horizontal mixing Circulation. The sensitivity of the residual Circulation and its components from various processes to changes in forcing is investigated by varying the Simpson number (nondimensional longitudinal buoyancy gradient) and the unsteadiness parameter (nondimensional tidal frequency), as well as the bed roughness and the width of the estuary. For relatively weak salinity gradient forcing, the tidal straining Circulation dominates the residual exchange Circulation in support of classical Estuarine Circulation (up-estuary flow near the bed and down-estuary flow near the surface). The strength of the longitudinal Estuarine Circulation clearly increases with increased salinity gradient forcing. However, when the Simpson number exceeds 0.15, the relative contributions of both gravitational Circulation and advectively driven Circulation to Estuarine Circulation increase substantially. Lateral residual Circulation is relatively weak for small Simpson numbers and becomes flood oriented (divergent flow near the bed and convergent flow near the surface) for larger Simpson numbers because of increasing contributions from gravitational and advectively driven Circulation. Increasing the unsteadiness number leads to decreased longitudinal and lateral residual Circulation. Although changes in bed roughness result in relatively small changes in residual Circulation, results are sensitive to the width of the estuary, mainly because of changes in residual exchange Circulation driven by tidal straining.

Jian Shen - One of the best experts on this subject based on the ideXlab platform.

  • nonlinearity of subtidal Estuarine Circulation in the pearl river estuary china
    Frontiers in Marine Science, 2021
    Co-Authors: Jian Shen, Dongxiao Wang, Lin Luo, Bo Hong
    Abstract:

    The Pearl River Estuary (PRE) is a bell-shaped estuary with a narrow deep channel and wide shoals. This unique topographic feature leads different dynamics of the subtidal Estuarine Circulation (SEC) in the PRE compared with a narrow and straight estuary. In this study, the nonlinear dynamics of the SEC in the PRE under mean circumstance are analyzed by using a validated 3-D numerical model. Model results show that the nonlinear advections reach leading order in the along-channel momentum balance. Modulated by tide, the nonlinear advections show significant temporal variations as they have much larger values during spring tide than that during neap tide. Unlike straight and narrow estuaries, both tidally and cross-sectionally averaged axial and lateral advections play important roles in driving the SEC in the PRE in which the axial advection dominates the nonlinear effect. But the two nonlinear terms balance each other largely resulting in a reduced nonlinear effect. Despite, the total nonlinear advection is still comparable with other terms and it acts as the baroclinic pressure to reinforce the SEC, especially during ebb tide, suggesting a flood-ebb asymmetry of the nonlinear dynamics in the PRE. In addition, diagnostic analyses of the along-channel vorticity budget show that nonlinear advections also make significant contribution to drive the lateral Circulation in the PRE as Coriolis and baroclinic pressure terms, indicating complex dynamics of the Circulation in the PRE.

  • transport of riverine material from multiple rivers in the chesapeake bay important control of Estuarine Circulation on the material distribution
    Journal of Geophysical Research, 2017
    Co-Authors: Jian Shen
    Abstract:

    Driven by Estuarine Circulation, material released from lower Chesapeake Bay tributaries has the potential to be transported to the upper Bay. How far and what fraction of the material from tributaries can be carried to the upper estuary have not been quantitatively investigated. For an estuary system with multiple tributaries, the relative contribution from each tributary can provide valuable information for source assessment and fate prediction for riverine materials and passive moving organisms. We conducted long-term numerical simulations using multiple passive tracers that are independently released in the headwater of five main rivers (i.e., Susquehanna, Potomac, Rappahannock, York, and James Rivers) and calculated the relative contribution of each river to the total material in the mainstem. The results show that discharge from Susquehanna River exerts the dominant control on the riverine material throughout the entire mainstem. Despite the smaller contribution from the lower-middle Bay tributaries to the total materials in the mainstem, materials released from these rivers have a high potential to be transported to the middle-upper Bay through the bottom inflow by the persistent Estuarine Circulation. The fraction of the tributary material transported to the upper Bay depends on the location of the tributary. Materials released near the mouth are subject to a rapid flushing process, small retention time, and strong shelf current. Our results reveal three distinct spatial patterns for materials released from the main river, tributary, and coastal oceans. This study highlights the important control of Estuarine Circulation over horizontal and vertical distributions of materials in the mainstem.

  • Water residence time in Chesapeake Bay for 1980-2012
    Journal of Marine Systems, 2016
    Co-Authors: Jian Shen
    Abstract:

    Abstract Concerns have grown over the increase of nutrients and pollutants discharged into the estuaries and coastal seas. The retention and export of these materials inside a system depends on the residence time (RT). A long-term simulation of time-varying RT of the Chesapeake Bay was conducted over the period from 1980 to 2012. The 33-year simulation results show that the mean RT of the entire Chesapeake Bay system ranges from 110 to 264 days, with an average value of 180 days. The RT was larger in the bottom layers than in the surface layers due to the persistent stratification and Estuarine Circulation. A clear seasonal cycle of RT was found, with a much smaller RT in winter than in summer, indicating materials discharged in winter would be quickly transported out of the estuary due to the winter-spring high flow. Large interannual variability of the RT was highly correlated with the variability of river discharge (R 2  = 0.92). The monthly variability of RT can be partially attributed to the variability of Estuarine Circulation. A strengthened Estuarine Circulation results in a larger bottom influx and thus reduces the RT. Wind exerts a significant impact on the RT. The upstream wind is more important in controlling the lateral pattern of RT in the mainstem.

  • a numerical model study of the transport timescale and change of Estuarine Circulation due to waterway constructions in the changjiang estuary china
    Journal of Marine Systems, 2010
    Co-Authors: Ya Wang, Jian Shen
    Abstract:

    Abstract A three-dimensional hydrodynamic model was developed for the Changjiang Estuary and adjacent coastal sea to study the transport timescale and change of Estuarine Circulation due to human activates. The model was calibrated with measured tidal current and salinity forced by observed freshwater discharge and tides. The tracer age was introduced to measure the transport timescale and evaluate the influence of the man-made construction on Estuarine Circulation through a series of numerical experiments under various hydrodynamic conditions. The results show that it takes about 23 and 35 days, respectively, for dissolved substances to be transported from the Xuliujing to the mouth of the Estuary (122°30′ E) under high and low discharge conditions. The transport time increases significantly in the upper portion of the North Passage and the South Passage due to the diversion of the freshwater discharge from the upstream North Channel and the increase of the friction downstream. However, the rate of increasing transport time along the estuary decreases farther towards the mouth of the Estuary, presumably resulting from the enhancement of gravitational Circulation. Two major physical mechanisms that contribute to the transport timescale, tide and river discharge are analyzed and quantified. The results indicate that freshwater discharge is one of the dominant factors controlling the transport timescale in the Changjiang Estuary. The man-made construction has a significant impact on both horizontal Estuarine Circulation and gravitational Circulation, which ultimately influences the Estuarine transport processes. The transport time increased about 50% by man-made constructions, especially near the turbidity maximum. The simulation results provide useful information for understanding the change of transport process and Circulation caused by the man-made construction in the Changjiang Estuary.

  • The Influence of Wind on the Water Age in the Tidal Rappahannock River
    Marine Environmental Research, 2009
    Co-Authors: Wenping Gong, Jian Shen, Bo Hong
    Abstract:

    Wind plays an important role in regulating mixing/stratification, Estuarine Circulation, and transport timescale in estuaries. A three-dimensional model was used to investigate the effect of wind on transport time by using the concept of water age (WA) in the tidal Rappahannock River, a western tributary of the Chesapeake Bay, USA. The model was calibrated for water level, current, and salinity. A series of experiments regarding the effects of wind on WA was conducted under various dynamic conditions. The effect of wind on transport timescale depends strongly on the competition between the wind and buoyancy forcings, and on the pre-status of the Circulation. A down-estuary wind generally decreases WA along the estuary. An up-estuary wind increases WA substantially because it changes the vertical mixing and Estuarine Circulation more significantly. When the buoyancy forcing increases, the up-estuary wind effect decreases whereas the down-estuary wind effect increases. A 2-day period wind pulse with a maximum speed of 15 ms can alter WA for 3 days; but the wind influence on WA lasts up to 40 days in the simulation. Both local and non-local wind forcings alter WA distribution. The local wind enhances vertical mixing and changes the gravitational Circulation in the downstream portion of the estuary whereas it enhances transport in the freshwater portion of the estuary. Consequently, the local wind has a significant impact on WA distribution. In contrast, the non-local wind does not change the gravitational Circulation significantly by imposing setup (setdown) of water level at the open boundary, resulting in a lesser impact on WA distribution.

Parker Maccready - One of the best experts on this subject based on the ideXlab platform.

  • Estuarine Circulation mixing and residence times in the salish sea
    Journal of Geophysical Research, 2021
    Co-Authors: Parker Maccready, Neil S Banas, Ryan M Mccabe, Samantha A Siedlecki, Marvin Lorenz, Sarah N Giddings, Julia Bos, S L Albertson, Soizic Garnier
    Abstract:

    A realistic numerical model is used to study the Circulation and mixing of the Salish Sea, a large, complex Estuarine system on the United States and Canadian west coast. The Salish Sea is biologic...

  • Numerical issues of the Total Exchange Flow (TEF) analysis framework for quantifying Estuarine Circulation
    Copernicus Publications, 2019
    Co-Authors: M. Lorenz, Parker Maccready, K. Klingbeil, H. Burchard
    Abstract:

    For more than a century, Estuarine exchange flow has been quantified by means of the Knudsen relations which connect bulk quantities such as inflow and outflow volume fluxes and salinities. These relations are closely linked to Estuarine mixing. The recently developed Total Exchange Flow (TEF) analysis framework, which uses salinity coordinates to calculate these bulk quantities, allows an exact formulation of the Knudsen relations in realistic cases. There are however numerical issues, since the original method does not converge to the TEF bulk values for an increasing number of salinity classes. In the present study, this problem is investigated and the method of dividing salinities, described by MacCready et al. (2018), is mathematically introduced. A challenging yet compact analytical scenario for a well-mixed Estuarine exchange flow is investigated for both methods, showing the proper convergence of the dividing salinity method. Furthermore, the dividing salinity method is applied to model results of the Baltic Sea to demonstrate the analysis of realistic exchange flows and exchange flows with more than two layers.

  • the Estuarine Circulation
    Annual Review of Fluid Mechanics, 2014
    Co-Authors: Rockwell W Geyer, Parker Maccready
    Abstract:

    Recent research in estuaries challenges the long-standing paradigm of the gravitationally driven Estuarine Circulation. In estuaries with relatively strong tidal forcing and modest buoyancy forcing, the tidal variation in stratification leads to a tidal straining Circulation driven by tidal variation in vertical mixing, with a magnitude that may significantly exceed the gravitational Circulation. For weakly stratified estuaries, vertical and lateral advection are also important contributors to the tidally driven residual Circulation. The apparent contradiction with the conventional paradigm is resolved when the Estuarine parameter space is mapped with respect to a mixing parameter M that is based on the ratio of the tidal timescale to the vertical mixing timescale. Estuaries with high M values exhibit strong tidal nonlinearity, and those with small M values show conventional Estuarine dynamics. Estuaries with intermediate mixing rates show marked transitions between these regimes at timescales of the spri...

  • a model study of the salish sea Estuarine Circulation
    Journal of Physical Oceanography, 2011
    Co-Authors: David A Sutherland, Parker Maccready, Neil S Banas, Lucy F Smedstad
    Abstract:

    ArealistichindcastsimulationoftheSalishSea,whichencompasses theEstuarinesystemsofPugetSound, the Strait ofJuan de Fuca, and the Strait of Georgia, is described for the year 2006. The model shows moderate skill when compared against hydrographic, velocity, and sea surface height observations over tidal and subtidal time scales. Analysis of the velocity and salinity fields allows the structure and variability of the exchange flow to be estimated for the first time from the shelf into the farthest reaches of Puget Sound. This study utilizes the total exchange flow formalism that calculates volume transports and salt fluxes in an isohaline framework, which is then compared to previous estimates of exchange flow in the region. From this analysis, residence time distributions are estimated for Puget Sound and its major basins and are found to be markedly shorter than previous estimates. The difference arises from the ability of the model and the isohaline method for flux calculations to more accurately estimate the exchange flow. In addition, evidence is found to support the previously observed spring‐neap modulation of stratification at the Admiralty Inlet sill. However, the exchange flow calculated increases at spring tides, exactly opposite to the conclusion reached from an Eulerian average of observations.

Patrick F Cummins - One of the best experts on this subject based on the ideXlab platform.

  • fortnightly modulation of the Estuarine Circulation in juan de fuca strait
    Journal of Marine Research, 2000
    Co-Authors: Diane Masson, Patrick F Cummins
    Abstract:

    Riverine discharge into the Strait of Georgia sets up a well-defined Estuarine Circulation within Juan de Fuca Strait, the main path for the freshwater outflow to the continental shelf. At the landward end of Juan de Fuca Strait, the water flows through narrow channels in which strong tidal currents are known to induce significant mixing of the water column, and a spring-neap modulation of the Estuarine exchange. A three-dimensional prognostic numerical model has been developed to study the Circulation around Vancouver Island, British Columbia. In a series of simulations, the Estuarine Circulation within Juan de Fuca Strait is established by the Fraser River freshwater discharge. A fortnightly modulation is imposed on the mixing over the various sills to simulate the spring-neap tidal mixing regime. The resulting variation in the Estuarine Circulation is found to be largely limited to the eastern section of Juan de Fuca Strait, in the vicinity of the sills. Data from current meter moorings and surface salinity data from lighthouse stations compare favorably with the model results. The effect of local wind forcing on the Estuarine exchange is also examined. The model is capable of simulating those rare events during which a concurrence of river freshet, neap tide and northwest wind allows a stronger pulse of fresh surface water to escape relatively unmixed into the eastern end of Juan de Fuca Strait. The disturbance then propagates along the northern shore of the strait as a first mode internal Kelvin wave. Finally, the effect of the fortnightly modulation on the export of freshwater onto the continental shelf is examined. It is found that small amplitude coastal trapped waves are generated near the mouth of Juan de Fuca. However, this fortnightly signal is weak in comparison to the energetic wind-induced variations typically found over the shelf.