On the Tide-Induced Property Flux: Can It Be Locally Countergradient?*

Ou, Hsien-Wang; Chen, Dake

doi:10.1175/1520-0485(2000)030<1472:ottipf>2.0.co;2

Cited by 10 publications

(16 citation statements)

References 5 publications

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“…At sections A and C, the strongly tidal oscillatory salt transport occurs near the interface of the inflow layer and outflow layer. This is different from the structure caused by tidal shear dispersion, which is negative near boundaries and positive away from boundaries (Larsen 1977;Ou et al 2000;Bowen and Geyer 2003).…”

Section: B Mechanisms Of F T During Neap Tidementioning

confidence: 60%

“…For the shear dispersion mechanism, the mixing perpendicular to the shear causes the phase shift between tidal velocity and salinity that vary through the cross section in such a way as to produce a net upstream salt transport (Larsen 1977;Ou et al 2000;Bowen and Geyer 2003). The phase shift is greater than 908 in the slow-moving fluid near boundaries and less than 908 away from boundaries, inducing negative tidal oscillatory salt transport near boundaries and positive tidal oscillatory salt transport away from boundaries (Fig.…”

Section: Mechanisms Of F T During Spring Tidementioning

confidence: 99%

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Mechanisms of Tidal Oscillatory Salt Transport in a Partially Stratified Estuary

Wang

Geyer

Engel

et al. 2015

Journal of Physical Oceanography

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Tidal oscillatory salt transport, induced by the correlation between tidal variations in salinity and velocity, is an important term for the subtidal salt balance under the commonly used Eulerian method of salt transport decomposition. In this paper, its mechanisms in a partially stratified estuary are investigated with a numerical model of the Hudson estuary. During neap tides, when the estuary is strongly stratified, the tidal oscillatory salt transport is mainly due to the hydraulic response of the halocline to the longitudinal variation of topography. This mechanism does not involve vertical mixing, so it should not be regarded as oscillatory shear dispersion, but instead it should be regarded as advective transport of salt, which results from the vertical distortion of exchange flow obtained in the Eulerian decomposition by vertical fluctuations of the halocline. During spring tides, the estuary is weakly stratified, and vertical mixing plays a significant role in the tidal variation of salinity. In the spring tide regime, the tidal oscillatory salt transport is mainly due to oscillatory shear dispersion. In addition, the transient lateral circulation near large channel curvature causes the transverse tilt of the halocline. This mechanism has little effect on the cross-sectionally integrated tidal oscillatory salt transport, but it results in an apparent left-right cross-channel asymmetry of tidal oscillatory salt transport. With the isohaline framework, tidal oscillatory salt transport can be regarded as a part of the net estuarine salt transport, and the Lagrangian advective mechanism and dispersive mechanism can be distinguished.

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Section: B Mechanisms Of F T During Neap Tidementioning

confidence: 60%

Section: Mechanisms Of F T During Spring Tidementioning

confidence: 99%

Mechanisms of Tidal Oscillatory Salt Transport in a Partially Stratified Estuary

Wang

Geyer

Engel

et al. 2015

Journal of Physical Oceanography

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“…Vertical tidal oscillatory shear dispersion results from the straining and vertical mixing of the salinity field by vertically sheared oscillatory tidal currents (Okubo 1967;Larsen 1977;Fischer et al 1979;Ou et al 2000). The longitudinal dispersion rate associated with tidal oscillatory shear dispersion is maximal when the tidal period matches the vertical mixing time scale.…”

Section: B Tidal Oscillatory Salt Fluxmentioning

confidence: 99%

“…The contour interval is 5 ϫ 10 Ϫ1 kg m Ϫ2 s Ϫ1 in (c) and 2 ϫ 10 Ϫ1 kg m spatial structure is consistent with tidal oscillatory shear dispersion. The flux near the bottom boundary layer is countergradient (negative) as a consequence of tidal currents near the bottom leading currents higher up in the water column (Larsen 1977;Ou et al 2000). The salt flux increases toward the surface where the tidal currents are stronger, and the area-integrated value is downgradient (positive).…”

mentioning

confidence: 99%

Mechanisms Driving the Time-Dependent Salt Flux in a Partially Stratified Estuary*

Lerczak

Geyer

Chant

2006

Journal of Physical Oceanography

182

234

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The subtidal salt balance and the mechanisms driving the downgradient salt flux in the Hudson River estuary are investigated using measurements from a cross-channel mooring array of current meters, temperature and conductivity sensors, and cross-channel and along-estuary shipboard surveys obtained during the spring of 2002. Steady (subtidal) vertical shear dispersion, resulting from the estuarine exchange flow, was the dominant mechanism driving the downgradient salt flux, and varied by over an order of magnitude over the spring-neap cycle, with maximum values during neap tides and minimum values during spring tides. Corresponding longitudinal dispersion rates were as big as 2500 m 2 s Ϫ1 during neap tides. The salinity intrusion was not in a steady balance during the study period. During spring tides, the oceanward advective salt flux resulting from the net outflow balanced the time rate of change of salt content landward of the study site, and salt was flushed out of the estuary. During neap tides, the landward steady shear dispersion salt flux exceeded the oceanward advective salt flux, and salt entered the estuary. Factor-of-4 variations in the salt content occurred at the spring-neap time scale and at the time scale of variations in the net outflow. On average, the salt flux resulting from tidal correlations between currents and salinity (tidal oscillatory salt flux) was an order of magnitude smaller than that resulting from steady shear dispersion. During neap tides, this flux was minimal (or slightly countergradient) and was due to correlations between tidal currents and vertical excursions of the halocline. During spring tides, the tidal oscillatory salt flux was driven primarily by oscillatory shear dispersion, with an associated longitudinal dispersion rate of about 130 m 2 s Ϫ1.

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“…Ralston and Stacy [2005] discuss the importance of lateral circulation to salt flux and observed that dispersion due to vertical shear was a maximum during stratified ebbs. Larsen [1977] and Ou et al [2000] show the cross-sectional structure of shear dispersion, associating localized up-gradient tidal fluxes with bottom or side boundary layers in which the current phase leads the rest of the channel. Bowen and Geyer [2003] found shear dispersion responsible for three quarters of the tidal salt flux in a section of their modeled Hudson that had little along channel variation in bathymetry.…”

Section: Introductionmentioning

confidence: 99%

Spatial and temporal variability of tide-induced salt flux in a partially mixed estuary

Engel¹

2009

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Mechanisms for the tidal component of salt flux in the Hudson River estuary are investigated using a 3D numerical model. Variations with river discharge, fortnightly tidal forcing, and along channel variability are explored. Four river discharge conditions were considered: 1200 m 3 s -1 , 600 m 3 s -1 , 300 m 3 s -1 , 150 m 3 s -1 . Tide-induced residual salt flux was found to be variable along the channel, with locations of counter-gradient flux during both neap and spring tide. The magnitude of tidal salt flux scales with the river flow and has no clear dependence on the spring-neap tidal forcing. The diffusive fraction, ν, has a value of -0.25 to 0.46 in the lower estuary, increasing to -0.23 to 1 near the head of salt. The phase lag between tidal salinity and velocity is analyzed at three cross-sections with: large positive, negative, and weak tidal salt flux. The composite Froude number, G 2 , is calculated along the channel and indicates nearly ubiquitous supercritical flow for maximum flood and ebb during both neap and spring tides. Subcritical flow occurs during slack water and at geographically locked locations during neap floods. Application of two-layer, frictional hydraulic theory reveals how variations in channel width and depth generate tidal asymmetries in cross-sectional salinity, the key ingredient of tidal salt flux.3 4

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On the Tide-Induced Property Flux: Can It Be Locally Countergradient?*

Cited by 10 publications

References 5 publications

Mechanisms of Tidal Oscillatory Salt Transport in a Partially Stratified Estuary

Mechanisms of Tidal Oscillatory Salt Transport in a Partially Stratified Estuary

Mechanisms Driving the Time-Dependent Salt Flux in a Partially Stratified Estuary*

Spatial and temporal variability of tide-induced salt flux in a partially mixed estuary

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