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1.
Physical and dynamical oceanography of Liverpool Bay 总被引：1，自引：1，他引：0
The UK National Oceanography Centre has maintained an observatory in Liverpool Bay since August 2002. Over 8 years of observational
measurements are used in conjunction with regional ocean modelling data to describe the physical and dynamical oceanography
of Liverpool Bay and to validate the regional model, POLCOMS. Tidal dynamics and plume buoyancy govern the fate of the fresh
water as it enters the sea, as well as the fate of its sediment, contaminants and nutrient loads. In this context, an overview
and summary of Liverpool Bay tidal dynamics are presented. Freshwater forcing statistics are presented showing that on average
the bay receives 233 m3 s − 1. Though the region is salinity controlled, river input temperature is shown to significantly modulate the plume buoyancy
with a seasonal cycle. Stratification strongly influences the region’s dynamics. Data from long-term moored instrumentation
are used to analyse the stratification statistics that are representative of the region. It is shown that for 65% of tidal
cycles, the region alternates between being vertically mixed and stratified. Plume dynamics are diagnosed from the model and
are presented for the region. The spring–neap modulation of the plume’s westward extent, between 3.5 °W and 4°W, is highlighted. The rapid eastward erosion of the plume during spring tides is identified as a potentially important freshwater
mixing mechanism. Novel climatological maps of temperature, salinity and density from the CTD surveys are presented and used
to validate numerical simulations. The model is found to be sensitive to the freshwater forcing rates, temperature and salinities.
The existing CTD survey grid is shown to not extend sufficiently near the coast to capture the near coastal and vertically
mixed component the plume. Instead the survey grid captures the westward spreading, shallow and transient, portion of the
plume. This transient plume feature is shown in both the long-term averaged model and observational data as a band of stratified
fluid stretching between the mouth of the Mersey towards the Isle of Man. Finally the residual circulation is discussed. Long-term
moored ADCP data are favourably compared with model data, showing the general northward flow of surface water and southward
trajectory of bottom water. 相似文献
2.
An idealised three-dimensional numerical model of the Rhine region of fresh water influence (ROFI) was set up to explore the effect of stratification on the vertical structure of the tidal currents. Prandle’s dynamic Ekman layer model, in the case of zero-depth-averaged, cross-shore velocities, was first used to validate the response of the numerical model in the case of barotropic tidal flow. Prandle’s model predicted rectilinear tidal currents with an ellipse veering of up to 2%. The behaviour of the Rhine ROFI in response to both a neap and a spring tide was then investigated. For the given numerical specifications, the Rhine plume region was well mixed over the vertical on spring tide and stratified on neap tide. During spring conditions, rectilinear tidal surface currents were found along the Dutch coast. In contrast, during neap conditions, significant cross-shore currents and tidal straining were observed. Prandle’s model predicted ellipse veering of 50%, and was found to be a good indicator of ellipticity magnitude as a function of bulk vertical eddy viscosity. The modelled tidal ellipses showed that surface currents rotated anti-cyclonically whereas bottom currents rotated cyclonically. This caused a semi-diurnal cross-shore velocity shearing which was 90° out of phase with the alongshore currents. This cross-shore shear subsequently acted on the horizontal density gradient in the plume, thereby causing a semi-diurnal stratification pattern, with maximum stratification around high water. The same behaviour was exhibited in simulations of a complete spring–neap tidal cycle. This showed a pattern of recurring stratification on neaps and de-stratification on springs, in accordance with observations collected from field campaigns in the 1990’s. To understand the increase in ellipticities to 30% during neaps and the precise shape of the vertical ellipse structure, stratification has to be taken into account. Here, a full three-dimensional numerical model was employed, and was found to represent the effect of de-coupling of the upper and lower layers due to a reduction of mixing at the pycnocline. 相似文献
3.
Liverpool Bay is a region of freshwater influence which receives significant freshwater loading from a number of major English
and Welsh rivers. Strong tidal current flow interacts with a persistent freshwater-induced horizontal density gradient to
produce strain-induced periodic stratification (SIPS). Recent work (Palmer in Ocean Dyn 60:219–226, 2010; Verspecht et al. in Geophys Res Lett 37:L18602, 2010) has identified significant modification to tidal ellipses in Liverpool Bay during stratification due to an associated reduction
in pycnocline eddy viscosity. Palmer (Ocean Dyn 60:219–226, 2010) identified that this modification results in asymmetry in flow in the upper and lower layers capable of permanently transporting
freshwater away from the Welsh coastline via a SIPS pumping mechanism. Observational data from a new set of observations from
the Irish Sea Observatory site B confirm these results; the measured residual flow is 4.0 cm s−1 to the north in the surface mixed layer and 2.4 cm s−1 to the south in the bottom mixed layer. A realistically forced 3D hydrodynamic ocean model POLCOMS succeeds in reproducing
many of the characteristics of flow and vertical density structure at site B and is used to estimate the transport of water
through a transect WT that runs parallel with the Welsh coast. Model results show that SIPS is the dominant steady state,
occurring for 78.2% of the time whilst enduring stratification exists only 21.0% of the year and enduring mixed periods, <1%.
SIPS produces a persistent offshore flow of freshened surface water throughout the year. The estimated net flux of water in
the surface mixed layer is 327 km3 year
−1, of which 281 km3 year−1 is attributable to SIPS periods. Whilst the freshwater component of this flux is small, the net flux of freshwater through
WT during SIPS is significant, the model estimates 1.69 km3 year−1 of freshwater to be transported away from the coast attributable to SIPS periods equivalent to 23% of annual average river
flow from the four catchment areas feeding Liverpool Bay. The results show SIPS pumping to be an important process in determining
the fate of freshwater and associated loads entering Liverpool Bay. 相似文献
4.
Matthew Robert Palmer 《Ocean Dynamics》2010,60(2):219-226
Understanding the fate of freshwater runoff and corresponding nutrient and pollution loads is of critical importance for the
development of accurate predictive models and coastal management tools. A key element of such studies is the identification
and understanding of the interaction between stratification and current structure. This paper presents a new series of measurements
made in the Liverpool Bay region of freshwater influence (ROFI) during spring 2004 where freshwater-maintained horizontal
density gradients and strong tidal currents interact to produce strain-induced periodic stratification (SIPS). During stratification,
tidal current profiles are significantly modified such that the tidal flow deviates from the otherwise rectilinear E–W axis
generating counter rotating upper and lower mixed layers. This feature has often been reported for the Rhine ROFI but not
previously identified in Liverpool Bay despite previous investigation at this site. Investigation of an ongoing long-term
dataset collected nearby reveals this process to be a common feature throughout the year. Liverpool Bay is shown to maintain
three different regimes, long term mixed, long term stratified, and a transitional state when SIPS occurs. The phase of SIPS
relative to the tide results in a residual flow away from the Welsh coastline in the upper water column of 2.3–3.6 cm s−1 with a counterflow in the lower layer of 2.8–3.1 cm s−1 towards the coast. 相似文献
5.
A three-dimensional hydrodynamic model is used to investigate intra-tidal and spring–neap variations of turbulent mixing, stratification and residual circulation in the Chesapeake Bay estuary. Vertical profiles of salinity, velocity and eddy diffusivity show a marked asymmetry between the flood and ebb tides. Tidal mixing in the bottom boundary layer is stronger and penetrates higher on flood than on ebb. This flood–ebb asymmetry results in a north–south asymmetry in turbulent mixing because tidal currents vary out of phase between the lower and upper regions of Chesapeake Bay. The asymmetric tidal mixing causes significant variation of salinity distribution over the flood–ebb tidal cycle but insignificant changes in the residual circulation. Due to the modulation of tidal currents over the spring–neap cycle, turbulent mixing and vertical stratification show large fortnightly and monthly fluctuations. The stratification is not a linear function of the tidal-current amplitude. Strong stratification is only established during those neap tides when low turbulence intensity persists for several days. Residual circulation also shows large variations over the spring–neap cycle. The tidally averaged residual currents are about 50% stronger during the neap tides than during the spring tides. 相似文献
6.
Field observations of flow and sediment transport in a tributary channel through intertidal mudflats indicate that suspended sediment was closely linked to advection and dispersion of a tidal salinity front. During calm weather when tidal forcing was dominant, high concentrations of suspended sediment advected up the mudflat channel in the narrow region between salty water from San Francisco Bay and much fresher runoff from the small local watershed. Salinity and suspended sediment dispersed at similar rates through each tidal inundation, such that during receding ebbs the sediment pulse had spread spatially and maximum concentrations had decreased. Net sediment transport was moderately onshore during the calm weather, as asymmetries in stratification due to tidal straining of the salinity front enhanced deposition, particularly during weaker neap tidal forcing. Sediment transport by tidal forcing was periodically altered by winter storms. During storms, strong winds from the south generated wind waves and temporarily increased suspended sediment concentrations. Increased discharge down the tributary channels due to precipitation had more lasting impact on sediment transport, supplying both buoyancy and fine sediment to the system. Net sediment transport depended on the balance between calm weather tidal forcing and perturbations by episodic storms. Net transport in the tributary channel was generally off-shore during storms and during calm weather spring tides, and on-shore during calm weather neap tides. 相似文献
7.
Observations of semidiurnal internal tidal currents from three moorings deployed on the continental shelf off central Chile during summer and winter of 2005 are reported. The spectra of the baroclinic currents showed large peaks at the semidiurnal band with a dominant counterclockwise rotation, which was consistent with internal wave activity. The amplitude of the barotropic tidal currents varied according to the spring–neap cycle following the sea level fluctuations. In contrast, the amplitudes of the internal tide showed high spatial-temporal variability not directly related to the spring–neap modulation. Near the middle of the continental shelf and near the coast (San Vicente Bay) the variance of the semidiurnal baroclinic current is larger than the variance of its barotropic counterpart. The vertical structure of the baroclinic tidal current fluctuations was similar to the structure of the first baroclinic internal wave mode. In general, in the three study sites the variance of the baroclinic current was larger near the surface and bottom and tended to show a minimum value at mid depths. Kinetic energy related to semidiurnal internal waves was larger in winter when stratification of the water column was stronger. During summer, upwelling and the decrease of freshwater input from nearby rivers reduced the vertical density stratification. The amplitude of the semidiurnal internal tide showed a tendency to be enhanced with increasing stratification as observed in other upwelling areas. The continental shelf break and submarine canyons, which limit the continental shelf in the alongshore direction, represent near-critical slopes for the semidiurnal period and are suggested to be the main internal tide generation sites in the study region. 相似文献
8.
《Continental Shelf Research》1999,19(9):1143-1159
The Oder river discharge into the Pomeranian Bight of the Baltic Sea was investigated in a combined study using satellite data, numerical modelling and shipborne measurements. The aim was to understand the dynamical processes forming the freshwater distribution patterns during the prevailing winds. From an analysis of typical distribution patterns of the river discharge in relation to the main wind directions and in comparison to seasonal wind statistics, the two main transport directions were determined. The prevailing westerly winds produce an onshore transport and a downwind coastal jet which transports the river water along the Polish coast, in certain cases over a distance of 300 km to the Gdansk Bay. During a period of stable westerly winds in June 1994, the calculated time scale for a water transport over 250 km corresponded to the observed time of 12 d. In spring, the period of maximum river runoff, easterly winds dominate and transport occurs along the German coast into the Arkona Sea. The river water is guided by upwelling processes in front of the Polish coast. During occasional north-easterly winds stable plumes form in front of the Swine river mouth; this occurred in May 1991 for several days. The numerical model showed that the stability of the plume is caused by an interaction between the alignment of the coast, the large-scale circulation in the north, the buoyancy of the freshwater and the Coriolis effect. The underlying anticyclonic eddy is indicated by warm rings in a high resolution Landsat Thematic Mapper scene. From the different datasets the range of the spatial and temporal scales of a stable plume were determined. The volume varied between 0.14 and 0.9 km3, and the suspended matter and chlorophyll load between 1120 and 7200 t and 2.8 and 18 t, respectively. These values are important for ecological budget calculations in turnover process studies. 相似文献
9.
In this paper SST imagery and a three-dimensional numerical model of a river plume were employed to detect upwelling induced by tidal straining in the Rhine ROFI (region of fresh water influence). Previous studies have shown that the Rhine ROFI in the North Sea exhibits strong cross-shore density gradients that compete with tidal and wind mixing to establish stratification. During neap periods with low mixing energy an area measuring 30 km offshore by 100 km alongshore becomes stratified. When the ROFI is stratified strong cross-shore currents are observed, with surface currents rotating anti-cyclonically and bottom currents rotating cyclonically. The cross-shore currents interact with the cross-shore density gradients to produce a semi-diurnal cycle of stratification. Due to continuity requirements imposed by the proximity of the coast, the offshore-directed surface currents and onshore-directed bottom currents should lead to coastal upwelling. 相似文献
10.
Residence time of an estuary can be used to estimate the rate of removal of freshwater and pollutants from river inflow. In this study, a calibrated three‐dimensional hydrodynamic model was used to determine residence time in response to the change of freshwater input in Apalachicola Bay. The bay is about 40 km long and 7 km wide, with an average 3 m water depth. Through hydrodynamic model simulations, the spatial and temporal salinity and the total freshwater volume in the bay were calculated. Then the freshwater fraction method was used to estimate the residence time. Results indicate that the residence time in Apalachicola Bay typically ranges between 3 and 10 days for the daily freshwater input ranging from 177 m3/s to 4561 m3/s. Regression analysis of model results shows that an exponential regression equation can be used to correlate the estuarine residence time to changes of freshwater input. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
11.
The spatial and diurnal tidal variability of dissolved organic carbon (DOC) concentrations and the composition of dissolved
organic matter (DOM), as evaluated by high-temperature catalytic oxidation and excitation–emission matrix combined with parallel
factor analysis (EEM–PARAFAC), respectively, were determined in Liverpool Bay. EEM–PARAFAC modeling resulted in six fluorescent
components characterized as terrestrial humic-like (two), microbial humic-like (two), and protein-like (two). The spatial
distributions of DOC and the four humic-like components were negatively correlated with salinity in the high-salinity waters
observed in this study (30.41–33.75), suggesting that terrestrial DOM was conservatively distributed. The spatial patterns
of protein-like components were largely different from those of DOC, humic-like components, and chlorophyll a, suggesting that these distributions were the combined result of production and degradation in the bay in addition to river
inputs. These findings suggest that the DOM dynamics in Liverpool Bay are strongly controlled by river-dominated allochthonous
DOM inputs with some less significant contributions of autochthonous DOM within the bay. In addition, the temporal variations
of DOM associated with the diurnal tidal cycles were determined at one inshore (31.34–32.24 salinity) and one offshore (33.64–33.75
salinity) station in the bay. Negative linear relationships between salinity and DOM characteristics, i.e., DOC, humic-like,
and protein-like components, were observed at the inshore station. In contrast, no relationship was observed at the offshore
station, suggesting that the export of DOM through rivers and possibly tidal flats have a noticeable influence on DOM concentration
and composition up to a relatively elevated salinity of around 33 in Liverpool Bay. 相似文献
12.
Patrick L. Murphy Amy F. Waterhouse Tyler J. Hesser Allison M. Penko Arnoldo Valle-Levinson 《Continental Shelf Research》2009
Spatial and temporal variability of the subtidal exchange flow at West Pass, an inlet at the entrance to a subtropical lagoon (St. Andrew Bay, Florida), was studied using moored and towed current velocity profiles and hydrographic data. Towed and hydrographic measurements were captured over one diurnal tidal cycle to determine intratidal and spatial changes in flow. Hydrographic profiles over the tidal cycle showed that tidal straining modified density stratification asymmetrically, thus setting up the observed mean flow within the inlet. During the towed survey, the inlet's mean flow had a two-layer exchange structure that was moderately frictional and weakly influenced by Coriolis accelerations. Moored current profiles revealed the additional contribution to the dynamics from centrifugal accelerations. Along channel residual flows changed between unidirectional and exchange flow, depending on the forcing from the along-estuary wind stress and, to a lesser extent, the spring–neap tidal cycle. Increases in vertical shear in the along channel subtidal flow coincided with neap tides and rain pulses. Lateral subtidal flows showed the influence on the dynamics of centrifugal accelerations through a well-developed two-layer structure modulated in magnitude by the spring–neap tidal cycle. 相似文献
13.
Liverpool Bay, a region of freshwater influence subject to semi-diurnal and enduring periods of stratification, is home to
a long-term coastal observatory. The observatory provides a new array of data which include vertical profiles of velocity
from an Acoustic Doppler Current Profiler and a high frequency radar system (which provides measurements of surface currents).
Using this dataset in conjunction with an analytical potential energy model that uses advances in the formulation of a freshwater
buoyancy term, the processes controlling stratification can be assessed. The results indicate that a depth-resolving freshwater
buoyancy term should be used for the calculation stratification. Advection, in addition to depth-mean straining, is an important
process affecting the stratification in Liverpool Bay. Specifically, when semi-diurnal stratification occurs, the two terms
are in phase whilst when enduring stratification occurs, they are out of phase. The phase of the advective component, and
thus its influence relative to depth-mean straining, was found to be a function of the vertical variation of the horizontal
density gradient. 相似文献
14.
Naomi Greenwood David J. Hydes Claire Mahaffey Andrew Wither Jon Barry David B. Sivyer David J. Pearce Susan E. Hartman Olga Andres Helen E. Lees 《Ocean Dynamics》2011,61(12):2181-2199
This paper presents data for the temporal and spatial distribution of nutrients in Liverpool Bay between 2003 and 2009 and
an analysis of inputs of nutrients from the major rivers. The spatial distribution of winter nutrient concentrations are controlled
by the region of freshwater influence (ROFI) in Liverpool Bay through the mixing of riverine freshwater and Irish Sea water,
with strong linear relationships between nutrient concentration and salinity between December and February. The location of
highest spring and summer phytoplankton biomass reflects the nutrient distributions as controlled by the ROFI. Analysis of
7 years of data showed that the seasonal cycle of winter maximum nutrient concentrations in February and drawdown in April/May
is a recurrent feature of this location, with the timing of the drawdown varying by several weeks between years. A comparison
of observed nutrient concentrations in Liverpool Bay with those predicted from inputs from rivers has been presented. Nutrient
concentrations in the rivers flowing into Liverpool Bay were highly variable and there was reasonable agreement between predicted
freshwater nutrient concentrations using data from this study and riverine nutrient concentrations weighted on the basis of
river flow, although the exact nature of mixing between the rivers could not be determined. Predicted Irish Sea nutrient concentrations
in the winter were lower than those reported for the input waters of the North Atlantic, supporting findings from previous
work that nitrogen is lost through denitrification in the Irish Sea. 相似文献
15.
Luiz Bruner de Miranda Alessandro Luvizon Bérgamo Belmiro Mendes de Castro 《Ocean Dynamics》2005,55(5-6):430-440
Observations of thermohaline properties and currents were undertaken in the Curimataú River estuary (6°18′S), Rio Grande do
Norte state (RN), Brazil, during consecutive neap–spring tidal cycles in the austral autumn rainy season. Highly asymmetric
neap tide along channel velocities (−0.4 to 0.9 m s−1) and highly stratified conditions were generated by an increase of the buoyancy energy from the freshwater input (R
iE≈5.6). During the spring-tidal cycle the river discharge decreased and the longitudinal velocity components were higher, less
asymmetrical (−0.8 to 1.1 m s−1) and semidiurnal, associated with moderately stratified conditions (R
iE≈0.1) due to the increase of the kinetic tidal energy forcing mechanism. The overall salinity variation from surface to bottom
during two tidal cycles was from 20.5 to 36.3 and 29 to 36.7 in the neap and spring tide experiments, respectively; in the
last experiment, the tropical water (TW) mass intrusion was enhanced. The net salt transport reversed from down to up estuary
during the neap and spring tide experiments, respectively, varied from 6.0 to –2.0 kg m−1 s−1, an indication of changes in the main forcing of the estuary dynamics. Evaluation of a classical steady analytical model,
in comparison with nearly steady experimental vertical profiles of velocity, shows an agreement classifiable as reasonably
fair. 相似文献
16.
Data are presented on long-term salinity behaviour in San Francisco Bay, California. A two-level, width averaged model of the tidally averaged salinity and circulation has been written in order to interpret the long-term (days to decades) salinity variability. The model has been used to simulate daily averaged salinity in the upper and lower levels of a 51 segment discretization of the Bay over the 22-yr period 1967–1988. Monthly averaged surface salinity from observations and monthly-averaged simulated salinity are in reasonable agreement. Good agreement is obtained from comparison with daily averaged salinity measured in the upper reaches of North Bay.The salinity variability is driven primarily by freshwater inflow with relatively minor oceanic influence. All stations exhibit a marked seasonal cycle in accordance with the Mediterranean climate, as well as a rich spectrum of variability due to extreme inflow events and extended periods of drought. Monthly averaged salinity intrusion positions have a pronounced seasonal variability and show an approximately linear response to the logarithm of monthly averaged Delta inflow. Although few observed data are available for studies of long-term salinity stratification, modelled stratification is found to be strongly dependent on freshwater inflow; the nature of that dependence varies throughout the Bay. Near the Golden Gate, stratification tends to increase up to very high inflows. In the central reaches of North Bay, modelled stratification maximizes as a function of inflow and further inflow reduces stratification. Near the head of North Bay, lowest summer inflows are associated with the greatest modelled stratification. Observations from the central reaches of North Bay show marked spring-neap variations in stratification and gravitational circulation, both being stronger at neap tides. This spring-neap variation is simulated by the model. A feature of the modelled stratification is a hysteresis in which, for a given spring-neap tidal range and fairly steady inflows, the stratification is higher progressing from neaps to springs than from springs to neaps.The simulated responses of the Bay to perturbations in coastal sea salinity and Delta inflow have been used to further delineate the time-scales of salinity variability. Simulations have been performed about low inflow, steady-state conditions for both salinity and Delta inflow perturbations. For salinity perturbations a small, sinusoidal salinity signal with a period of 1 yr has been applied at the coastal boundary as well as a pulse of salinity with a duration of one day. For Delta inflow perturbations a small, sinusoidally varying inflow signal with a period of 1 yr has been superimposed on an otherwise constant Delta inflow, as well as a pulse of inflow with a duration of one day. Perturbations in coastal salinity dissipate as they move through the Bay. Seasonal perturbations require about 40–45 days to propagate from the coastal ocean to the Delta and to the head of South Bay. The response times of the model to perturbations in freshwater inflow are faster than this in North Bay and comparable in South Bay. In North Bay, time-scales are consistent with advection due to lower level, up-estuary transport of coastal salinity perturbations; for inflow perturbations, faster response times arise from both upper level, down-estuary advection and much faster, down-estuary migration of isohalines in response to inflow volume continuity. In South Bay, the dominant time-scales are governed by tidal dispersion. 相似文献
17.
Modelling travel and residence times in the eastern Irish Sea 总被引：2，自引：0，他引：2
The Irish Sea, which lies between 51 degrees N-56 degrees N and 2 degrees 50'W-7 degrees W, provides a sheltered environment to exploit valuable fisheries resource. Anthropogenic activity is a real threat to its water quality. The majority of freshwater input down rivers flows into the eastern Irish Sea. The structure of the water circulation was not well understood during the planning of Sellafield nuclear plant outfall site in the eastern Irish Sea. A three-dimensional primitive equation numerical model was applied to the Irish Sea to simulate both barotropic and baroclinic circulation within the region. High accuracy was achieved with regard to the prediction of both tidal circulation and surface and nearbed water temperatures across the region. The model properly represented the Western Irish Sea Gyre, induced by thermal stratification and not known during planning Sellafield. Passive tracer simulations based on the developed hydrodynamic model were used to deliver residence times of the eastern Irish Sea region for various times of the year as well as travel times from the Sellafield outfall site to various locations within the Irish Sea. The results indicate a strong seasonal variability of travel times from Sellafield to the examined locations. Travel time to the Clyde Sea is the shortest for the autumnal tracer release (90 days); it takes almost a year for the tracer to arrive at the same location if it is released in January. Travel times from Sellafield to Dublin Bay fall within the range of 180-360 days. The average residence time of the entire eastern Irish Sea is around 7 months. The areas surrounding the Isle of Man are initially flushed due to a predominant northward flow; a backwater is formed in Liverpool Bay. Thus, elevated tracer concentrations are predicted in Liverpool Bay in the case of accidental spills at the Sellafield outfall site. 相似文献
18.
The turbulent kinetic energy dissipation rate, ε, in tidal seas is maximum at the bottom during full flood and during full ebb, i.e. when tidal currents are strongest. In
coastal regions with tides similar to a Kelvin wave, this coincides with high water and low water. If there is a freshwater
source at the coast, stratification in such a region will be most stable at high water and least at low water. Measurements
of ε in the Rhine region of freshwater influence performed by previous studies have revealed bottom maxima at both high and low
water. In addition, a maximum in the upper half of the water column was found around high water, which cannot be explained
by tidal shear at the bottom, convective instabilities or wind mixing. This study investigates the dissipation rate and relevant
physical properties in the Rhine region of freshwater influence by means of three-dimensional numerical simulations using
the General Estuarine Transport Model and idealised conditions. The measurements are well reproduced; two distinct peaks of
ε are evident in the upper layer shortly before and after high water. These maxima turn out to be due to strong peaks in the
alongshore shear occurring when the fore- and the back-front of the plume transit the water column. 相似文献
19.
Acoustic Doppler current profiles and current meter data are combined with wind observations to describe the transport of water leaving Florida Bay and moving onto the inner shelf on the Atlantic side of the Florida Keys. A 275-day study in the Long Key Channel reveals strong tidal exchanges, but the average ebb tide volume leaving Florida Bay is 19% greater than the average flood tide volume entering the bay. The long-term net outflow averages 472 m3 s−1. Two studies in shelf waters describe the response to wind forcing during spring and summer months in 2004 and during fall and winter months in 2004–2005. During the spring–summer study, southeasterly winds have a distinct shoreward component, and a two-layer pattern appears. Surface layers move shoreward while near-bottom layers move seaward. During the winter study, the resultant wind direction is parallel to the Keys and to the local isobaths. The entire water column moves in a nearly downwind direction, and across-shelf transport is relatively small. During the summer wet season, Florida Bay water should be warmer, fresher, and thus less dense than Atlantic shelf waters. Ebbing bay water should move onto the shelf as a buoyant plume and be held close to the Keys by southeasterly winds. During the winter dry season, colder and saltier Florida Bay water should leave the tidal channels with relatively high density and be concentrated in the near-bottom layers. But little across-shelf flow occurs with northeasterly winds. The study suggests that seasonally changing wind forcing and hydrographic conditions serve to insulate the reef tract from the impact of low-quality bay water. 相似文献
20.
The response of the Chesapeake Bay to river discharge under the influence and absence of tide is simulated with a numerical model. Four numerical experiments are examined: (1) response to river discharge only; (2) response to river discharge plus an ambient coastal current along the shelf outside the bay; (3) response to river discharge and tidal forcing; and (4) response to river discharge, tidal forcing, and ambient coastal current. The general salinity distribution in the four cases is similar to observations inside the bay. Observed features, such as low salinity in the western side of the bay, are consistent in model results. Also, a typical estuarine circulation with seaward current in the upper layer and landward current in the lower layer is obtained in the four cases. The two cases without tide produce stronger subtidal currents than the cases with tide owing to greater frictional effects in the cases with tide. Differences in salinity distributions among the four cases appear mostly outside the bay in terms of the outflow plume structure. The two cases without tide produce an upstream (as in a Kelvin wave sense) or northward branch of the outflow plume, while the cases with tide produce an expected downstream or southward plume. Increased friction in the cases with tide changes the vertical structure of outflow at the entrance to the bay and induces large horizontal variations in the exchange flow. Consequently, the outflow from the bay is more influenced by the bottom than in the cases without tide. Therefore, a tendency for a bottom-advected plume appears in the cases with tide, rather than a surface-advected plume, which develops in the cases without tide. Further analysis shows that the tidal current favors a salt balance between the horizontal and vertical advection of salinity around the plume and hinders the upstream expansion of the plume outside the bay. 相似文献