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1.
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   

2.
An experimental study of temperature cycles and the heat budget in the Duplin River, a tidal creek bordered by extensive intertidal salt marshes, was carried out in late summer of 2003 and spring of 2004 near Sapelo Island on the central Georgia coast in the southeastern US. Three water masses are identified with differing temperature and salinity regimes, the characteristics of which are dictated by channel morphology, tidal communication with the neighboring sound, ground water hydrology, the extent of local intertidal salt marshes and side channels and the spring–neap tidal cycle (which controls both energetic mixing and, presumably, ground water input). For the first experiment, heat budgets are constructed for the upper (warmer) and lower (cooler) areas of the Duplin River showing the diminishing importance of tidal advection away from the mouth of the creek along with the concomitant increase in the importance of both direct atmospheric fluxes and of interactions with the marsh and side creeks. The second experiment, in the spring of 2004, reexamines the heat budget on seasonal and daily averaged scales revealing the decreased importance of advective fluxes relative to direct atmospheric fluxes on this scale but the constant importance of marsh/creek interactions regardless of time scale or season. Short period temperature fluctuations which affect larval development are examined and analogies are drawn to use heat to understand the marsh as a source of sediment, carbon and other nutrients.  相似文献   

3.
Observations of surface velocity data from August 2002 to February 2004 were collected by a series of four long-range high-frequency (HF) radars along the coast of New Jersey. The shelf observations of the central Mid-Atlantic Bight (MAB) were compared to historical observations of surface flow characteristics in the area. The time-averaged spatial mean velocity of 4 cm/s in the down-shelf along-shelf direction and 3 cm/s in the offshore across-shelf direction compared very well to historical surface measurements in the study region. However, as the spatial resolution of the data set revealed, this simple measure masked significant spatial variations in the overall and seasonal mean flow structures. Three regions – the south bank of the Hudson Shelf Valley, the southern New Jersey inner shelf (LEO-15) region, and the region offshore of the Delaware Bay mouth (southwest corner) – had mean flows that favor offshore transport of surface water. In terms of temporal variability, maps of the principle axes showed that the across-shelf (minor) axis contribution was not insignificant in the surface layer ranging from 0.3 to 0.9 of along-shelf (major) axis and that there were seasonal differences in orientation and ellipticity. Analysis of the spatial changes in the temporal and spatial correlation scales over the shelf showed that shelf position, in addition to site separation, contributed to the differences in these properties. Furthermore, observations over the Hudson Shelf Valley region suggested that this was a region of transition in which the orientation of along- and across-shelf components begin to change.  相似文献   

4.
We examine multi-year conductivity-temperature-depth (CTD) data to better understand temperature and salinity variability over the central Bering Sea shelf. Particular consideration is given to observations made annually from 2002 to 2007 between August and October, although other seasons and years are also considered. Vertical and horizontal correlation maps show that near-surface and near-bottom salinity anomalies tend to fluctuate in phase across the central shelf, but that temperature anomalies are vertically coherent only in the weakly or unstratified inner-shelf waters. We formulate heat content (HC) and freshwater content (FWC) budgets based on the CTD observations, direct estimates of external fluxes (surface heat fluxes, ice melt, precipitation (P), evaporation (E) and river discharge), and indirect estimates of advective contributions. Ice melt, P−E, river discharge, and along-isobath advection are sufficient to account for the mean spring-to-fall increase in FWC, while summer surface heat fluxes are primarily responsible for the mean seasonal increase in HC, although interannual variability in the HC at the end of summer appears related to variability in the along-isobath advection during the summer months. On the other hand, FWC anomalies at the end of summer are significantly correlated with the mean wind direction and cross-isobath Ekman transport averaged over the previous winter. Consistent with the latter finding, salinities exhibit a weak but significant inverse correlation between the coastal and mid-shelf waters. The cross-shelf transport likely has significant effect on nutrient fluxes and other processes important to the functioning of the shelf ecosystem. Both the summer and winter advection fields appear to result from the seasonal mean position and strength of the Aleutian Low. We find that interannual thermal and haline variability over the central Bering Sea shelf are largely uncoupled.  相似文献   

5.
The geomorphology of the southern Yellow Sea（SYS） is characterized by offshore radial sand ridges（RSR）.An offshore tidal channel（KSY Channel） is located perpendicular to the coast,comprised of a main and a tributary channel separated by a submarine sand ridge（KSY Sand Ridge） extending seaward.In order to investigate the interactions among water flow,sediment transport,and topography,current velocity and suspended sediment concentration（SSC） were observed at 11 anchor stations along KSY Channel in RSR during a spring tide cycle.High resolution bottom topography was also surveyed.Residual currents and tidally averaged suspended sediment fluxes were calculated and analyzed by using the decomposition method.Results suggested that the water currents became stronger landward but with asymmetrical current speed and temporal duration of flood and ebb tides.Residual currents showed landward water transport in the nearshore channel and a clockwise circulation around the KSY Sand Ridge.Tidally-averaged SSC also increased landward along the channel.The main mechanisms controlling SSC variations were resuspension and horizontal advection,with spatial and temporal variations in the channel,which also contributed to sediment redistribution between channels and sand ridges.Residual flow transport and the tidal pumping effect dominated the suspended sediment flux in the KSY Channel.The KSY Sand Ridge had a potential southward migration due to the interaction between water flow,sediment transport,and topography.  相似文献   

6.
The physical processes affecting the development of seasonal hypoxia over the Louisiana-Texas shelf were examined using a high-resolution, three-dimensional, unstructured-grid, Finite Volume Coastal Ocean Model (FVCOM). The model was forced with the observed freshwater fluxes from the Mississippi and Atchafalaya Rivers, surface winds, heat fluxes, tides and offshore conditions. The simulations were carried out over a six-month period, from April to September 2002, and the model performance was evaluated against several independent series of observations that included tidal gauge data, Acoustic Doppler Current Profiler (ADCP) data, shipboard measurements of temperature and salinity, vertical salinity and sigma-t profiles, and satellite imagery. The model accurately described the offshore circulation mode generated over the Louisiana-Texas shelf by the westerly winds during summer months, as well as the prevalent westward flow along the coast caused by the easterly winds during the rest of the study period. The seasonal cycle of stratification also was well represented by the model. During 2002, the stratification was initiated in early spring and subsequently enhanced by the intensity and phasing of riverine freshwater discharges. Strong stratification persisted throughout the summer and was finally broken down in September by tropical storms. The model simulations also revealed a quasi-permanent anticyclonic gyre in the Louisiana Bight region formed by the rotational transformation of the Mississippi River plume, whose existence during 2002 was supported by the satellite imagery and ADCP current measurements. Model simulations support the conclusion that local wind forcing and buoyancy flux resulting from riverine freshwater discharges were the dominant mechanisms affecting the circulation and stratification over the inner Louisiana-Texas shelf.  相似文献   

7.
Halifax Harbour is located on the Atlantic coast of Nova Scotia, Canada. It is one of the world’s largest, ice-free natural harbours and of great economic importance to the region. A good understanding of the physical processes controlling tides, flooding, transport and dispersion, and hydrographic variability is required for pollution control and sustainable development of the Harbour. For the first time, a multi-nested, finite difference coastal ocean circulation model is used to reconstruct the three-dimensional circulation and hydrography of the Harbour and its variability on timescales of hours to months for 2006. The model is driven by tides, wind and sea level pressure, air-sea fluxes of heat, and terrestrial buoyancy fluxes associated with river and sewage discharge. The predictive skill of the model is assessed by comparing the model simulations with independent observations of sea level from coastal tide gauges and currents from moored instruments. The simulated hydrography is also compared against a new monthly climatology created from all available temperature and salinity observations made in the Harbour over the last century. It is shown that the model can reproduce accurately the main features of the observed tides and storm surge, seasonal mean circulation and hydrography, and wind driven variations. The model is next used to examine the main physical processes controlling the circulation and hydrography of the Harbour. It is shown that non-linear interaction between tidal currents and complex topography occurs over the Narrows. The overall circulation can be characterized as a two-layer estuarine circulation with seaward flow in the thin upper layer and landward flow in the broad lower layer. An important component of this estuarine circulation is a relatively strong, vertically sheared jet situated over a narrow sill connecting the inner Harbour to the deep and relatively quiescent Bedford Basin. Local wind driven variability is strongest in winter as expected but it is also shown that a significant part of the temperature and salinity variability is driven by physical processes occurring on the adjacent inner continental shelf, especially during storm and coastal upwelling events.  相似文献   

8.
Current meter data from a 411-day study are used to characterize the movement of water through Tavernier Creek, a tidal channel in the Upper Florida Keys that connects northeastern Florida Bay with the narrow continental shelf on the Atlantic Ocean side of the Keys. The record reveals active tidal and nontidal exchanges. Strongest flood and ebb current speeds commonly reach 50 cm s−1. Low-frequency exchanges are highly coherent with the across-shelf component of local wind stress over time scales in excess of 2.5 days. Bay-shelf exchanges are investigated in four ways. Current measurements made while a drogue was tracked from one end of the creek to the other provide a relationship that can be applied to the time series of current meter data. Results suggest that ocean water reaches the bay end of the creek on 92% of the floods, and bay water reaches the ocean end on 94% of the ebbs. The Eulerian tidal excursion calculated from the amplitude of the M 2 tidal constituent is 1.42 times the length of the channel, and half-tidal cycle Eulerian displacements are commonly 1.5 times the length of the channel. Salinity measurements over a 165-day period document the arrival of bay and ocean water at a study site at about the midpoint of Tavernier Creek. Results suggest that the creek becomes completely flushed after about 450,000 m3 of water have entered from either end. Histograms of ebb and flood volume transports indicate that half-tidal cycle transports are commonly between 800,000 and 1,100,000 m3. The long-term movement of water through Tavernier Creek is a net outflow from Florida Bay. Results support the idea that Tavernier Creek serves as an effective conduit for exchanging bay and ocean water, and especially for draining the northeast corner of Florida Bay.  相似文献   

9.
TOPEX/Poseidon/Jason1 (T/P/J) sea surface height (SSH) measurements along tracks 91 and 15, crossing the wide West Florida continental shelf (WFS), were used to estimate seasonal across-shelf SSH gradients. SSH gradients and the knowledge that geostrophic flow approximately follows the isobaths enable estimation of the seasonal along-isobath geostrophic flows. The calculated along-isobath geostrophic flows are southeastward from December to March and northwestward in June, August, and September. The along-isobath geostrophic component of the flow is most likely small during the remaining months and, thus, not discernable in T/P/J SSH measurements. In agreement with previous theoretical, modeling, and observational work, the mid-shelf seasonal surface flow appears to be driven largely by the seasonal along-shore wind stress. Theory for flow driven by seasonal heat flux suggests negligible flow near the surface and on the bulk of the shelf away from the shelf break.  相似文献   

10.
Hydrographic variability on the Alabama shelf just outside of Mobile Bay, a major source of river discharge into the Gulf of Mexico, is examined using time series of water column temperature and surface and bottom salinity from a mooring site with a depth of 20 m in conjunction with a series of across-shelf CTD surveys. The time series data show variability in a range of time scales. The density variation is affected by both salinity and temperature, with its relatively strong annual signal mostly determined by temperature and its year to year variability mostly determined by salinity. Seasonal mean structures of temperature, salinity, and density show a transition from estuarine to shelf conditions in which three regions with distinct seasonal characteristics in their horizontal and vertical gradient structures are identified. Correlation analysis with the available forcing functions demonstrates the influence of Mobile Bay on the variability at the mooring site. At low frequencies, river discharge from Mobile Bay has a varying influence on salinity, which is absent during the periods with unusually low discharge. At shorter synoptic time scales, both the estuarine response to the across-shelf wind stress and the shelf response to the along-shelf wind stress are significantly correlated with temperature/salinity variability: the former becoming important for the surface layer during winter whereas the latter for the bottom layer during both winter and summer. These forcing functions are important players in determining the estuarine-shelf exchange, which in turn is found to contribute to the shelf hydrographic structure.  相似文献   

11.
The formation and dynamics of turbidity maxima (TM) in the Pearl River estuary (PRE) are not well understood but the existence of TM in the estuary have great potential engineering and environmental impacts. Based on the measurements of two large-scale field surveys conducted in the wet season of 1978 (July of 1978) and the dry season of 1979 (March of 1979), the existence and seasonal variations of TM in the PRE focusing on the two major natural navigation channels have been studied. The sediment transport fluxes over two consecutive tidal cycles have been analyzed in detail. The analysis results reveal that the formation mechanisms of TM in the estuary are rather complex. In general, gravitational circulation, tidal trapping, and sediment resuspension and deposition processes are the primary TM formation mechanisms in the PRE. The clockwise back flow pattern around Lingding Island also leads to the formation of TM in the West channel of the PRE. The occurrence of TM far upstream of the salt water wedge is the result of the complex hydrodynamic and sediment transport processes generated by the runoff of the major rivers.  相似文献   

12.
Data from time series of transects made over a tidal period across a section of the upper Chesapeake Bay, USA, reveal the influence of lateral dynamics on sediment transport in an area with a deep channel and broad extents of shallower flanks. Contributions to lateral momentum by rotation (Coriolis plus channel curvature), cross channel density gradients and cross channel surface slope were estimated, and the friction and acceleration terms needed to complete the balance were compared to patterns of observed lateral circulation. During ebb, net rotation effects were larger because of river velocity and reinforcement of Coriolis by curvature. During flood, stratification was greater because of landward advection of strong vertical density gradients. Together, the ebb intensified lateral circulation and flood intensified stratification focused sediment and sediment transport along the left side of the estuary (looking seaward). The tendency for greater stratification on flood and net sediment flux toward the left-hand shoal are contrary to more common models which, in the northern hemisphere, predict greater resuspension on flood and move sediment toward the right-hand shoal. These tidal asymmetries interact with the lateral circulation to focus net sediment flux on the left side of the estuary, and to produce net ebb directed sediment transport at the surface of the same order of magnitude as net flood directed sediment transport at the bottom.  相似文献   

13.
Non-linear tidal dynamics are investigated in a network that consists of a semi-enclosed main channel and a secondary channel at an arbitrary position. The water motion, governed by the one-dimensional shallow water equations, is forced by an incoming tidal wave. Solutions are obtained with the method of characteristics. The overall aim is to quantify and understand the spatial structure of different tidal harmonics (the principal tide and its non-linear overtides) and of tidal asymmetry for both the vertical and the horizontal tide in the main channel for different locations of the secondary channel. This is of practical interest in the context of possible construction of secondary channels to reduce tidal range in estuaries. Moreover, tidal asymmetry is an important factor in driving net sediment transport. Analysis of the different tidal harmonics shows that their characteristics are similar to those obtained with an earlier linear model. In particular, amplitudes of the harmonics are reduced landward of the secondary channel if the latter is positioned less than a quarter wavelength of the respective tidal wave away from the landward boundary. Thus, the distortions of the tide due to the presence of the secondary channel are generated locally and afterwards propagate through the network. Tidal asymmetry is quantified by examining tidal range, flood-to-ebb ratio and the duration of the falling tide and the duration between maximum flood and maximum ebb. A spatial non-uniform reduction in tidal range is observed that shows very localised increase and decrease depending on the position of the secondary channel. The changes in the velocity characteristics induce changes in net sediment transport. It turns out that the direction of the peak current, derived from the flood-to-ebb ratio, is not sensitive to the position of the secondary channel, whereas the duration between flood and ebb can change from more to less than half the tidal cycle. However, the changes in the velocity asymmetries are confined to a small region.  相似文献   

14.
The validation and subsequent application of the current three-dimensional numerical hydrodynamic model of Chesapeake Bay is presented. The numerical model solves conservation equations for water mass, momentum, salinity, and heat on a boundary-fitted grid in the horizontal plane and a Cartesian z-grid in the vertical. A generalized ADI finite difference scheme is employed in conjunction with mode splitting technique, solving external and the internal modes. The 10-year boundary conditions including tide, slinity, temperature, wind, heat exchange coefficient, river and non-point source flows were constructed. Model validation was accomplished by demonstrating the model's ability to reproduce observed data over time scales ranging from tidal to seasonal periods. The major parameters compared include tidal elevation, intra-tidal and residual velocities, salinity, temperature, stratification, and flux calculated through the Bay mouth.After validation, the model was applied to simulate bay hydrodynamics for the 10 years of 1985–94. These results were used to drive the three-dimensional water quality model of Chesapeake Bay, which is discussed in a companion paper.  相似文献   

15.
The broad (～500 km) southeastern Bering Sea continental shelf contains three fronts; outer (shelf break, ～170-m depth), middle (～100-m depth), and inner (～50-m depth). The shelf break and inner fronts appear to be analogous to similar fronts reported from other mid-latitude continental shelves; extensively studied examples are from the mid-Atlantic bight, off Nova Scotia, and around the British Isles. The middle front may have counterparts on the broad North Sea and East China Sea shelves.One-month current and temperature records from either side of the middle front, ～150 km landward from the shelf break, showed convergence in the layers deeper than 30 m in both the cross-shelf flow field and heat flux. The convergence was ～3 cm s?1, so an average upwelling at ～1 × 10?3cm s?1 and divergence in the surface layer were required to maintain continuity. Variations in the degree of convergence arose primarily from 1 to 5-day fluctuations in sub-tidal flow across the outer shelf domain seaward of the front.Diffusive landward heat flux was dominated by tidal scales. Horizontal eddy conductivities describing the flux were ～1 ? 106 on the landward side and ～5 × 106cm2 s?1 on the seaward side, and were less in the layers above the bottom layer. Advective flux by the mean flow was the same order as diffusive flux, but landward in the bottom layer and seaward in the mid-water column layers, in agreement with deductions from water mass analyses. Frontal effects reduced the net cross-shelf heat flux beneath ～30 m by about 50%. The observation of a flow convergence in the middle of a broad, flat continental shelf poses an important question of dynamics.  相似文献   

16.
Measurements of velocity and density profiles were used to describe the tidal and mean flow structure across and along a sill in Refugio Channel, a fjord-like inlet in Southern Chile (43.9°S). These are the first oceanographic measurements of any kind effected in Refugio Channel. Current profiles were obtained with a 307.2-kHz acoustic Doppler current profiler during two semidiurnal cycles along a repeated triangular circuit. Two along-channel transects formed the sides of the triangle that crossed the sill and were identified as the western and eastern transects. One cross-channel transect, the base of the triangle, was located on the seaward side of the sill. Density profiles were obtained at the corners of the triangle. The longitudinal mean flow in the western transect showed a two-layer exchange structure over the landward side of the sill. The structure of net seaward flow at the surface and landward flow at depth was disrupted by the sill in such a way that over the seaward side of the sill, only seaward flow was observed throughout the water column. This likely resulted from the blocking of landward net flow by the sill. In the eastern transect, two-layer exchange dominated over most of the transect and was consistent with the observed density profiles. Over the seaward side of the sill, a surface layer, ∼10m deep, flowed landward as a third layer. This feature should have been caused by river input further seaward (to the north) and produced a surface convergence region over the sill. In terms of tidal flows, the greatest tidal current amplitudes were 40cm s−1 over the sill as the flow accelerated through the reduced cross-sectional area of the channel. Near-surface flow convergences were identified over both along-channel transects.  相似文献   

17.
The continental shelf to the southwest of the British Isles is an area of intense tidal current activity. Although most of the sediment is coarse, it still contains a small proportion of fine (<63 μm) material, consisting mainly of biogenic debris. Coccoliths in particular are both common and widespread and they are found in the adjacent slope sediments. The results presented here show that coccolithophorid diversity is high in the deeper parts of Whittard Canyon, the abyssal plain, and on a small area of shelf. Diversity decreases away from the ocean towards the land. Coccoliths and coccospheres have also been recorded from surface water samples. Apart from the landward movement of oceanic water shown by the coccolithophorids, movement of material on the bottom from shelf to slope is shown by the distribution of benthic diatom frustules and mollusc shell chips cut by clionid sponges. Thus, a study of the fine sediment reveals details of sediment transport and water movement.  相似文献   

18.
A three-dimensional shelf circulation model is used to examine the effect of seasonal changes in water-column stratification on the tidal circulation over the Scotian Shelf and Gulf of St. Lawrence. The model is driven by tidal forcing specified at the model’s lateral open boundaries in terms of tidal sea surface elevations and depth-averaged currents for five major tidal constituents (M2, N2, S2, K1, and O1). Three numerical experiments are conducted to determine the influence of baroclinic pressure gradients and changes in vertical mixing, both associated with stratification, on the seasonal variation of tidal circulation over the study region. The model is initialized with climatological hydrographic fields and integrated for 16 months in each experiment. Model results from the last 12 months are analyzed to determine the dominant semidiurnal and diurnal tidal components, M2 and K1. Model results suggest that the seasonal variation in the water-column stratification affects the M2 tidal circulation most strongly over the shelf break and over the deep waters off the Scotian Shelf (through the development of baroclinic pressure gradients) and along Northumberland Strait in the Gulf of St. Lawrence (through changes in vertical mixing and bottom stress). For the K1 constituent, the baroclinic pressure gradient and vertical mixing have opposing effects on the tidal circulation over several areas of the study region, while near the bottom, vertical mixing appears to play only a small role in the tidal circulation.  相似文献   

19.
基于大气边界层动量、感热和水汽通量的基本方程,定量地计算了波动海面的动量、感热和水汽通量。首先,应用Prandtl的混合长概念,推导出贴海面大气层中风速、位温和比湿的涡动交换率及其贴海面层厚度,并且证明了波面上位温或比湿贴海面层厚度与速度贴海面层厚度的比值,和平面上的相应比值完全相等。随后,利用空气动力学理论讨论了贴海面动量、感热和水汽输送的参数化问题。最后,对现有五种理论模式进行了比较说明。  相似文献   

20.
The effects of large-scale interventions in the North Passage of the Yangtze Estuary (the Deep Waterway Project, DWP) on the along-channel flow structure, suspended sediment distribution and its transport along the main channel of this passage are investigated. The focus is explaining the changes in net sediment transport in terms of physical mechanisms. For this, data of flow and suspended sediment concentration (SSC), which were collected simultaneously at several locations and at different depths along the main channel of the North Passage prior to and after the engineering works, were harmonically analyzed to assess the relative importance of the transport components related to residual (time-mean) flow and various tidal pumping mechanisms. Expressions for main residual flow components were derived using theoretical principles. The SSC revealed that the estuarine turbidity maximum (ETM) was intensified due to the interventions, especially in wet seasons, and an upstream shift and extension of the ETM zone occurred. The amplitude of the M 2 tidal current considerably increased, and the residual flow structure was significantly altered by engineering works. Prior to the DWP, the residual flow structure was that of a gravitational circulation in both seasons, while after the DWP, there was seaward flow throughout the channel during the wet season. The analysis of net sediment transport reveals that during wet seasons and prior to the DWP, the sediment trapping was due to asymmetric tidal mixing, gravitational circulation, tidal rectification, and M 2 tidal pumping, while after the DWP, the trapping was primarily due to seaward transport caused by Stokes return flow and fresh water discharge and landward transport due to M 2 tidal pumping and asymmetric tidal mixing. During dry seasons, prior to the DWP, trapping of sediment at the bottom relied on landward transports due to Stokes transport, M 4 tidal pumping, asymmetric tidal mixing, and gravitational circulation, while after the DWP the sediment trapping was caused by M 2 tidal pumping, Stokes transport, asymmetric tidal mixing, tidal rectification, and gravitational circulation.  相似文献   

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