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1.
A numerical shelf circulation model was developed for the Scotian Shelf, using a nested-grid setup consisting of a three-dimensional baroclinic inner model embedded inside a two-dimensional barotropic outer model. The shelf circulation model is based on the Princeton Ocean Model and driven by three-hourly atmospheric forcing provided by a numerical weather forecast model and by tidal forcing specified at the inner model's open boundaries based on pre-calculated tidal harmonic constants. The outer model simulates the depth-mean circulation forced by wind and atmospheric pressure fields over the northwest Atlantic Ocean with a horizontal resolution of 1/12°. The inner model simulates the three-dimensional circulation over the Gulf of St. Lawrence, the Scotian Shelf, and the adjacent slope with a horizontal resolution of 1/16°. The performance of the shelf circulation model is assessed by comparing model results with oceanographic observations made along the Atlantic coast of Nova Scotia and in the vicinity of Sable Island (on the Scotian Shelf) during two periods: October 2000–March 2001 and April–June 2002. Analysis of model results on Sable Island Bank indicates that tidal currents account for as much as ∼80% of the total variance of near-bottom currents, and currents driven by local winds account for ∼30% of the variance of the non-tidal near-bottom currents. Shelf waves generated remotely by winds and propagating into the region also play an important role in the near-bottom circulation on the bank.  相似文献   

2.
Ocean circulation influences nearly all aspects of the marine ecosystem. This study describes the water circulation patterns on time scales from hours to years across Torres Strait and adjacent gulfs and seas, including the north of the Great Barrier Reef. The tridimensional circulation model incorporated realistic atmospheric and oceanographic forcing, including winds, waves, tides, and large-scale regional circulation taken from global model outputs. Simulations covered a hindcast period of 8 years (i.e. 01/03/1997–31/12/2004), allowing the tidal, seasonal, and interannual flow characteristics to be investigated. Results indicated that the most energetic current patterns in Torres Strait were strongly dominated by the barotropic tide and its spring-neap cycle. However, longer-term flow through the strait was mainly controlled by prevailing winds. A dominant westward drift developed in summer over the southeasterly trade winds season, which then weakened and reversed in winter over the northwesterly monsoon winds season. The seasonal flow through Torres Strait was strongly connected to the circulation in the north of the Great Barrier Reef, but showed little connectivity with the coastal circulation in the Gulf of Papua. Interannual variability in Torres Strait was highest during the monsoon period, reflecting variability in wind forcing including the timing of the monsoon. The characteristics of the circulation were also discussed in relation to fine sediment transport. Turbidity level in Torres Strait is expected to peak at the end of the monsoon, while it is likely to be at a low at the end of the trade season, eventually leading to a critically low bottom light level which constitutes a severe risk of seagrass dieback.  相似文献   

3.
This paper examines the subtidal circulation and associated variability in the Gulf of St. Lawrence, the Scotian Shelf, and the Gulf of Maine (GSL-SS-GOM) in 1988–2004 based on results produced by a nested-grid shelf circulation model. The model has a fine-resolution child model (～ (1/12)°) embedded inside a coarse-resolution parent model (～ (1/4)°) of the northwest Atlantic. The combination of the semi-prognostic method and the spectral nudging method is used to reduce the model seasonal bias and drift. The child model reproduces the general features of the observed circulation and hydrography over the study region during the study period. The child model results demonstrate that the time-mean circulation in the GSL is affected by the time-mean atmospheric forcing and inflow through the Strait of Belle Isle. The temporal variability in atmospheric forcing affects the outflow through western Cabot Strait, which in turn affects the transport of the Nova Scotian Current and the gulf-wide cyclonic circulation in the GOM. The simulated seasonal variability of salinity in the top 30 m of the GSL-SS-GOM is mainly affected by the equatorward advection of low-salinity waters from the lower St. Lawrence Estuary to the GOM through the Scotian Shelf. The simulated intraseasonal variability of circulation in the GSL is affected by the variability in the estuarine circulation in response to the temporal variability in atmospheric forcing. On the Scotian Shelf, the intraseasonal variability is mainly driven by the variability of wind forcing and mesoscale and nonlinear dynamics over the shelf break and slope region. The interannual variability in the simulated temperature and salinity are spatially coherent in the intermediate waters in the GSL, which is caused partially by the local response to atmospheric variability and partially by variabilities over the southern Newfoundland Shelf that enter the GSL through the eastern Cabot Strait. By comparison, on the Scotian Shelf, the interannual variability of simulated circulation is affected by anomalies produced by the nonlinear dynamics which are advected equatorwards by the shelf break jet.  相似文献   

4.
The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.  相似文献   

5.
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   

6.
《Continental Shelf Research》1999,19(9):1221-1245
This paper presents some recent results of drifters released on the West Florida Shelf during 1996–1997 and compares with the numerical model results of the wind-driven circulation. Using satellite tracked surface drifters during the one year period from February 1996 to February 1997, a drifter free region, called the “forbidden zone”, is found over the southern portion of the West Florida Shelf. This finding is consistent with historical drift bottle data and with a recent numerical model study of the West Florida Shelf circulation response to climatological wind forcing. Direct drifter simulations by numerical model during March 1996 show a good agreement with both the in situ ADCP current observation and drifter observation. Three mechanisms are proposed for the observed Lagrangian features. The primarily dynamic mechanism is the along-shore wind forcing, which induces a coastal jet that tends to leave the coast and the bottom onshore and near surface offshore transports. The second one is the convergent coastal geometry and bottom topography for the southward flow in central shelf near Tampa Bay that enforces the coastal jet and the bottom and near surface transport. The last is a kinematic one, simply due to the short along-shore Lagrangian excursion, driven by the typical synoptic weather systems. Thus near surface shelf waters over the north may not reach the southern coast of the West Florida. Implication is that surface hazard such as oil spill that may occur outside of the southern West Florida shelf may not greatly impact the southern coastal region except Florida Keys. However, the biological and chemical patches over the north that may occur in the water column such as red tides still can easily reach the southern coastal region through the subsurface and bottom waters.  相似文献   

7.
The ocean circulation on Australia's Northern Shelf is dominated by the Monsoon and influenced by large-scale interannual variability. These driving forces exert an ocean circulation that influences the deep Timor Sea Passage of the Indonesian Throughflow, the circulation on the Timor and Arafura Shelves and, further downstream, the Leeuwin Current. Seasonal maxima of northeastward (southwestward) volume transports on the shelf are almost symmetric and exceed 106 m3/s in February (June). The associated seasonal cycle of vertical upwelling from June to August south of 8.5°S and between 124°E and 137.5°E exceeds 1.5×106 m3/s across 40 m depth. During El Niño events, combined anomalies from the seasonal means of high regional wind stresses and low inter-ocean pressure gradients double the northeastward volume transport on the North Australian Shelf to 1.5×106 m3/s which accounts for 20% of the total depth-integrated transport across 124°E and reduce the total transport of the Indonesian Throughflow. Variability of heat content on the shelf is largely determined by Pacific and Indian Ocean equatorial wind stress anomalies with some contribution from local wind stress forcing.  相似文献   

8.
Maps of satellite-derived estimates of monthly averaged chlorophyll a concentration over the northern West Florida Shelf show interannual variations concentrated near the coastline, but also extending offshore over the shelf in a tongue-like pattern from the Apalachicola River during the late winter and early spring. These anomalies are significantly correlated with interannual variability in the flow rate of the Apalachicola River, which is linked to the precipitation anomalies over the watershed, over a region extending 150–200 km offshore out to roughly the 100 m isobath. This study examines the variability of the Apalachicola River and its impacts on the variability of water properties over the northern West Florida Shelf. A series of numerical model experiments show that episodic wind-driven offshore transport of the Apalachicola River plume is a likely physical mechanism for connecting the variability of the river discharge with oceanic variability over the middle and outer shelf.  相似文献   

9.
This study examines seasonal circulation, hydrography, and associated spatial variability over the inner shelf of the northern South China Sea (NSCS) using a nested-grid coastal ocean circulation model. The model external forcing consists of tides, atmospheric forcing, and open boundary conditions based on the global ocean circulation and hydrography reanalysis produced by the Hybrid Coordinate Ocean model. Five numerical experiments are conducted with different combinations of external forcing functions to examine main physical processes affecting the seasonal circulation in the study region. Model results demonstrate that the monthly mean circulation in the study region features the Guangdong Coastal Current (GCC) over coastal waters and the South China Sea Warm Current (SCSWC) in the offshore deep waters. The GCC produced by the model flows nearly southwestward in winter months and northwestward in summer months, which agrees with previous studies. The SCSWC flows roughly northeastward and is well defined in summer months. In winter months, by comparison, the SCSWC is superseded by the southwestward strong wind-driven currents. Analysis of model results in five different experiments demonstrates that the monthly mean circulation over coastal and inner shelf waters of the NSCS can be approximated by barotropic currents forced by the southwestward monsoon winds in winter months. In summer months, by comparison, the monthly mean circulation in the study region is affected significantly by baroclinic dynamics associated with freshwater runoff from the Pearl River and advection of warm and saline waters carried by the SCSWC over the NSCS.  相似文献   

10.
The sea level and the barotropic, frictional circulation response for the New York Bight are used to demonstrate the effects of external sea-level forcing, bathymetry, and variable friction. The governing equation is the steady, integrated vorticity equation and is computed by finite differencing over a curvilinear grid conforming to the 10- and 100-m isobaths and extending for 250 km alongshore. The boundary conditions are based on the hypothesis that the dynamics of the shelf are driven by the external sea-level gradient and the coastal no-flux condition; and consequently the conditions at the lateral boundaries are dependent thereon. Therefore, the external sea-level slope must be independently specified, and the lateral boundary conditions must be dependently generated. The diabathic component of the external sea slope forces the calm wind circulation by its effect on the transport through the upstream boundary; and the parabathic component has also an important modifying effect by forcing a shelf convergent transport. The parabathic sea slope at the coast is independent of its offshore value, being instead a direct product of the coastal boundary condition.The bottom friction is expressed as related to the sea level through a bottom length parameter and a veer angle, both of which are taken to increase shoreward. An additional bottom stress component, related to the surface stress, is determined for bottom depths less than the Ekman depth. Such bottom stress variability produces significant alterations in the nearshore flow field, over the constant bottom stress formulation, by reducing it and causing it to veer downgradient and downwind in the nearshore.The model is forced by different wind directions and the results are discussed. The circulations generally conform to the observed mean flow patterns, but with several smaller-scale features. The strong bathymetric feature of the Hudson Shelf Valley causes a polarized up- and downvalley flow for winds with an eastward or westward component, respectively. Under mean westerly winds, there is a divergence in the shelf valley flow at about the 60-m isobath. The Apex gyre existing off the western tip of Long Island becomes more extensive for winds changing from northeast to southwest. Mean flow reversals (to the northeast) occur off both Long Island and New Jersey for wind directions changing counterclockwise from northwest to southeast and from west to east, respectively. Southeastward transport over the outer New Jersey shelf tends to be enhanced by wind and external sea-level conditions; and the transport over the New Jersey midshelf, particularly in the lee of the shelf valley, tends to be weak and variable also under these mean conditions.  相似文献   

11.
Marine circulation above the northern Brazilian continental shelf is subject to energetic forcing factors of various origins: high water buoyancy fluxes induced by the Amazon River freshwater discharge, a strong coastal current associated with a mesoscale current (North Brazil Current (NBC)), a forcing by semidiurnal tide and by Northeast or Southeast trade winds according to the season. Using a three-dimensional (3-D) hydrodynamic numerical model (MOBEEHDYCS), and realistic bathymetry and coastline of the northern Brazilian shelf, this paper aims at studying the influence of some specific physical processes on the morphology of the Amazon plume. The very large volume discharge (180 000 m3/s on average) and the weak effect of Coriolis force are additional characteristics of the studied system, which induce a particular dynamics. The various forcing factors are successively introduced into the model in order to simulate and to determine their respective influences upon the plume extent and the hydrodynamics at the shelf scale. Simulation reveal that the coastal current is at the origin of the permanent northwestward Amazon plume extension while wind effect can either reinforce or moderate this situation. The tide intervenes also to modify the position of the salinity front: a horizontal migration of salinity front is observed under its action.  相似文献   

12.
The Pearl River Estuary (PRE) in South China's Guangdong Province is a subtropical estuary with highly irregular topography and dynamically complicated circulations. A nested-grid coastal circulation modelling system is used in this study to examine dynamic responses of the PRE to tides, meteorological forcing and buoyancy forcing. The nested-grid modelling system is based on the Princeton Ocean Model and consists of three downscaling subcomponents: including an outer-most model with a coarse horizontal resolution of ~7 km for simulating tidally forced and wind-driven surface elevations and depth-mean currents over the China Seas from Bohai Sea to the northern South China Sea and an innermost model with a fine resolution of ~1.2 km for simulating the 3D coastal circulation and hydrography over the PRE and adjacent coastal waters. Model results during the winter northeast monsoon surge in January and super typhoon Koryn in June of 1993 are used to demonstrate that the 3D coastal circulation and hydrographic distributions in the PRE are affected by tides, winds and buoyancy forcing associated with river discharge from the Pearl River with significant seasonal and synoptic variabilities.  相似文献   

13.
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   

14.
Coastal polynyas around Antarctica are the place of intense air–sea exchanges which eventually lead to the formation of high-salinity shelf waters (HSSW) over continental shelves. Here, the influence of atmospheric forcing on coastal polynyas in the Ross Sea is studied by contrasting the response of a regional ocean/sea-ice circulation model to two different atmospheric forcing sets. A first forcing (DFS3) is based on ERA40 atmospheric surface variables and satellite products. A second forcing (MAR) is produced on the basis of ERA40 with a dynamical downscaling procedure. As compared to DFS3, MAR forcing is shown to improve substantially the representation of small-scale patterns of coastal winds with stronger katabatic winds along the coast. The response of the ocean/sea-ice model to the two forcing sets shows that the MAR forcing improves substantially the geographical distribution of polynyas in the Ross Sea. With the MAR forcing, the polynya season is also shown to last longer with a greater ice-production rate. As a consequence, a greater flow of dense water out of the polynyas is found with the MAR forcing and the properties of HSSW are notably improved as compared to the DFS3 forcing. The factors contributing to the activity of Terra Nova Bay and Ross Ice Shelf polynyas in the model are studied in detail. The general picture that emerges from our simulations is that the properties of HSSW are mostly set by brine rejection when the polynya season resume. We found that coastal polynyas in the Ross Sea export about 0.4 Sv of HSSW which then flows along three separate channels over the Ross Shelf. A 6-month time lag is observed between the peak of activity of polynyas and the maximum transport across the sills in the channels with a maximum transport of about 1 Sv in February. This lag corresponds to the time it takes to the newly formed HSSW to spread from the polynya to the sills (at a speed of nearly 2 cm s−1).  相似文献   

15.
Despite their harmful effects in the upper atmosphere, anthropogenic chlorofluorocarbons dissolved in seawater are extremely useful for studying ocean circulation and ventilation, particularly in remote locations. Because they behave as a passive tracer in seawater, and their atmospheric concentrations are well-mixed, well-known, and have changed over time, they are ideal for gaining insight into the oceanographic characteristics of the isolated cavities found under Antarctic ice shelves, where direct observations are difficult to obtain. Here we present results from a modeling study of air–sea chlorofluorocarbon exchange and ocean circulation in the Ross Sea, Antarctica. We compare our model estimates of oceanic CFC-12 concentrations along an ice shelf edge transect to field data collected during three cruises spanning 16 yr. Our model produces chlorofluorocarbon concentrations that are quite similar to those measured in the field, both in magnitude and distribution, showing high values near the surface, decreasing with depth, and increasing over time. After validating modeled circulation and air–sea gas exchange through comparison of modeled temperature, salinity, and chlorofluorocarbons with field data, we estimate that the residence time of water in the Ross Ice Shelf cavity is approximately 2.2 yr and that basal melt rates for the ice shelf average 10 cm yr−1. The model predicts a seasonal signature to basal melting, with highest melt rates in the spring and also the fall.  相似文献   

16.
Bo Yang  Jinyu Sheng 《Ocean Dynamics》2008,58(5-6):375-396
This study examines main physical processes affecting the three-dimensional (3D) circulation and hydrographic distributions over the inner Scotian Shelf (ISS) in June and July 2006 using a nested-grid coastal ocean circulation modeling system known as the NCOPS-LB. The nested-grid system has five relocatable downscaling submodels, with the outermost submodel of a coarse horizontal resolution of (1/12)° for simulating storm surges and barotropic shelf waves over the Eastern Canadian shelf and the innermost submodel of a fine resolution of ～180 m for simulating the 3D coastal circulation and hydrography over Lunenburg Bay of Nova Scotia in the default setup. The NCOPS-LB is driven by meteorological and astronomical forcing and used to study the storm-induced circulation over the ISS during tropical storm Alberto. Model results demonstrate that the coastal circulation and hydrographic distributions over the ISS are affected significantly by tides, local wind forcing, and remotely generated coastal waves during the study period.  相似文献   

17.
Observations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70–98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70–80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80–98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.  相似文献   

18.
The Adriatic Sea general circulation model coupled to a third generation wave model SWAN and a sediment transport model was implemented in the Adriatic Sea to study the dynamics of the sediment transport and resuspension in the northern Adriatic Sea (NAS) during the Bora event in January 2001. The bottom boundary layer (BBL) was resolved by the coupled model with high vertical resolution, and the mechanism of the wave–current interaction in the BBL was also represented in the model. The study found that, during the Bora event of 13–17 January 2001, large waves with significant wave height 2 m and period of 5 s were generated by strong winds in the northwestern shelf of the Adriatic where the direction of wave propagation was orthogonal to the current. The combined motion of the wave and current in the BBL increased the bottom stress over the western Adriatic shelf, resulting in stronger sediment resuspension there. Combining stronger bottom resuspension and strong upward vertical flux of resuspended sediments due to turbulent mixing, the model predicted that sediment concentration near the Po River was much higher than that predicted by the model run without wave forcing. The study also shows that wave–current interaction in the BBL reduced the western Adriatic Coastal Currents (WACCs) in the shallower north. It is concluded that wave forcing significantly changed the sediment distributions and increased the total horizontal fluxes over the western shelf. These results signified wave effect on sediment flux and distribution in the NAS, and suggested that waves cannot be neglected in the study of dynamics of sediment transport and resuspension in the shallow coastal seas. By including the tidal forcing in the coupled model, we also examined the effect of tides on the sediment transport dynamics in the NAS.  相似文献   

19.
We studied the circulation on the coastal domain of the Amazon Shelf by applying the hydrodynamic module of the estuarine and coastal ocean model and sediment transport. The first barotropic experiment aimed to explain the major bathymetric effects on tides and those generated by anisotropy in sediment distribution. We analyzed the continental shelf response of barotropic tides under realistic bottom stress parametrization (C d ), considering sediment granulometry obtained from a faciologic map, where river mud deposits and reworked sediments areas are well distinguished, among others classes of sediments. Very low C d values were set in the fluid mud regions off the Amapá coast (1.0 10???4), in contrast to values around 3.5 10???3 for coarser sediment regions off the Pará coast. Three-dimensional experiments represented the Amazon River discharge and trade winds, combined to barotropic tide influences and induced vertical mixing. The quasiresonant response of the Amazon Shelf to the M2 tide acts on the local hydrodynamics by increasing tidal admittance, along with tidal forcing at the shelf break and extensive fluid mud regions. Harmonic analysis of modeled currents agreed well with the analysis of the AMASSEDS observational data set. Tidal-induced vertical shear provided strong homogenization of threshold waters, which are subject to a kind of hydraulic control due to the topographic steepness. Ahead of the hydraulic jump, the low-salinity plume is disconnected from the bottom and acquires negative vorticity, turning southeastward. Tides act as a generator mechanism and topography, via hydraulic control, acts as a maintainer mechanism for the low-salinity frontal zone positioning. Tidally induced southeastward plume fate is overwhelmed by northwestward trade winds so that they, along with background circulation, probably play the most important role on the plume fate and variability over the Amazon Shelf.  相似文献   

20.
Two very high-frequency radars (VHFR) operating on the Opal coast of eastern English Channel provided a nearly continuous 35-day long dataset of surface currents over a 500 km2 area at 0.6–1.8 km resolution. Argo drifter tracking and CTD soundings complemented the VHFR observations, which extended approximately 25 km offshore. The radar data resolve three basic modes of the surface velocity variation in the area, that are driven by tides, winds and freshwater fluxes associated with seasonal river discharge. The first mode, accounting for 90% of variability, is characterized by an along-shore flow pattern, whereas the second and third modes exhibit cross-shore, and eddy-like structures in the current velocity field. All the three modes show the dominant semi-diurnal variability and low-frequency modulation by the neap-spring tidal cycle. Although tidal forcing provides the major contribution to variability of local currents, baroclinicity plays an important role in shaping the 3D velocity field averaged over the tidal cycle and may strongly affect tracer dynamics on larger time scales. An empirical orthogonal function (EOF) decomposition and a spectral rotary analysis of the VHFR data reveal a discontinuity in the velocity field occurring approximately 10 km offshore which was caused by the reversal in the sign of rotation of the current vector. This feature of local circulation is responsible for surface current convergence on ebb, divergence on flood and strong oscillatory vertical motion. Spectral analysis of the observed currents and the results of the Agro drifter tracking indicate that the line of convergence approximately follows the 30-m isobath. The most pronounced feature of the radar-derived residual circulation is the along-coast intensification of surface currents with velocity magnitude of 0.25 m/s typical for the Regions of Freshwater Influence (ROFI). The analysis has provided a useful, exploratory examination of surface currents, suggesting that the circulation off the Opal coast is governed by ROFI dynamics on the hypertidal background.  相似文献   

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