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1.
In situ observations were combined with 3D modeling to gain understanding of and to quantify the suspended sediment transport in the Gulf of Lions (NW Mediterranean Sea). The outputs of a hydrodynamic–sediment transport coupled model were compared to near-bottom current and suspended sediment concentration measurements collected at the head of seven submarine canyons and at a shallow shelf site, over a 6-month period (November 2003–May 2004). The comparisons provide a reasonable validation of the model that reproduces the observed spatial and time variations. The study period was marked by an unusual occurrence of marine storms and high river inputs. The major water and sediment discharges were supplied by the Rhone, the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December 2003. A second major storm, with moderate flooding, occurred in February 2004. The estimate of river input during the studied period was 5.9 Mt. Our study reveals (i) that most of the particulate matter delivered by the Rhone was entrapped on the prodelta, and (ii) that marine storms played a crucial role on the sediment dispersal on the shelf and the off-shelf export. The marine storms occurring in early December 2003 and late February 2004 resuspended a very large amount of shelf sediment (>8 Mt). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Sediment deposition took place in the middle part of the shelf, between 50 and 100 m depth. Resuspended sediments and river-borne particles were transported to the southwestern end of the shelf by a cyclonic circulation induced by these onshore winds and exported towards the Catalan shelf and into the Cap de Creus Canyon which incises the slope close to the shore. Export taking place mostly during marine storms was estimated to reach 9.1 Mt during the study period.  相似文献   

2.
A 1-year survey of sediment dynamics on the Têt inner-shelf in the south-western part of the Gulf of Lions was conducted as part of the EUROSTRATAFORM program (European Margin Strata Formation) from October 2004 to November 2005. Several bottom instruments (ADCP, wave gauge and altimeters) were deployed at 28 m water depth on the Têt prodelta to measure forcing responsible for sediment erosion and transport on the inner-shelf.  相似文献   

3.
Motivated by our puzzling high-resolution radar observations of surface vortices in the nearly rectangular Gulf of Eilat/Aqaba, northern Red Sea, we propose and explore the driven cavity approach to this geophysical phenomenon. While the lid-driven cavity has long been considered a benchmark problem in computational fluid dynamics, its oceanographic context has not been considered. Despite the additional effects of rotation and stratification, our modeling demonstrates that when the fluid within a cavity geometrically similar to the Gulf of Eilat is driven by the external current, an interior vortex can develop as in our observations. Furthermore, the Eilat vortices appear only under relatively calm conditions, adding evidence to the intriguing possibility of their simple shear-driven origin.  相似文献   

4.
The ECOLOPHY experiments aimed at investigating physical exchanges between coastal and open sea. They were carried out in June and December 2005 over the shelf-break in the North-eastern part of the Gulf of Lions (Northwestern Mediterranean Sea). This area is considered to be the generation zone for the eddy and meandering structures of the Northern Current (NC). The objective of the present work is to examine mesoscale variability of this coastal slope current in the light of available data. Numerical modeling is used to support the field data analysis. ADCP current measurements over a one-year period show that mesoscale activity is maximal in late winter, correlating with the seasonal variability of the NC and, also, partly with local winds. Measured currents exhibit mesoscale fluctuations with periods ranging from 3 to 30 days, in agreement with previous analyses. Fluctuations of periods longer than 10 days are found to be mainly oriented in the direction of the mean current, whereas more frequently observed high frequency fluctuations tend to be oriented cross-slope, suggesting a relationship with the NC mesoscale meandering. Moreover, trajectories of surdrift buoys launched in the NC vein exhibit mesoscale phenomena, such as current meanders or eddies and on-shelf intrusions. Numerical modeling provides a synoptic point of view and is used hereafter to support physical interpretation of punctual eulerian or lagrangian measurements. Therefore, modeled hydrodynamic fields are used to analyze surdrift buoy trajectories and computed vertically averaged current and Ertel potential vorticity provide a better understanding of these behaviors.  相似文献   

5.
Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35 km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5 mg/l. In Cap de Creus canyon head (located ∼14 km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20 mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists.  相似文献   

6.
The Northern Current (hereafter NC), the major current in the North-Western Mediterranean (hereafter NWM) basin, has been largely investigated in the litterature for its mesoscale features. Its behaviour in the Var region can strongly condition the downstream flow along the Gulf of Lions shelf and Spain coast, making this zone a key area. However, the sub-mesoscale dynamics of the area and its potential impacts on the rest of the flow are not well known. This work reveals the potential interest of better simulating high-resolution dynamics in a restricted area and how this could improve the circulation representation in a larger area. To address this question, a very high resolution configuration (1/192°) nested in an already existing high-resolution configuration (1/64°) has been developed, using the NEMO model. Comparisons with observations show that the very high-resolution changes only weakly the mean NC characteristics but can significantly modify individual mesoscale events such as eddies and meanders occurring in the zoomed area. Furthermore, the coastal dynamics and episodic intrusions of a NC secondary branch inside a semi-enclosed bay appear to be significantly enhanced. In a second stage, the assessment of the feedback of this improved dynamics on the regional mesoscale dynamics is shown, this being allowed by the two-way coupling option of the embedded configuration using AGRIF.  相似文献   

7.
Coastal mesoscale eddies were evidenced during a high-frequency radar campaign in the Gulf of Lions (GoL), northwestern Mediterranean Sea, from June 2005 to January 2007. These anticyclonic eddies are characterized by repeated and intermittent occurrences as well as variable lifetime. This paper aims at studying the link between these new surface observations with similar structures suggested at depth by traditional acoustic Doppler current profiler measurements and investigates the eddy generation and driving mechanisms by means of an academic numerical study. The influence of the wind forcing on the GoL circulation and the eddy generation is analyzed, using a number of idealized configurations in order to investigate the interaction with river discharge, buoyancy, and bathymetric effects. The wind forcing is shown to be crucial for two different generation mechanisms: A strong northerly offshore wind (Mistral) generates a vortex column due to the bathymetric constraint of a geostrophic barotropic current, which can surface after the wind relaxes; a southerly onshore wind generates a freshwater bulge from the Rhône river discharge, which detaches from the coast and forms a well-defined surface anticyclonic eddy based on buoyancy gradients. These structures are expected to have important consequences in terms of dispersion or retention of biogeochemical material at local scales.  相似文献   

8.
Previous work in the Gulf of Lions (western Mediterranean Sea) has suggested that significant amounts of sediment escape through the western part of this tectonically passive margin, despite it being far removed from the primary sediment source (the Rhone River, ∼160 km to the NE). The primary mechanism behind this export is hypothesized to be the interaction of a regional, southwestward sediment-transport path with a canyon deeply incising the southwestern part of the shelf, Cap de Creus Canyon.  相似文献   

9.
Abstract

Unstable waves in a western boundary current are investigated in a full three-dimensional, numerical model. A numerical integration is carried out which traces the evolution of a growing wave on an initially uniform current with vertical shear. As indicated in earlier analytic studies based on simpler 2-layer models (Orlanski, 1969) the current is baroclinically unstable for the observed parameter range of the Gulf Stream.

Large meanders of the jet in the western boundary current are noticeable within 10 days. Finite amplitude effects, which can be investigated by the numerical model, reduce the growth rate of the disturbance by nearly an order of magnitude compared to linear theory. Comparison with observations indicate that the meanders of the Florida Current between Miami and Hatteras are probably baroclinically unstable waves.  相似文献   

10.
Recent oceanographic observations and a retrospective analysis of nutrients and hydrography over the past five decades have revealed that the principal source of nutrients to the Gulf of Maine, the deep, nutrient-rich continental slope waters that enter at depth through the Northeast Channel, may have become less important to the Gulf's nutrient load. Since the 1970s, the deeper waters in the interior Gulf of Maine (>100 m) have become fresher and cooler, with lower nitrate (NO3) but higher silicate (Si(OH)4) concentrations. Prior to this decade, nitrate concentrations in the Gulf normally exceeded silicate by 4–5 μM, but now silicate and nitrate are nearly equal. These changes only partially correspond with that expected from deep slope water fluxes correlated with the North Atlantic Oscillation, and are opposite to patterns in freshwater discharges from the major rivers in the region. We suggest that accelerated melting in the Arctic and concomitant freshening of the Labrador Sea in recent decades have likely increased the equatorward baroclinic transport of the inner limb of the Labrador Current that flows over the broad continental shelf from the Grand Banks of Newfoundland to the Gulf of Maine. That current system now brings a greater fraction of colder and fresher deep shelf waters into the Gulf than warmer and saltier offshore slope waters which were previously thought to dominate the flux of nutrients. Those deep shelf waters reflect nitrate losses from sediment denitrification and silicate accumulations from rivers and in situ regeneration, which together are altering the nutrient regime and potentially the structure of the planktonic ecosystem.  相似文献   

11.
This study focuses on the interaction between mid depth vortices and surface jets and fronts in a three-layer quasi-geostrophic model. Such vortices may be regarded as an idealisation of meddies, eddies of Mediterranean Water in the Northeastern Atlantic Ocean, interacting with the Azores j t and front. Successively, a single vortex, a vortex doublet and a vortex pair (in the middle layer) are studied. When a single vortex is considered, the jet has a critical effect of its motion, temporarily slowing down its zonal drift and accelerating it meridionally as the vortex crosses the front. On the contrary, if the vortex does not cross the front, it can drift fairly rapidly along it. The merger of a vortex doublet (two like-signed vortices) below a surface jet is possible whatever the relative position of this doublet with respect to the jet axis. Nevertheless, doublets initially located below the front, will undergo stronger shear and merger efficiency will be diminished. The merged vortex will be circled at the surface by a large meander of the jet. Finally, eastward jet-dipole interaction experiments are performed with various orientations of the vortex dipoles. Eastward propagating dipoles below the jet follow it without deformation. Southeastward drifting dipoles finally join the previous evolution. Southward and southwestward directed dipoles cross the surface jet southeastward. The presence of meanders initially on the jet does not prevent its crossing by a single vortex. Characteristics of the surface jet meanders are also described for a possible remote detection of this process.  相似文献   

12.
Hydrodynamical Modeling Of Oceanic Vortices   总被引：1，自引：0，他引：1  
Mesoscale coherent vortices are numerous in the ocean.Though they possess various structures in temperature and salinity,they are all long-lived, fairly intense and mostly circular. Thephysical variable which best describes the rotation and the density anomaly associated with coherent vortices is potential vorticity. It is diagnostically related to velocity and pressure, when the vortex is stationary. Stationary vortices can be monopolar (circular or elliptical) or multipolar; their stability analysis shows thattransitions between the various stationary shapes are possible when they become unstable. But stable vortices can also undergo unsteady evolutions when perturbed by environmental effects, likelarge-scale shear or strain fields, -effect or topography. Changes in vortex shapes can also result from vortex interactions. such as the pairing, merger or vertical alignment of two vortices, which depend on their relative polarities and depths. Such interactions transfer energy and enstrophy between scales, and are essential in two-dimensional and in geostrophic turbulence. Finally, in relation with the observations, we describe a few mechanisms of vortex generation.  相似文献   

13.
Winter observations of shelf and slope hydrography and currents in the inner Gulf of Tehuantepec are analysed from two field studies in 1989 and 1996 to specify the variability of near-shore conditions under varying wind stress. During the winter period frequent outbursts of ‘Norte’ winds over the central Gulf result in persistent alongshore inflows along both its eastern and western coasts. Wind-induced variability on time scales of several days strongly influences the shelf currents, but has greater effect on its western coast because of the generation and separation of anticyclonic eddies there. The steadier inflow (∼0.2 m s−1) on the eastern shelf is evident in a strong down-bowing of shallow isosurfaces towards the coast within 100 km of shore, below a wedge of warmer, fresher and lighter water. This persistent entry of less saline (33.4–34.0), warmer water from the southeast clearly originates in buoyancy input by rivers along the Central American coast, but is augmented by a general shoreward tendency (0.2 m s−1) in the southeastern Gulf. The resultant shallow tongue of anomalous water is generally swept offshore in the head of the Gulf and mixed away by the strong outflow and vertical overturning of the frequent ‘Norte’ events but during wind relaxations the warm, low-salinity coastal flow may briefly extend further west. In the head of the Gulf, flow is predominantly offshore (<0.2 m s−1) as the alongshore component alternates eastward and westward in association with elevation or depression, respectively, of the pycnocline against the shore. More saline, open ocean water is introduced from the north-western side of the Gulf by the inflow along the west coast. During extended wind relaxations, the flow becomes predominantly eastward beyond the shelf while nearshore the coastally trapped buoyant inflow from the southeast penetrates across the entire head of the gulf at least as far as its western limit. On the basis of these and other recent observations, it seems that the accepted view of a broad, persistent Costa Rica Coastal Current (CRCC) is the result of averaging over many relatively sparse observations and that the instantaneous CRCC is a highly variable and convoluted flow around and between constantly changing eddies. The buoyancy-driven shelf current reported here forms a hitherto unrecognized, but major, component of this CRCC system.  相似文献   

14.
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   

15.
Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300 m depth from November 2003 to March 2004. Two major storm events producing waves Hs>6 m coming from the E–SE sector took place, the first on 3–4 December 2003 (max Hs: 8.4 m) and the second on 20–22 February 2004 (max Hs: 7 m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events.  相似文献   

16.
Eddy resolving modelling of the Gulf of Lions and Catalan Sea   总被引：1，自引：1，他引：0  
The generation process of strong long-lived eddies flowing southwestwards along the Catalan slope was revealed through numerical modelling and in situ observations. Careful analyses of a particular event in autumn 2007 demonstrated a link between a “LATEX” eddy, which remained in the southwestern corner of the Gulf of Lions and a “CATALAN” eddy, which moved along the Catalan Shelf, since the death of the former gave birth to the latter. The origin of such eddies was found to be an accumulation of potential energy in the southwestern corner of the Gulf of Lions: under the influence of the negative wind stress curl associated with the Tramontane, a warm and less dense water body can be isolated and fed by a coastal current carrying warm water from the Catalan Sea. In summer, this structure can grow and intensify to generate a strong anticyclonic eddy. After a long period of Tramontane, a burst of southeasterlies and northerlies appeared to detach the “LATEX” eddy, which flowed out of the Gulf of Lions, migrating along the Catalan continental slope and continued into the Balearic Sea as the “CATALAN” eddy.  相似文献   

17.
Conductivity-temperature-depth (CTD) observations taken in the Great Australian Bight (GAB) during ORV Franklin cruise Fr 07/94 in July 1994 indicated the presence of a dense bottom layer at the head of the GAB, which flowed along the sea floor towards the shelf-break as a gravity current The north central region of the GAB was stratified with a maximum salinity difference of between 0.4 and 0.5. The outflow was confined to the shelf and was directed in a south-easterly direction with little evidence of cross-shelf transport. The flow exhibited a well-defined bottom interface evident from the head of the GAB to near the mouth of Spencer Gulf (SG), where the surface-bottom salinity difference was about 0.3. The mean thickness of the outflow was about 15 m. An estimate of the speed of the outflow at the discharge over the shelf-break was made using the zero entrainment assumption. This yielded a speed of <16 cm s−1, which remarkably was consistent with near bottom current meter measurements (16 cm s−1) on the continental shelf edge, reported south of the Eyre Peninsula. A mass budget analysis indicated that the outflow, which probably is partially maintained by the gravity current and partly by a wind-driven circulation would exist over the period, July–December, with a peak transport of about 106 m3s−1 (1 Sverdrup) which is approximately twenty times that of the bottom outflow from the adjoining Spencer Gulf.  相似文献   

18.
Turbulence measurements in fine-scale phytoplankton layers (∼1 to ∼10 m) in the Gulf of Aqaba (Red Sea) were used to evaluate mechanisms of layer formation, maintenance, and breakdown. Simultaneous profiles of chlorophyll a (Chl a) fluorescence and temperature microstructure were measured in the upper 40 m of a 430 m water column over a 16-d period, using a Self Contained Autonomous MicroProfiler (SCAMP). Layers of concentrated phytoplankton were identified in 95 of the 456 profiles. The layers were situated in density stratified regions between 15 and 38 m depth and were characterized by intensities of 0.1 to 0.35 μg Chl a L−1 (as much as two times background concentrations) and an average thickness of 10 m. We show that turbulent mixing and isopycnal displacements associated with internal waves modulated the thickness of the layers. Variations in mixing rates within layers were connected to the vertical structure of the stratified turbulence and the stage of layer development. The breakdown of a persistent phytoplankton layer was tied to strong turbulent mixing at the base of the surface mixed layer, which encroached on the layer from above. Hydrographic observations and scaling analysis suggest that the layers most likely formed in horizontal intrusions from the adjacent coastal region. The cross-shore propagation of phytoplankton-rich intrusions may have important implications for the trophic state of offshore planktonic communities.  相似文献   

19.
The logarithmic layer of double-averaged (in time and space) streamwise velocity profiles obtained from field measurements made in the Swiss rivers, Venoge and Chamberonne is parameterized and discussed. Velocity measurements were made using a 3D Acoustic Doppler Velocity Profiler. Both riverbeds are hydraulically rough, composed of coarse gravel, with relative submergences (h/D50) of 5.25 and 5.96, respectively. From the observations, the flow may be divided into three different layers: a roughness layer near the bed, an equivalent logarithmic layer and a surface or outer layer. It was found that a logarithmic law can describe the double-averaged profiles in the layer 0.30 < z/h < 0.75. The parameterization of the logarithmic law is discussed. Special emphasis is given to the geometric parameters roughness and zero-displacement heights and to the equivalent von Karman constant.  相似文献   

20.
Measurements of Pb isotopes in aerosols, precipitation, and size-fractionated particulate matter from the Gulf of Maine, Scotian Shelf and Labrador Sea are used to investigate the source of Pb. The 206Pb/207Pb ratio in aerosols and precipitation collected at New Castle, NH suggests that anthropogenic Pb is a mixture of US and Canadian sources. 206Pb/207Pb ratios in >53 μm particulate matter from the Gulf of Maine and Scotian Shelf slope waters are consistent with contaminant Pb inputs predominantly from US and Canadian sources, while in shelf waters 206Pb/207Pb ratios in >0.4 μm and >53 μm particles are consistent with a mixture of US and Canadian sources, as well as Pb associated with resuspended surface sediment. 206Pb/207Pb ratios in particulate matter (>0.4 μm, 10-53 μm, and >53 μm) from Labrador Sea surface waters range from 1.165 to 1.211 and are a mixture of Pb derived from ore compositions consistent with Broken Hill, Australia and southeast Missouri, US sources.  相似文献   

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