首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Investigations of multi-layer current regime, variations in sea level and wave parameters using a bottom-mounted RDCP (Recording Doppler Current Profiler) during 20 December 2006–23 May 2007 were integrated with surveys on changes of shorelines and contours of beach ridges at nearby Harilaid Peninsula (Saaremaa Island). A W-storm with a maximum average wind speed of 23 m s−1 occurred on 14–15 January with an accompanying sea level rise of at least 100 cm and a significant wave height of 3.2 m at the 14 m deep RDCP mooring site. It appeared that in practically tideless Estonian coastal waters, Doppler-based “vertical velocity” measurements reflect mainly site-dependent equilibrium between resuspension and sedimentation. The mooring site, 1.5 km off the Kelba Spit of Harilaid, was located in the accumulation zone, where downward fluxes dominated and fine sand settled. As a result of storms in January and April, the distal part of the accumulative gravel spit advanced by 50 m, whereas a 30–50 m retreat of the shoreline in the western and northern parts occurred at Cape Kiipsaare. The location of the beach ridges shows that the development of the spit occurs through relatively short-period but infrequent storm events, roughly 2–3 times each decade.  相似文献   

2.
An Acoustic Doppler Current Profiler (ADCP) was moored at the deep-sea site of the ANTARES neutrino telescope near Toulon, France, thus providing a unique opportunity to compare high-resolution acoustic and optical observations between 70 and 170 m above the sea bed at 2475 m. The ADCP measured downward vertical currents of magnitudes up to 0.03 m s−1 in late winter and early spring 2006. In the same period, observations were made of enhanced levels of acoustic reflection, interpreted as suspended particles including zooplankton, by a factor of about 10 and of horizontal currents reaching 0.35 m s−1. These observations coincided with high light levels detected by the telescope, interpreted as increased bioluminescence. During winter 2006 deep dense-water formation occurred in the Ligurian subbasin, thus providing a possible explanation for these observations. However, the 10-20 days quasi-periodic episodes of high levels of acoustic reflection, light and large vertical currents continuing into the summer are not direct evidence of this process. It is hypothesized that the main process allowing for suspended material to be moved vertically later in the year is local advection, linked with topographic boundary current instabilities along the rim of the ‘Northern Current’.  相似文献   

3.
Year-long moorings were deployed across the Alaskan Stream near Samalga Pass (169°W) on two occasions, first in 2001–2002 (5 moorings) and again in 2003–2004 (3 moorings). Currents were measured throughout the water column, and temperature and salinity were measured at selected depths. Satellite altimetry and satellite-tracked drifters revealed a well defined Alaskan Stream, with the largest near-surface average speeds (>60 cm s−1) and highest eddy kinetic energy just upstream from the mooring sites. Excluding periods when large eddies disrupted the flow, transport in the Alaskan Stream ranged from 10 to 30×106 m3 s−1. The estimated mean transport in 2001–2002 was 19×106 m3 s−1, and in 2003–2004 was 21×106 m3 s−1. Large (diameter>200 km), anti-cyclonic eddies were not uncommon in the vicinity of Samalga Pass (14 times in 20 year period, 1992–2012). Although there were no such eddies observed during the period 2000–2003, one of the largest ever recorded eddies occurred in spring 2004. In addition, smaller eddies occurred on several occasions. Eddies disrupted the flow, shifting the Alaskan Stream farther off shore and were clearly evident in both the satellite imagery and the mooring data. Other energetic events, which were less evident in the satellite records, but clearly evident in the mooring measurements, also disrupted the flow. In addition to the moorings in the Alaskan Stream, pressure gauges were placed in Samalga Pass and a single mooring measuring currents was placed in the Aleutian North Slope Current (ANSC) in the Bering Sea. The alongshore, near-surface flow measured at the moorings deployed on the 1000-m isobaths in the Alaskan Stream and the ANSC were significantly correlated with the bottom pressure time series. In addition, at periods longer than 14 days, the bottom pressure measured at the mooring sites in Samalga Pass was significantly correlated with the sea surface height measured by the satellites. The eddy kinetic energies measured from the satellites and from moorings were also significantly correlated.  相似文献   

4.
The dynamic response of the mooring line will be a dominant factor to consider in their use for the station keeping of a wave energy converter (WEC). Due to the relatively small size of WECs and their being moored in relatively shallow waters the effect of waves, tide and current can be of greater significance than for other floating offshore systems. Axial line stretching and high-frequency ‘top-end’ dynamics can importantly modify damping and top-end loading.If a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere and this will have great significance for the loading experienced by the line. One must also consider how the mooring system might change the response of the WEC and so alter its ability to extract power from the waves. Unlike a typical offshore system, the design of moorings for a WEC device must consider reliability and survivability, and the need to ensure efficient energy conversion.The design and operation of a chain mooring for a WEC is considered here. Generic experimental measurements of mooring line damping were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The measurements were conducted on a single mooring line for surge motions and include the study of axial stretching and high top-end dynamics. The laboratory procedures were designed to resemble tests undertaken earlier at ‘full’ scale in 24 m water depth. The measurements were also compared with numerical studies. The experimental findings for WEC devices, supports the conclusion that dynamic mooring line motion will be an important variable, needing to be considered carefully within the design.  相似文献   

5.
Airborne hyperspectral and thermal infrared imagery collected over the Florida Current provide a view of the disintegration of a Sargassum drift line in 5 m s−1 winds. The drift line consists mostly of rafts 20-80 m2 in size, though aggregations larger than 1000 m2 also occur. Rafts tend to be elongated, curved in the upwind direction, and 0.1-0.5 °C warmer than the surrounding ocean surface. Long weed ‘trails’ extending upwind from the rafts are evidence of plants dropping out and being left behind more rapidly drifting rafts. The raft line may be a remnant of an earlier Sargassum frontal band, which is detectible as an upwind thermal front and areas of submerged weed. Issues are identified that require future field measurements.  相似文献   

6.
  相似文献   

7.
Turbidity and sediment transport in a muddy sub-estuary   总被引：2，自引：0，他引：2  
Sub-estuaries, i.e. tidal creeks and also larger estuaries that branch off the stem of their main estuary, are commonplace in many estuarine systems. Their physical behaviour is affected not only by tributary inflows, winds and tides, but also by the properties and behaviour of their main estuary. Measurements extending over more than an annual cycle are presented for the Tavy Estuary, a sub-estuary of the Tamar Estuary, UK. Generally, waves are small in the Tavy because of the short wind fetch. A several-hour period of up-estuary winds, blowing at speeds of between 7 and 10 m s−1, generates waves with significant wave heights of 0.25 m and a wave periodicity of 1.7 s that are capable of eroding the bed over the shallow, ca. 1.5 m-deep mudflats. Waves also influence sedimentation within and near salt marsh areas. An estuarine turbidity maximum (ETM) occurs in the Tavy's main channel, close to the limit of salt intrusion at HW. Suspended particulate matter (SPM) concentrations typically are less than 40 mg l−1 at HW, although concentrations can exceed 80 mg l−1 when tides and winds are strong. Flood-tide SPM inputs to the Tavy from the Tamar are greater during high runoff events in the River Tamar and also at spring tides, when the Tamar has a high-concentration ETM. Higher SPM concentrations are experienced on the mudflats following initial inundation. Without wave resuspension, this is followed by a rapid decrease in SPM for most of the tide, indicating that the mudflats are depositional at those times. SPM concentrations on the mudflats again increase sharply prior to uncovering. Peak ebb tidal speeds at 0.15 m above the mudflat bed can exceed 0.26 m s−1 at spring tides and 0.4 m s−1 following high runoff events, which are sufficient to cause resuspension. Time-series measurements of sediment bed levels show strong seasonal variability. Higher and lower freshwater flows are associated with estimated, monthly-mean sediment transport that is directed out of, or into, the upper sub-estuary, respectively. Seasonal sediment transfers between the estuary and its sub-estuary are discussed.  相似文献   

8.
We consider an alternative to traditional high-modulus synthetic electro-optical-mechanical (EOM) mooring cables that are being used in single-point moorings for deep-ocean observatories. The alternative cable design is based on using low-modulus nylon or polyester fibers as the strength member. High-modulus EOM cables such as those that use Vectran fibers as the strength member are usually constructed with the conductors and optical fibers in the core and the strength member on the outside. The key aspect of the new design is that the strength member is placed in the center of the cable and the conductors and fibers are wrapped around the outside at a high helix angle to accommodate stretching of the center-strength-member. A comparison of the static and dynamic responses of moorings constructed with nylon, polyester, and Vectran EOM cables (for mooring scopes of 1.1 and 1.2 and deployment depths of 1800, 3000, and 5000 m) shows that the maximum total tensions of moorings made with nylon EOM cables are lowest under all conditions. Differences between the nylon and the Vectran EOM cable moorings are due principally to the differences in the dynamic tensions. Differences between the nylon and the polyester EOM cable moorings are due mainly to differences in static tensions caused by the higher specific gravity of polyester fibers. Reduction in the scope of all the moorings from 1.2 to 1.1 resulted in significantly higher tensions for the polyester and Vectran EOM cable moorings but only slightly higher tensions for the nylon EOM cable moorings  相似文献   

9.
The SAGE iron addition experiment was conducted from R.V. Tangaroa east of South Island, New Zealand, in late March-early April 2004. A desktop survey of climatological data was completed before the experiment, providing information to inform site selection and experiment design. The desktop survey is presented here in updated and enhanced form in order to explain the site selection and describe the conditions expected at the site during the experiment in comparison with those actually encountered.The experiment site was in Subantarctic waters between the Subtropical and Subantarctic Fronts. These waters are characterised by high surface macronutrient concentration, low iron concentration and low chlorophyll. The preferred site based on the desktop survey was in the vicinity of 173.5°E, 47.5°S, in Southern Bounty Trough. The actual release location was chosen immediately before the release and was 112 km to the northwest of this at 172°32′E, 46°44′S. The surface water here has typically come from the southwest (over the northern Campbell Plateau) or the southeast (through Pukaki Gap) and the mean current is directed towards ENE at ∼0.1 m s−1. The release location is well removed from regions of high eddy kinetic energy to the east (where the Subantarctic Front reaches its northern limit) and the west (where fine-scale instabilities develop on the Southland Front, which flows along the continental shelf). Typical conditions at the release site at the end of March are: surface temperature 12 °C; mixed layer depth 40 m; surface chlorophyll concentration ∼0.3 mg m−3; surface photosynthetically active radiation (PAR) 23 E m−2 d−1; surface nutrient concentrations 8-10 mmol m−3 (nitrate), 0.5-0.8 mmol m−3 (phosphate), 1-2 mmol m−3 (silicate) and 0.1-0.5 nM (iron); 99th percentile wind speed 19-21 m s−1. At this time of year, surface PAR is well below its summer maximum, the mixed layer is beginning its seasonal deepening and the silicate concentration is at its seasonal minimum. These factors may have limited the phytoplankton response to iron addition and were compounded in March-April 2004 by strong winds early in the experiment (substantially exceeding the 99th percentile in speed), lower than the average SST, larger than the average mixed layer depth, silicate concentration at the bottom end of the expected range and initially low PAR.  相似文献   

10.
Determination of internal wave properties from X-Band radar observations   总被引：2，自引：0，他引：2  
The application of nautical X-Band radars to measure internal wave (IW) properties is investigated. A methodology based on the use of Radon transform (RT) techniques to detect internal wave related features from backscatter image sequences is introduced to compute properties such as direction of propagation, non-linear velocity (c0), distance between solitons (Lcc) and number of solitons per packet. The proposed methodology was applied to several events recorded by a ship-mounted X-Band radar system (WaMoS) during the NLIWI experiment in 2006. Results from the comparisons to simultaneous measurements taken at neighboring oceanographic moorings indicated that c0 can be estimated with a RMS error of 0.06 m s−1, which corresponds to a mean relative error of −1.4%. Similarly, Lcc can be estimated with a RMS error of 98 m, which is associated with a mean relative error of 14.6%. This latter error estimate however is likely to be overestimated, because it reflects strongly the separation between sampling stations as Lcc was shown to be highly dependent on propagation distance. The accuracy of the results shows that X-Band systems are well suited to measure internal wave properties offering some advantages over SAR and other in situ devices.  相似文献   

11.
An analytical model is proposed for predicting the dynamics of instrumented oceanographic surface moorings made up of a combination of wire rope and compliant synthetic rope. The model simplifies the problem by treating only the vertical motion of the buoy and the longitudinal motion of the mooring line and attached instruments. It is demonstrated using full-scale experimental data and numerical simulations, that the simplified model captures all of the important dynamic effects and gives accurate predictions of the dynamic tension at the top of the mooring line. The model shows that the total mass and damping of the instruments and wire rope that make up the stiff upper half of the mooring are the major sources of the dynamic tension. Damping of the instruments becomes a significant factor in larger sea states, especially near the peak frequency of the wave spectrum. Elastic stretching of the wire and synthetic rope make up approximately 10% of the total response. This is based on a coefficient of friction equal to 0.003 which fits the experimental data best.  相似文献   

12.
  相似文献   

13.
Using inverse methods a circulation for a new section along 32°S in the Indian Ocean is derived with a maximum in the overturning stream function (or deep overturning) of 10.3 Sv at 3310 m. Shipboard and Lowered Acoustic Doppler Current Profiler (ADCP) data are used to inform the choice of reference level velocity for the initial geostrophic field. Our preferred solution includes a silicate constraint (−312 ± 380 kmol s−1) consistent with an Indonesian throughflow of 12 Sv. The overturning changes from 12.3 Sv at 3270 m when the silicate constraint is omitted to 10.3 Sv when it is included. The deep overturning varies by only ±0.7 Sv as the silicate constraint varies from +68 to −692 kmol s−1, and by ±0.3 Sv as the net flux across the section, driven by the Indonesian throughflow, varies from −7 to −17 Sv with an appropriately scaled silicate flux constraint. Thus, the overturning is insensitive to the size of the Indonesian throughflow and silicate constraint within their apriori uncertainties. We find that the use of the ADCP data adds significant detail to the horizontal circulation. These resolved circulations include the Agulhas Undercurrent, deep cyclonic gyres and deep fronts, features evidenced by long term integrators of the flow such as current meter and float measurements as well as water properties.  相似文献   

14.
Understanding the movement of marine fish larvae in coastal habitats requires an assessment of active swimming abilities. The critical speed (U-crit) and endurance swimming of late-stage larvae of Diplodus capensis and Sarpa salpa (Family Sparidae), common inshore recreational linefish species, were measured in a laboratory swimming chamber. Postflexion and settlement-stage larvae were collected from the wild in a small bay on the warm temperate coast of South Africa. Larvae were allowed to acclimate in captivity and were tested soon after capture. For the endurance tests a speed of 18 cm s−1 was selected, as this approximated the mean current speed observed in the coastal environment of the area. The mean U-crit value (maximum swimming speed) for D. capensis (19 cm s−1) was similar to that of S. salpa (18 cm s−1), and similarly mean endurance (km swum) for S. salpa (8 km) was similar to that of D. capensis (6 km). The increase in critical speed and endurance swimming abilities with standard length was best described by a linear relationship. At lengths between 12 and 15 mm BL, D. capensis was the better swimmer, whereas S. salpa was the better swimmer between 15 and 16 mm BL. Of all the larvae that swam at critical speed, 90% were in an inertial environment. These swimming speeds exceed the modal current velocities observed in the shallow nearshore of the study region where these larvae occur abundantly. These swimming abilities provide larvae with the potential to influence their dispersal trajectories and ultimately influence their distribution in their nearshore nursery areas.  相似文献   

15.
Turbidity limits gas exchange in a large macrotidal estuary   总被引：1，自引：0，他引：1  
In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO2 in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO2 and floating chamber flux measurements. Values of k increased with wind speed and were significantly greater when water currents and wind were in opposing directions. At low wind speeds (<1 m s−1), k increased with water current velocities (0–1.5 m s−1) following an exponential trend. The latter was a good proxy for the Y-intercept in a generic equation for k versus wind speed in estuaries. We also found that, in this turbid estuary, k was significantly lower at high turbidity. The presence of suspended material in great concentrations (TSS > 0.2 g L−1) had a significant role in attenuating turbulence and therefore gas exchange. This result has important consequences for modeling water oxygenation in estuarine turbidity maxima. For seven low turbidity estuaries previously described in the literature, the slope of the linear regression between k and wind speed correlates very well with the estuary surface area due to a fetch effect. In the Gironde Estuary, this slope follows the same trend at low turbidity (TSS < 0.2 g L−1), but is on average significantly lower than in other large estuaries and decreases linearly with the TSS concentration. A new generic equation for estuaries is proposed that gives k as a function of water current velocity, wind speed, estuarine surface area and TSS concentration.  相似文献   

16.
We investigated the movement of the Kuroshio axis on the northeast shelf of Taiwan associated with the passage of typhoons, using sea surface current data observed by the ocean radar system on Yonaguni and Ishigaki islands. First, we examined daily Kuroshio axis variation on the northeast shelf of Taiwan during typhoon events. The ocean radar data showed that the Kuroshio axis moved onto the shelf after passages of typhoons. The Kuroshio moved onto the shelf and stayed there after the passage of Typhoon Hai-Tang; while the Kuroshio maintained this pattern, southerly wind blew continuously for 4 days. The mean current speed northeast of Taiwan after the typhoon's passage increased by 18 cm s−1. In addition, the sea level difference between two satellite altimetry tracks east of Taiwan increased by 14.4 cm. These results suggest that coastal upwelling east of Taiwan caused by the southerly wind generated an east–west sea level difference that, in turn, generated a northward geostrophic current. This current could have enhanced the Kuroshio east of Taiwan, and pushed it onto the shelf.  相似文献   

17.
Deep-circulation flow at mid-latitude in the western North Pacific   总被引：1，自引：1，他引：1  
Direct current measurements with five moorings at 27–35°N, 165°E from 1991 to 1993 and with one mooring at 27°N, 167°E from 1989 to 1991 revealed temporal variations of deep flow at mid-latitude in the western North Pacific. The deep-circulation flow carrying the Lower Circumpolar Deep Water from the Southern Ocean passed 33°N, 165°E northwestward with a high mean velocity of 7.8 cm s−1 near the bottom and was stable enough to continue for 4–6 months between interruptions of 1- or 2-months duration. The deep-circulation flow expanded or shifted intermittently to the mooring at 31°N, 165°E but did not reach 35°N, 165°E although it shifted northward. The deep-circulation flow was not detected at the other four moorings, whereas meso-scale eddy variations were prominent at all the moorings, particularly at 35°N and 29°N, 165°E. The characteristics of current velocity and dissolved oxygen distributions led us to conclude that the deep-circulation flow takes a cyclonic pathway after passing through Wake Island Passage, passing 24°N, 169.5–173°E and 30°N, 168–169°E northward, proceeds northwestward around 33°N, 165°E, and goes westward through the south of the Shatsky Rise. We did not find that the deep-circulation flow proceeded westward along the northern side of the Mid-Pacific Seamounts and eastward between the Hess Rise and the Hawaiian Ridge toward the Northeast Pacific Basin.  相似文献   

18.
During November 2000–June 2002, both direct current measurements from deployment of a line of five moorings and repeated CTD observations were conducted along the Oyashio Intensive observation line off Cape Erimo (OICE). All the moorings were installed above the inshore-side slope of the Kuril-Kamchatka Trench. Before calculating the absolute volume transports, we compared vertical velocity differences of relative geostrophic velocities with those of the measured velocities. Since both the vertical velocity differences concerned with the middle three moorings were in good agreement, the flows above the continental slope are considered to be in thermal wind balance. We therefore used the current meter data of these three moorings, selected among all five moorings, to estimate the absolute volume transports of the Oyashio referred to the current meter data. As a result, we estimated that the southwestward absolute volume transports in 0–1000 db are 0.5–12.8 × 106 m3/sec and the largest transport is obtained in winter, January 2001. The Oyashio absolute transports in January 2001, crossing the OICE between 42°N and 41°15′ N from the surface to near the bottom above the continental slope, is estimated to be at least 31 × 106 m3/sec. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
Field data were analyzed from a simultaneous deployment of two 3D-ACM WAVE instruments; one on a fixed seabed frame in the nearshore zone, and the other further offshore on a taut-wire mooring. An intercomparison of measurements of vertical and horizontal wave-orbital currents with pressures was used to evaluate the velocity sensor response under field conditions. Results using the fixed frame have validated the measured horizontal wave-orbital velocities, but found the vertical velocities to be less coherent with the pressure time-series. The influence of the instrument mooring system on the velocity measurements was investigated. The oscillation of the taut-wire mooring was found to influence the magnitude of the measured horizontal wave-orbital velocities and induce a phase lag between velocity and sea-surface elevation. Examination of other data from similar taut-wire moorings indicates a systematic relationship between the length of the mooring cable and the measured phase lag, consistent with the behavior of the mooring system considered as a forced, linearly damped oscillator. A comparison was made between the spectra of wave direction derived from both velocity and pressure data with that derived solely from velocity data. The results show a high coherence for the fixed mooring, but significant directional variability in the higher frequencies (>0.13 Hz) on the taut-wire mooring we employed, which we attribute to the mooring oscillation. The analysis further indicates that on taut-wire moorings, the spectra of wave direction should be resolved solely from velocity data. Using these findings, directional wave spectra were produced for the nearshore and offshore sites from 233 coincident events over a two-month period, and these data are presented in a time-averaged spectral format  相似文献   

20.
The knowledge of juvenile fish growth in extreme environmental conditions is a key to the understanding of adaptive responses and to the relevant management of natural populations. The juvenile growth of an extreme euryhaline tilapia species, Sarotherodon melanotheron (Cichlidae), was examined across a salinity gradient (20–118) in several West African estuarine ecosystems. Juveniles were collected during the reproduction period of two consecutive years (2003 and 2004) in six locations in the Saloum (Senegal) and Gambia estuaries. Age and growth were estimated using daily otolith microincrements. For each individual, otolith growth rates showed three different stages (slow, fast, decreasing): around 4 ± 0.5 μm d−1 during the first five days, 9 ± 0.5 μm d−1 during the next 15 days and 4 ± 0.50 μm d−1 at 60 days. Growth modelling and model comparisons were objectively made within an information theory framework using the multi-model inference from five growth models (linear, power, Gompertz, von Bertalanffy, and logistic). The combination of both the model adjustment inspection and the information theory model selection procedure allowed identification of the final set of models, including the less parameterised ones. The estimated growth rates were variable across spatial scales but not across temporal scales (except for one location), following exactly the salinity gradient with growth decrease towards the hypersaline conditions. The salinity gradient was closely related to all measured variables (condition factor, mean age, multi-model absolute growth rate) demonstrating the strong effect of hypersaline environmental conditions—induced by climate changes—on fish populations at an early stage.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号