共查询到20条相似文献，搜索用时 31 毫秒
1.
Lagrangian flow patterns in the vicinity of Cape Hatteras are examined using the tracks of 42 drifters drogued at 10 m depth and initially deployed over Georges Bank. The drifters predominantly move southwestward over the continental shelf and slope. North of Cape Hatteras, the drifters become entrained in the Gulf Stream and are carried eastward into the central Atlantic Ocean. There are two types of entrainment, abrupt and gradual. The first is characterized by a rapid change in drifter speed and an abrupt shift in drifter direction to the east. During such entrainment events, the radius of curvature of the drifter track is less than 30 km. The second type of entrainment is characterized by a gradual change in drifter direction with little change in speed. The radius of curvature of drifter tracks during such entrainment events is large (typically 50 km). The latter type occurs more frequently in summer and fall, when stratification is stronger. The drifter tracks further reveal that entrainment from the shelfbreak front/slope water system into the Gulf Stream may occur a significant distance north of Cape Hatteras, occasionally as far north as 38 °N, 200 km north of Cape Hatteras. Only two drifter tracks extend along the shelf past Diamond Shoals into the South Atlantic Bight. Four drifters are ejected from the Gulf Stream and recirculate over the slope. The observed time scale of recirculation ranges over 1–3 months. These results suggest that there are a variety of processes that determine the maximum southward penetration of Mid-Atlantic Bight shelf water before entrainment into the Gulf Stream as well as the cross-slope speed of entrainment. 相似文献
2.
J.D. Wang 《Estuarine, Coastal and Shelf Science》1998,46(6):901-915
Recent observations using moored current meters, shipboard ADCP transects, salinity mapping and drifters have been used to study the residual circulation including wind drift in western Florida Bay.Rapid, nearly synoptic surveys of salinity over a large area was an effective tracer-mapping technique, when salinity gradients were sufficiently strong, and provided qualitative information on Lagrangian water motion for the entire study area. The salinity maps indicated a general south-eastward advection, which was only subordinate to tidal mixing in a narrow zone adjacent to the Florida Keys.Drifter data collected simultaneously, allowed quantitative estimates to be added to the transport pattern suggested by salinity maps. The selectively deployed drifters yielded estimates of total drift velocities. In addition, moored current meters and shipboard current profiling were used to determine the distribution of flow across the mouth of the bay facing the Gulf of Mexico and the transport through Long Key Channel, a major connection between the bay and the Atlantic Ocean.Analysis showed that from 64 to over 92% of the drifter trajectory variances could be explained by the combination of a local wind drift, expressed in terms of a wind drift factor multiplied by the surface shear velocity, and an ambient current. For a 1 m high drifter deployed at the surface of the water column, the wind drift factor was found to be approximately 0·125m, making the drift speed roughly equal to 0·45% of wind speed. The mean drifter speeds were linearly proportional to mean transport estimates derived from the current meter observations in Long Key Channel, enhancing confidence in both data sets.The total south-eastward directed residual current varied between 100 and 5000 m day−1and was weaker in summer than in winter, when southward winds associated with periodic passage of cold fronts boost the residual flow. The estimated contribution from local wind drift varied between 500 m day−1in summer to 1000 m day−1in winter. The remaining contribution to the observed Lagrangian residual circulation in western Florida Bay is caused by other forcing, including tidal rectification, remote wind forcing and large-scale current systems (the Gulf Stream and Florida Current systems). 相似文献
3.
Shinya Magome Tomohiro Yamashita Takeshi Kohama Atsushi Kaneda Yuichi Hayami Satoru Takahashi Hidetaka Takeoka 《Journal of Oceanography》2007,63(5):761-773
Jellyfish patch formation is investigated by conducting a drifter experiment combined with aerial photography of a sustained
patch of the moon jellyfish in Hokezu Bay, Japan. Jellyfish patches are aggregations of individuals that are caused by a combination
of swimming (active influence) and advection by currents (passive influence). The drifter experiment involved the injection
of 49 drifters around a distinct surface patch of jellyfish within an area of approximately 300 m × 300 m. The drifters’ motion,
caused only by the passive influence, was recorded in a series of 38 aerial photographs taken over approximately 1 h. The
ambient uniform current field larger than the patch scale was estimated from the movement of the centroid position of drifters,
while the distribution of horizontal divergence and relative vorticity around the patch was estimated from the time-derivative
in areas of triangles formed by the drifters. The centroid positions of both drifters and patches moved stably toward the
bay head at different speeds. The difference vector between the patch and drifter centroids was directed to the sun, and was
opposite to the ambient current. The distributions of vorticity and divergence around patches exhibited inhomogeneity within
the patch scale, and the drifters in this nonuniform current field aggregated near the convergence area within 1 h. The results
suggest that horizontal patch formation is predominantly influenced by passive factors at the surface of Hokezu Bay. Furthermore,
the upward swimming against downwelling may make sustained patch in surface layer. 相似文献
4.
《Ocean Modelling》2011,40(3-4):301-310
From a data set encompassing the years 1990–2008 pairs of surface drifters with maximum initial separations of 5, 10 and 25 km have been identified. Model trajectories have been calculated using the same initial positions and times as the selected pairs of surface drifters. The model trajectories are based on the TRACMASS trajectory code and driven by the ocean general circulation model NEMO. The trajectories are calculated off-line, i.e. with the stored velocity fields from the circulation model. The sensitivity of the trajectory simulations to the frequency of the stored velocity fields was tested for periods of 3 and 6 h as well as 5 days. The relative dispersion of the surface-drifter and model trajectories has been compared, where the latter was found to be too low compared to the relative dispersion of the drifters.Two low-order trajectory sub-grid parameterisations were tested and successfully tuned so that the total amplitude of the relative dispersion of the model trajectories is similar to that associated with the drifter trajectories. These parameterisations are, however, too simple for a correct simulation of Lagrangian properties such as the correlation time scales and the variance of the eddy kinetic energy.The importance of model-grid resolution is quantified by comparing the relative dispersion from an eddy-permitting and a coarse-resolution model, respectively. The dispersion rate is halved with the coarse grid. The consequences of the two-dimensionality of the trajectories is evaluated by comparing the results obtained with the 2D and the Lagrangian 3D trajectories. This shows that the relative dispersion is 15% stronger when the trajectories are freely advected with the 3D velocity field. 相似文献
5.
Ocean prediction systems rely on an array of assumptions to optimize their data assimilation schemes. Many of these remain untested, especially at smaller scales, because sufficiently dense observations are very rare. A set of 295 drifters deployed in July 2012 in the north-eastern Gulf of Mexico provides a unique opportunity to test these systems down to scales previously unobtainable. In this study, background error covariance assumptions in the 3DVar assimilation process are perturbed to understand the effect on the solution relative to the withheld dense drifter data. Results show that the amplitude of the background error covariance is an important factor as expected, and a proposed new formulation provides added skill. In addition, the background error covariance time correlation is important to allow satellite observations to affect the results over a period longer than one daily assimilation cycle. The results show the new background error covariance formulations provide more accurate placement of frontal positions, directions of currents and velocity magnitudes. These conclusions have implications for the implementation of 3DVar systems as well as the analysis interval of 4DVar systems. 相似文献
6.
In order to better estimate meso-scale variabilities in the energetic Kuroshio Extension (KE) region, simultaneous assimilation of drifter-derived velocity data, together with satellite and in situ hydrographic data, is attempted by using a high-resolution 4-dimensional variational data assimilation (4D-VAR) system. Our experimental results, both with and without assimilation of drifter data (Exp. Drf and Exp. Ref, respectively) for the period during Aug–Oct 2005, show that the reproduced fields in Exp. Drf better reflect the observed meso-scale features such as the KE meandering jet and associated eddies. The adjoint sensitivity analysis indicates that our 4D-VAR system has the ability to provide a more realistic timeseries of the meandering jet structures that play a key role in the intergyre exchange between the subtropical and subarctic gyres in the North Pacific. In addition, the observed information from the surface drifters works to improve the subsurface structure. These results illustrate the advantage of our 4D-VAR simultaneous assimilation with the addition of drifter-derived surface velocity information. 相似文献
7.
Shin’ichiro Kako Atsuhiko Isobe Shun Yoshioka Pil-Hun Chang Takeshi Matsuno Sang-Hyun Kim Joon-Soo Lee 《Journal of Oceanography》2010,66(2):161-174
To investigate technical issues associated with the particle-tracking numerical models frequently used to reproduce the behavior
of objects drifting in the actual ocean, the trajectories of satellite-tracked drifters released in 2003, 2004, and 2007 were
reproduced using a numerical model. In particular, the wind stress driving the surface currents which carried the drifters
has been computed using satellite-observed QuikSCAT/Seawinds data provided twice daily in conjunction with in-situ Ieodo-station
wind data. Although it is difficult to reproduce the trajectory of a single drifter using numerical models because of the
uncertainty induced by random-walk processes, the similarity between the modeled particle and observed buoy trajectories is
statistically significant, except for the experiment in 2007. In general, the satellite-derived wind field modified using
in situ data is likely to be able to reproduce observed drifter motion. However, it is found that the model is unable to reproduce
drifter trajectories in windy 2007. The numerical modeling result demonstrates that wind-induced leeway drift prevails in
drifter motion in 2007, in spite of the wind-resistant drogue attached to the drifters, and that this drift shows non-negligible
spatiotemporal variability, suggesting that leeway drift is not simply proportional to wind speeds, as in previous studies
have maintained. 相似文献
8.
Argos漂流浮标的若干观测结果 总被引：4，自引：0，他引：4
表层漂流浮标是一种利用Argos卫星系统定位与传送数据的海洋观测设备,它可以利用Lagrangian法则连续观测表层海流及表层水温。文章利用近年在南海和西太平洋投放的部分卫星跟踪表层漂流浮标所取得的观测资料,分析观测海域的表层海流特征及其漂移路径上的温度变化,得出以下结果。(1)由浮标的漂移轨迹看出,黑潮表层流路年际变异相当大。(2)2003年1月,黑潮表层水有入侵南海的趋势,似无西北太平洋表层水深入南海的迹象;夏季南海表层水由南海流出经吕宋海峡汇入黑潮主干。(3)秋季台湾东北海区存在一个强反气旋涡,空间尺度约270km。(4)黑潮主干在秋末冬初经过东海时明显呈弯曲流动,并形成许多小尺度的气旋式涡。(5)在九州西南海域,黑潮表层流并无分支北上进入对马暖流区。(6)2003年春季,黑潮在日本以南的弯曲不明显,并伴随有冷、暖涡产生,暖涡的强度和空间尺度都要比冷涡大得多。(7)由漂流浮标观测得到的的表层海水的温度分布明显呈日变化和季节变化的特征。在浮标漂移路径呈反气旋或气旋式转动的区域,表层水温对应出现高或低温区;但出现在台湾以东的低温区则与此时期的台风过境有关。 相似文献
9.
Argos表面漂流浮标在黑潮区的若干观测结果 总被引：3，自引：0，他引：3
利用近几年国家海洋局第二海洋研究所及国家海洋技术中心在南海和西北太平洋海域布放的部分卫星跟踪表面漂流浮标所取得的观测资料,分析了浮标流经海域的表层海流特征及浮标漂移路径上水温的变化。结果表明:2003年1月,黑潮表层水有入侵南海的趋势,夏季南海表层水经吕宋海峡流出,汇入黑潮主干;夏末冬初,黑潮主干经过东海时明显呈弯曲流动;2003年春季,日本以南海域黑潮弯曲不明显;台湾东北部海域存在一个强反气旋涡;表层海水的温度日变化和季节变化明显,在浮标漂移路径呈反气旋或气旋式转动的区域,对应出现了表层水温的高、低温区。 相似文献
10.
Development of means and methods of drifter technology applied to the problem of the Black Sea research 总被引：1，自引：0，他引：1
During the last ten years, autonomous drifting platforms for data collection (drifters) equipped with Argos satellite communication devices have become one of the main components for the observation system in the upper layer of the ocean and surface atmosphere. This article presents the results of the Black Sea drifter monitoring in 2002–2006 within a number of international programs and projects. During this period, about 70 drifters manufactured by NPF Marlin-Yug Ltd. (Sevastopol, Ukraine) were deployed in the western part of the Black Sea. The drifters were mainly standard barometric Lagrangian tracers and a part of them were principally new temperature profiling buoys. Unique long-term data were obtained about the circulation of the surface currents and the dynamics of the temperature stratification within the seasonal thermocline and the cold intermediate layer. 相似文献
11.
Richard Limeburner Robert C. Beardsley 《Deep Sea Research Part II: Topical Studies in Oceanography》1996,43(7-8)
Satellite-tracked drifters with drogues centered near-surface (5 m) and below the seasonal thermocline (50 m) were launched during late winter and spring of 1988 and 1989 in the northern Great South Channel in the western Gulf of Maine to investigate the regional circulation as part of the South Channel Ocean Productivity Experiment (SCOPEX). Many of the near-surface drifters became entrained in the clockwise gyre over Georges Bank, and eight drifters made a total of 16 complete circuits around the bank during the stratified season. The average recirculation period of these eight drifters was 48 days, and the average drifter speed around the bank was 12 cm s−1. There is no clear evidence from the drifter data that the strength of the clockwise gyre over the bank increased with time during the stratified season. On average, these drifters (i) followed a relatively narrow path around the bank, except over the eastern end of the bank where three preferred paths were observed, (ii) moved fastest over the northern and southern flanks of the bank, (iii) did not enter a core area of 3500 km2 centered at 41°17′N, 68°00′W, approximately 30 km southwest of the topographic center of the bank, and (iv) stopped circling the bank by the end of November, due in part to strong wind events that appeared to drive drifters off the bank. Curiously, none of the near-surface drifters moved from the southern flank of Georges Bank onto the New England shelf as might be expected from continuity of flow along the outer shelf; instead, the drifters that circled the bank tended to move off the bank along its southern flank. None of the drifters with drogues centered at 50 m appeared to recirculate around Georges Bank. 相似文献
12.
《Deep Sea Research Part II: Topical Studies in Oceanography》1999,46(11-12):2371-2403
Statistics of the near-surface circulation in the northeast Pacific Ocean were derived from the trajectories of nearly 100 surface drifters tracked between August 1990 and December 1995 as part of the World Ocean Circulation Experiment's (WOCE) Surface Velocity Program (SVP). Drifters were drogued within the mixed layer (15 m drogue depth) or near the top of the permanent halocline (120 m). All branches of the Alaskan Gyre were well-sampled at both depths, revealing a weak Subarctic Current, a bifurcation of the Subarctic Current near 48°N, 130°W at 15 m depth, and strong, variable flow in the Alaska Current and Alaskan Stream. At 120 m depth, northward flow in the Alaska Current occurred much farther offshore than within the mixed layer. The drifter trajectories revealed interannual variability, with evidence of an intensified Alaskan Gyre during the winters of 1991–92 and 1992–93 and more southerly transport during winter 1994–95. A minimum in eddy kinetic energy was found at both depths within the northern branch of the Subtropical Gyre. Eddy kinetic energies were nearly twice as high in the mixed layer compared to below, and were 2–3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. High eddy energies observed near the eastern perimeter of the Alaskan Gyre may be due to the offshore intrusion of eddies formed by coastal current instabilities.Taylor's theory of single-particle dispersion was applied to the drifter ensembles to estimate Lagrangian decorrelation scales and eddy diffusivities. Both the initial dispersion and random walk regimes were identified in the dispersion time series computed for several regions of both ensembles. The integral time scales and eddy diffusivities computed from the dispersion scale linearly with r.m.s. velocity, which is consistent with drifter studies from the Atlantic. An exception is the meridional integral time scales, which were nearly constant throughout the study area and at both drogue depths. The magnitudes of the derived eddy statistics are comparable to those derived from surface drifters in other parts of the world ocean. These are the first Lagrangian estimates of particle dispersion over a broad region of the near-surface North Pacific, and the consistency of the results with previous studies from the Atlantic lends credence to the idea that the simplifying assumptions of Taylor (1921) (Proceedings of the London Mathematical Society Series A 20, 196–221) are reasonably valid throughout the upper ocean. This bodes well for the effective parameterization of near-surface diffusivities in general circulation models. Finally, the drifter-derived velocity statistics were used to speculate on the source regions of waters of possible coastal origin observed at offshore stations during the field studies of the Canadian Joint Global Ocean Flux Study. 相似文献
13.
In the summer and fall of 2012, during the GLAD experiment in the Gulf of Mexico, the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) used several ocean models to assist the deployment of more than 300 surface drifters. The Navy Coastal Ocean Model (NCOM) at 1 km and 3 km resolutions, the US Navy operational NCOM at 3 km resolution (AMSEAS), and two versions of the Hybrid Coordinates Ocean Model (HYCOM) set at 4 km were running daily and delivering 72-h range forecasts. They all assimilated remote sensing and local profile data but they were not assimilating the drifter’s observations. This work presents a non-intrusive methodology named Multi-Model Ensemble Kalman Filter that allows assimilating the local drifter data into such a set of models, to produce improved ocean currents forecasts. The filter is to be used when several modeling systems or ensembles are available and/or observations are not entirely handled by the operational data assimilation process. It allows using generic in situ measurements over short time windows to improve the predictability of local ocean dynamics and associated high-resolution parameters of interest for which a forward model exists (e.g. oil spill plumes). Results can be used for operational applications or to derive enhanced background fields for other data assimilation systems, thus providing an expedite method to non-intrusively assimilate local observations of variables with complex operators. Results for the GLAD experiment show the method can improve water velocity predictions along the observed drifter trajectories, hence enhancing the skills of the models to predict individual trajectories. 相似文献
14.
15.
Hideo Kawai 《Journal of Oceanography》1985,41(3):157-166
To area-average the horizontal divergence and the vertical component of vorticity, three methods are proposed and examined. The polygon method is based on deformations of a polygon made by connecting drifters. They are deployed at widely different scales of 1 cm– 1,000 km and tracked using various procedures. The loop method is adopted when a drifter completes at least two loops of trajectory in a tidal vortex, a ring or a gyre. Even if data for a drifter completing only one loop is available, the vorticity can be calculated. The crossing method is applied to the GEK data on the circumference of a Kuroshio ring. The data which will be used to calculate them in Part 2 are summarized in tables. Offset dispositions of positive and negative divergences or vorticities on a horizontal plane and in a water column are shown. Probably, the vertical offset of vorticities does not occur in general. The area-turnover of a polygon of drifters are discussed. Sampling time-intervals, appropriate to the scale of the area, for the polygon and loop methods are examined. A first impression of Rhines' (1979) sketch has produced a misunderstanding that the polygon method would be useless because a limited number of drifters cannot follow such a complicated deformation of the material line over a long period. It is shown that adopting a short time-scale appropriate to the length scale furnishes a practical solution to the problem. 相似文献
16.
A group of 30 surface drifters, launched over a 4 day period as part of a US Navy exercise in early October 2007, are used to assess the predictability of trajectories in a confined geographic region at the northwestern edge of the Kuroshio north of Taiwan. Model trajectories were computed from archives of hourly hindcast velocities from the US Navy East Asian Seas (EAS16) model with 1/16° horizontal resolution. Three metrics are defined for comparing observed and modeled trajectories. All three metrics indicated that model trajectories separated from observations by roughly 15 km after the first 24 h on average. Because of the unique launch strategy for these drifters, with six repetitions of launches from four locations, the dependence of predictability on both launch time and launch location could be assessed separately. Predictive skill displayed only modest dependence on launch time, likely influenced by the passage of a typhoon near the experiment area a few days prior to the first drifter launch. Launch location was a much more reliable indicator of predictive skill, with trajectories for launches closest to the edge of the Kuroshio typically hardest to predict, and those for launches on the shelf, where currents tended to be weaker, predicted more accurately. Comparisons of skill metric statistics for modeled trajectories from hindcasts with and without tides suggested that tidal currents have only a small impact on predictive skill. The influence of archive time and space resolution was also studied using sets of model trajectories computed from hindcast archives that were systematically subsampled separately in space and time. Coarsening by up to a factor of eight in either space or time had little impact on predictive skill. Further coarsening degraded trajectory predictions, particularly when coarsening in time leads to an archive time step too large to adequately resolve the tides. While accurate trajectory predictions remain challenging for ocean models, skill assessments like the one presented here are important for developing error estimates for users of trajectory forecasts and for gaining new insight into potential sources of model errors. 相似文献
17.
The article proposes parallel implementation of the Ensemble Optimal Interpolation (EnOI) data assimilation (DA) method in eddy-resolving World Ocean circulation model. The results of DA experiments in North Atlantic with ARGO drifters are compared with the multivariate optimal interpolation (MVOI) DA scheme. The sensitivity of the model error, i.e., the difference between the model and observations depending on the number of ensemble elements, is also assessed and presented. The effectiveness of this method over the MVOI scheme is confirmed. The model outputs with and without assimilation are also compared with independent sea surface temperature data from ARMOR 3d. 相似文献
18.
We implement an approach for the accurate assimilation of Lagrangian data into regional general ocean circulation models. The forward model is expressed in Lagrangian coordinates and simulated float data are incorporated into the model via four-dimensional variational data assimilation. We show that forward solutions computed in Lagrangian coordinates are reliable for time periods of up to 100 days with phase speeds of 1 m/s and deformation radius of 35 km. The position and depth of simulated floats are assimilated into the viscous, Lagrangian shallow water equations. The weights for the errors in the model and data are varied and the assimilation results react appropriately. We show the effect of different spatial and temporal samplings of float data on all Lagrangian trajectories in the computational domain. At the end of the assimilation period, results from the Lagrangian shallow water equations could be interpolated and used as initial and boundary conditions in an Eulerian general ocean circulation model. 相似文献
19.
Argo drifters provide information of the vertical structure in the water column and have a potential for the improvement of
understanding phytoplankton primary production and biogeochemical cycles in combination with ocean color satellite data, which
can obtain the horizontal distribution of phytoplankton biomass in the surface layer. Our examples show that using Argo drifters
with satellite-measured horizontal distribution of phytoplankton biomass at the sea surface allow an improved understanding
of the development of the spring bloom. The other possible uses of Argo drifter are discussed. 相似文献
20.
In August 1998, a recurrent filament located near 42°N off Galicia was sampled as part of the OMEX-II project. Lagrangian and other observations were made on the shelf where the filament arose and offshore in the filament itself under upwelling favourable but fluctuating winds. The shelf drift experiment monitored a change from southward to weak northward net flow as the winds decreased to zero. Shipborne ADCP measurements showed that the shelf was supplying decreasing volumes of water to the filament as the wind speeds decreased. At the shelf edge the internal tide was larger than can be explained by local forcing and there were many unusually large high frequency internal waves with a quasi-sinusoidal form. Turbulence observations revealed enhanced dissipation rates and vertical eddy diffusion coefficients within the shelf thermocline (of order 1 cm2 s−1), which appeared to be caused by the breaking of internal wave. A second Lagrangian experiment was executed in the filament some 120 km offshore, which again coincided with a period of wind relaxation. Cross-sections revealed a double cold core and that the offshore flow was limited to a thin surface layer. Substantial onshore flow occurred below 50 m in the centre of the filament, while the strongest and deepest offshore flow coincided with its northern boundary. Turbulent kinetic energy dissipation rate measurements showed very weak mixing below 15 m in the filament core, but enhanced mixing at its boundaries. Four mixed layer drifters released in the filament initially indicated convergence at its southern boundary, marked by strong temperature and salinity contrasts. After the wind became more favourable for upwelling, the drifters accelerated. One drifter traced the full extent of the filament, while the other three escaped from it and began to circulate cyclonically over 28 days in a 100 km diameter loop back towards their release point. Although strong mesoscale activity linked the shelf and ocean regimes, offshore transport in the filament was weak at the time of the experiment and vertical and horizontal re-circulations on a variety of time scales were important. There was sufficient vertical mixing in the thermocline to cause it to thicken and draw some heat into the lower layers during the summer months on the shelf. The amount of heat involved was too little to have a significant impact on the development of a filament over a typical lifetime of a week. 相似文献