Dispersion of surface drifters and model-simulated trajectories

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.     

Current measurements with surface and subsurface drifters

An examination of behaviors of a subsurface drogue and a subsequent examination of current measurement with a drifter comprising the drogue as its important part are made in the channel between Oshima and Izu Peninsula. The drogue submerged to the anticipated depth of 300 m within 10 minutes after launching. Since then the drogue kept its depth and operated normally. From the comparison with the velocity measurement by the use of a currentmeter moored at a station in the vicinity of the drifter's track, it is verified that the drifter's motion well reflects the motion of a water parcel around the drogue.     

Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters

Surface drifters and subsurface floats drifting at depths near 800 m were used to study the pathways of warm, salty Indian Ocean water leaking into the South Atlantic that is a component of the upper limb of the Atlantic meridional overturning circulation (MOC). Four drifters and 5 floats drifted from the Agulhas Current directly into the Benguela Current. Others looped for various amounts of time in Agulhas rings and cyclones, which translated westward into the Atlantic, contributing a large part of Indian Ocean leakage. Agulhas rings translated into the Benguela Current, where they slowly decayed. Some large, blob-like Agulhas rings with irregular shapes were found in the southeastern Cape Basin. Drifter trajectories suggest these rings become more circular with time, eventually evolving into the circular rings observed west of the Walvis Ridge. Agulhas cyclones, which form on the north side of the Agulhas Current south of Africa, translated southwestward (to 6°E) and contributed water to the southern Cape Basin. A new discovery is a westward extension from the mean Agulhas retroflection measured by westward drifting floats near 41°S out to at least 5°W, with some floats as far west as 25°W. The Agulhas extension appears to split the South Atlantic Current (SAC) into two branches and to transport Agulhas water westward, where it is mixed and blended with eastward-flowing water from the western Atlantic. The blended mixture flows northeastward in the northern branch of the SAC and into the Benguela Current. Agulhas leakage transport was estimated from drifters and floats to be at least 15 Sv in the upper 1000 m, which is equivalent to the transport of the upper layer MOC. It is suggested that the major component of the upper layer overturning circulation in the Atlantic is Agulhas leakage in the form of Agulhas rings.     

Argos表面漂流浮标在黑潮区的若干观测结果

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。     

Currents in the Taiwan Strait as observed by surface drifters

The trajectories of 110 satellite-tracked surface drifters from 1989 to 2007 were analyzed to elucidate near-surface circulation in the Taiwan Strait. Although the summer circulation observed generally agrees with previous studies, several aspects of the winter circulation were revealed by the analyses. Unlike many earlier studies, which have suggested that a northward (southward) current prevails in the eastern (western) part of the Taiwan Strait during the northeast monsoon season, this study shows that almost all winter drifters that entered the Taiwan Strait eventually moved southward. Inside the Taiwan Strait, northward moving tracks can only be found in the Penghu Channel. After passing the Penghu Channel, the drifters were blocked by the northeast monsoon wind and the Yun-Chang Rise, and turned southward. None of the drifters flowed persistently northward through the Taiwan Strait in winter. In the southern Taiwan Strait, three typical patterns of circulation were observed for the winter trajectories—the “throughflow” pattern that enters the South China Sea flowing westward along the slope; the loop current pattern that circulates anticyclonically and returns to the Kuroshio; and the blocked intrusion pattern that penetrates into the Taiwan Strait through the Penghu Channel.     

Near-surface circulation of the northeast Pacific Ocean derived from WOCE-SVP satellite-tracked drifters

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.     

西北太平洋反气旋涡的Argos浮标观测结果分析

结合卫星高度计异常资料和2003年10月上旬投放在西北太平洋的25个Argos表层漂流浮标资料,分析观测海域的中尺度涡特征及浮标漂移路径上的温度和流速变化,结果表明:(1)7个浮标受强劲的黑潮流影响直接进入台湾岛以东黑潮表层的主流轴;(2)16个浮标在反气旋涡内旋转,并随中尺度涡向西运动,到达黑潮的东边界,由于中尺度涡旋的消亡,浮标脱离其影响后由黑潮带动向东海运动,浮标的移动轨迹呈螺线型;(3)仅有2个浮标在(123°E、20°N)附近通过吕宋海峡进入南海,且41490号浮标受台湾岛西南外海反气旋涡的影响作了2周旋转后再进入南海。比较分析表明,黑潮在冬季应该存在入侵南海的分支,但浮标能否顺利进入南海受多种随机因素控制,如风生流、潮流和波浪等。另外,西北太平洋向西传播的中尺度涡难以越过强劲的黑潮流屏障继续向西传播通过吕宋海峡进入南海。     

Creation of the database of monitoring of the Black Sea by using drifters

We consider some specific features of creation of the database according to the results of drifter experiments carried out in the Black Sea in 2001–2006. The general statistical information on all buoy studies in the sea is presented. The criteria used to filter the primary data are suggested. The principles of formation and structuring of the drifter database are presented. As an example, we describe the procedure and the results of comparison of model estimates with the data of contact measurements of the sea-surface temperature by drifting buoys.     

Lagrangian flow patterns north of Cape Hatteras using near-surface drifters

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.     

Design of small GPS drifters for current measurements in the coastal zone

Field observations of flows in coastal zone are scarce, but important for understanding the spatial variability of currents. The design of small, low-cost GPS drifters for collecting accurate Lagrangian data in the coastal zone is described. The drifters are intended for using in nearshore environments, lakes and estuaries over timescales of a few minutes up to several hours and are a low-cost alternative for applications which do not require drifter’s sea-going capability. Two field tests of GPS drifters in the south coast of Caspian Sea in Anzali port, Iran, in November 2008 and July 2009 were successful.     

Enhanced estimation of sonobuoy trajectories by velocity reconstruction with near-surface drifters

An investigation to improve trajectory prediction using Lagrangian data is presented. The velocity field of a data assimilating model, EAS-16, is corrected using drifter observations taken during an experiment off Taiwan. The results are tested using another independent Lagrangian data set provided by sonobuoys launched in the same area. The latter have instrument chains that extend well into the water column. Consequently the corrected model velocities were projected into the water column in order to calculate sonobuoy trajectories. The drifter and sonobuoy trajectories both show two distinct regimes in the considered area of approximately 1/2° square. One regime is dominated by shelf dynamics, the other by meandering of the Kuroshio, with a sharp boundary dividing the two. These two regimes are not reproduced by the trajectories of the EAS-16 model. When the drifter data are blended with the model velocities, synthetic sonobuoy trajectories track the observed ones much better, and the two regimes are clearly depicted. Two different methods for the velocity reconstruction are tested. One is based on a variational approach and the other on a normal mode decomposition. Both methods show qualitatively similar improvements in the prediction of sonobuoys trajectories, with a quantitative improvement in the total rms error of approximately 50% and 25%, respectively.     

Effect of divergence,stretching and nonnormal distribution on apparent diffusion of surface drifters

Kawai (1976, 1979) derived two simple relations among the apparent diffusivity (based on the geometric-average variance for principal axes of spreading) for many surface drifters in a patch and horizontal divergence and isotropic turbulent diffusivity for ambient water. By a new term medley diffusivity is meant an average of the products of the position coordinates relative to the centroid of drifters and the residual velocity, that is, the velocity left by subtracting the mean velocity and the velocity due to the linear velocity gradient from the velocity of a drifter. Regarding the medley diffusivity as the isotropic turbulent diffusivity that is weighted anisotropically according to drifters spreading along each of the principal axes, this paper derives a new relation with a form intermediate between the above two relations. Using the relation, this paper defines the new critical period within which the effect of horizontal divergence on the apparent diffusion surpasses that of isotropic turbulent diffusion, and discusses the time-scales on which the space-scale dependence of the squared divergence (Kawai, 1985b) is based. Stretching of a patch of drifters and deviation of drifters' position from the bivariate normal distribution enhance the effect of isotropic turbulent diffusion on the apparent diffusion. This stretching effect is equivalent to the shear effect on a horizontal plane. Remarks on reducing errors in the estimation of isotropic turbulent diffusivity are made.     

Comparison of methods for argo drifters data assimilation into a hydrodynamical model of the ocean

Different data assimilation methods such as an extended Kalman filter, the optimal interpolation method, and a method based on the Fokker-Planck equation applications are considered. Data from the ARGO drifters are assimilated into the HYCOM shallow water model (University of Miami, USA). Throughout the study, the schemes and methods of parallel computations with an MPI library are used. The results of the computations with assimilations are compared between themselves and with independent observations. The method based on the Fokker-Planck equation and the extended Kalman filter are preferable because they give better results than the optimal interpolation scheme. The various model characteristics of the ocean, such as the heat content fields and others, are analyzed after the data assimilation.     

Surface circulation in the southwestern Japan/East Sea as observed from drifters and sea surface height

The mean circulation of the surface layer of the southwestern Japan/East Sea (JES) was examined using current measurements collected at 15 m by satellite-tracked drifters and merged sea level anomalies from satellite altimeters. The study of circulation patterns in this paper focused on the inflow passing through the western channel of the Korea Strait from the East China Sea. Empirical Orthogonal Function (EOF) analysis of non-seasonal sea level anomalies revealed that significant energy in the circulation pattern of Ulleung Basin was controlled by the inflow conditions through the Korea Strait. Three circulation patterns were identified that depended on the initial relative vorticity of the inflow. When inflow had initially large negative vorticity, the flow gained more negative vorticity due to deepening of the bottom (stretching) and then turned right after entering the JES. The inflow then followed the path of the Tsushima Warm Current along the coast of Japan. When the inflow was strong, with a speed in excess of 55 cm/s and with a large positive vorticity, potential vorticity appeared to be conserved. In this case, the EKWC followed isobaths along the coast and then left the coast, following topographic features north of Ulleung-Do. The northward flowing jet developed inertial meandering after leaving the coast, which is a characteristic of many western boundary currents. The regular, bimonthly deployments of drifters in the western portion of the Korea Strait revealed that splitting or branching of the flow through the western channel of the Korea Strait occurred only 15% of the time. And splitting or branching rarely occurred during the fall and winter seasons, when the inflow splitting was previously reported in hydrographic surveys. The time-averaged circulation map of the EKWC and its seaward extension were considerably enhanced by using regularly sampled geostrophic velocities calculated from sea level anomalies to remove biases in the mean velocity that were caused by irregular spatial and temporal drifter observations. The East Korean Warm Current, a mean coastal current along the Korean coast, behaved like the simple model by Arruda et al. (2004) in which the generation of the Ulleung Warm Eddy and the meandering circulation pattern were well reproduced.     

Seasonal characteristics of the near-surface circulation in the northern South China Sea obtained from satellite-tracked drifters

The surface circulation of northern South China Sea (hereafter SCS) for the period 1987–2005 was studied using the data of more than 500 satellite-tracked drifters and wind data from QuikSCAT. The mean flow directions in the northern SCS except the Luzon Strait (hereafter LS) during the periods October~March was southwestward, and April~September northeastward. A strong northwestward intrusion of the Kuroshio through the LS appears during the October~March period of northeasterly wind, but the intrusion became weak between April and September. When the strong intrusion occurred, the eddy kinetic energy (EKE) in the LS was 388 cm²/s² which was almost 2 times higher than that during the weak-intrusion season. The volume transport of the Kuroshio in the east of the Philippines shows an inverse relationship to that of the LS. There is a six-month phase shift between the two seasonal phenomena. The volume transport in the east of the Philippines shows its peak sis-month earlier faster than that of the LS. The strong Kuroshio intrusion is found to be also related to the seasonal variation of the wind stress curl generated by the northeasterly wind. The negative wind stress curl in the northern part of LS induces an anticyclonic flow, while the positive wind stress curl in the southern part of LS induces a cyclonic flow. The northwestward Kuroshio intrusion in the northern part of LS happened with larger negative wind stress curl, while the westward intrusion along 20.5°N in the center of the LS occurred with weaker negative wind stress curl.     

Statistical description of the Black Sea near-surface circulation using drifters in 1999–2003

The near-surface circulation in the Black Sea is studied with the data of 54 satellite-tracked drifters in the period 1999–2003. The drifter trajectories confirm the prevalence of the Rim Current trapped on the continental slope (between water depths of 400 and 1800 m) along the periphery of the basin where sub-inertial speeds can reach 1 m/s. Some drifters were found to complete an entire basin loop with the Rim Current in 90–180 days. Meanders and loops in the tracks prove the existence of mostly anticyclonic circulation features inshore of the Rim Current, including strong signatures of the Batumi and Sevastopol eddies. They also reveal the presence of cyclonic and anticyclonic currents in most areas of the Black Sea. Pseudo-Eulerian statistics (averaged in 50-km bins), that is, mean currents and the corresponding velocity variances, show a strong and highly fluctuating signature of the Rim Current and the enhanced variability associated with the Batumi and Sevastopol eddies. The latter is also collocated with the branching of the Rim Current southwest of the Crimean Peninsula. It is shown that the kinetic energy is mainly in the mean for the Rim Current and in the velocity fluctuations elsewhere. Seasonal variability is also explored. The Rim Current tends to form a stronger single loop trapped on the continental slope in winter/spring, whereas in summer/fall the mean circulation is more meandering, recirculation cells appear in the central areas and the bifurcation southwest of Crimea is enhanced. There is some evidence of the reversal of sense of rotation of the currents in the Batumi Eddy region, changing from mainly anticyclonic in summer/fall to cyclonic in winter/spring. Mean residence times were calculated in the 50-km bins, with values ranging from a few days in the central basin to 8 days in the northwestern coastal area. Globally, the kinetic energy levels were found higher in winter–spring and lower in summer–fall, with a significant maximum in March. Single-particle Lagrangian statistics were computed for the entire basin, for the two extended seasons, and in selected local areas. Velocity variance, diffusivity and Lagrangian integral time scales are generally larger in the zonal direction. Globally, the velocity variance is 174 and 127 cm²/s² in the zonal and meridional directions, respectively, after the removal of the pseudo-Eulerian mean circulation. For the zonal direction, a diffusivity value of 4.5×10⁷ cm²/s and Lagrangian integral time and space scales of 3 days and 34 km were found. For the meridional direction, these statistics amount to 4.5×10⁷ cm²/s, 1.2 days and 12.2 km. Seasonal and geographical variations of these Lagrangian statistics were also assessed, showing variations between 1.9 (0.9) 10⁷ and 8.3 (2.0) 10⁷ cm²/s for the diffusivity in the zonal (meridional) direction. Integral time and space scales vary between 1 and 4.7 days, and 8.8 and 58 km, respectively. Seasonal differences are significant only in the zonal direction, where the diffusivity increases from 3.1×10⁷ to 5.9×10⁷ cm²/s and the integral scales vary from 2.1 days (24 km) to 3.8 days (44 km) from summer/fall to winter/spring.     

Surface current structure of the Tsushima Warm Current region in the Japan Sea derived by satellite-tracked surface drifters

We examined the surface current structure in the Tsushima Warm Current (TWC) region of the Japan Sea based on a dataset from satellite-tracked surface drifters, including our new deployments during 2004–2008. The gridded mean current velocity and mean kinetic energy (KE) fields calculated from all available drifter data revealed the structure and seasonality of the three branches of the TWC. By comparing the eddy kinetic energy (EKE) field to the KE field, we found four regions where the ratio of EKE to KE was high. In particular, this ratio became extremely high east of the Oki Islands located between the first branch of the TWC (FBTWC) and the second branch of the TWC (SBTWC). Our analysis suggests that the generation of alternate warm and cold eddies east of the Oki Island induced this high ratio. The occurrence of a warm or cold eddy was related to the route—either nearshore or offshore—of the SBTWC. When the offshore route of the SBTWC became dominant, a warm eddy was frequently generated east of the Oki Islands. In contrast, when the nearshore route of the SBTWC became dominant, a cold eddy was more likely to be generated.     

Investigating the impact of surface wave breaking on modeling the trajectories of drifters in the northern Adriatic Sea during a wind-storm event

An accurate numerical prediction of the oceanic upper layer velocity is a demanding requirement for many applications at sea and is a function of several near-surface processes that need to be incorporated in a numerical model. Among them, we assess the effects of vertical resolution, different vertical mixing parameterization (the so-called Generic Length Scale –GLS– set of k–ε, k–ω, gen, and the Mellor–Yamada), and surface roughness values on turbulent kinetic energy (k) injection from breaking waves.First, we modified the GLS turbulence closure formulation in the Regional Ocean Modeling System (ROMS) to incorporate the surface flux of turbulent kinetic energy due to wave breaking. Then, we applied the model to idealized test cases, exploring the sensitivity to the above mentioned factors. Last, the model was applied to a realistic situation in the Adriatic Sea driven by numerical meteorological forcings and river discharges. In this case, numerical drifters were released during an intense episode of Bora winds that occurred in mid-February 2003, and their trajectories compared to the displacement of satellite-tracked drifters deployed during the ADRIA02-03 sea-truth campaign.Results indicted that the inclusion of the wave breaking process helps improve the accuracy of the numerical simulations, subject to an increase in the typical value of the surface roughness z₀. Specifically, the best performance was obtained using α_CH = 56,000 in the Charnok formula, the wave breaking parameterization activated, k–ε as the turbulence closure model. With these options, the relative error with respect to the average distance of the drifter was about 25% (5.5 km/day). The most sensitive factors in the model were found to be the value of α_CH enhanced with respect to a standard value, followed by the adoption of wave breaking parameterization and the particular turbulence closure model selected.