Drifter Trajectories Research Articles

Observing Plumes and Overturning Cells with a New Coastal Bottom Drifter

AbstractA new platform, the Coastal Bottom Drifter, was designed and built to observe near-bottom environments in coastal regions. It is capable of observing properties by drifting near the bottom with a prescribed clearance or at a constant depth of up to 300 m. The platform can observe physical and biochemical parameters, such as temperature, salinity, oxygen, and velocities, and has the capacity to carry additional sensors to measure, for example, pH, turbidity, and nutrients. In addition, it can profile to the surface at chosen intervals and can be deployed for days or up to several months. The integrated Iridium communication allows the user to receive positions and data while the platform is surfaced, as well as send new missions to the instrument. The use of an acoustic bottom-tracking device allows the construction of a drifter trajectory while providing information about ocean circulation. Additionally, the ADCP provides information about suspended particles and possible sediment transport. These measurements are valuable in understanding coastal environments as well as the dominant physical processes that cause mixing and set the conditions for local biological activity. An example deployment in Apalachicola Bay shown in this study demonstrates the ability of the drifter to observe small-scale features, such as overturning cells and plumes of dense water, that are caused by local topography.

Enduring Lagrangian coherence of a Loop Current ring assessed using independent observations

Ocean flows are routinely inferred from low-resolution satellite altimetry measurements of sea surface height assuming a geostrophic balance. Recent nonlinear dynamical systems techniques have revealed that surface currents derived from altimetry can support mesoscale eddies with material boundaries that do not filament for many months, thereby representing effective transport mechanisms. However, the long-range Lagrangian coherence assessed for mesoscale eddy boundaries detected from altimetry is constrained by the impossibility of current altimeters to resolve ageostrophic submesoscale motions. These may act to prevent Lagrangian coherence from manifesting in the rigorous form described by the nonlinear dynamical systems theories. Here we use a combination of satellite ocean color and surface drifter trajectory data, rarely available simultaneously over an extended period of time, to provide observational evidence for the enduring Lagrangian coherence of a Loop Current ring detected from altimetry. We also seek indications of this behavior in the flow produced by a data-assimilative system which demonstrated ability to reproduce observed relative dispersion statistics down into the marginally submesoscale range. However, the simulated flow, total surface and subsurface or subsampled emulating altimetry, is not found to support the long-lasting Lagrangian coherence that characterizes the observed ring. This highlights the importance of the Lagrangian metrics produced by the nonlinear dynamical systems tools employed here in assessing model performance.

Marine debris pathways across the southern Indian Ocean

The distribution of marine debris both with the open oceans and along coastlines is largely governed by surface currents and thereby dependent upon seasonal circulation patterns and annual variability inherent in the major current systems. The recovery of a satellite-tracked current drifter on a South African beach some 22 months after release off Western Australia was an indicator of the ocean-wide processes operating in the southern Indian Ocean gyre. Using this single drift trajectory in combination with data from other current drifters released along the Western Australian coast between 2008 and 2013, from earlier satellite-tracked drifters and a selection of historical drift bottle returns, drift patterns across the southern Indian Ocean between Western Australia and east Africa were analysed to quantify inter-continental debris transport rates via the South Equatorial Current (northern east-west pathway) and the South Indian Ocean Current (southern west-east pathway). The northern east-west trans-ocean pathway can be considered in three segments: the complex and highly variable Leeuwin Current eddy system in the east, which can retain drifting debris for up to a year, the comparatively uniform westward travel in the South Equatorial Current, and the eddying components in the western boundary current system off southern Africa. A simple Eddy Activity Index is derived here to clearly distinguish the Leeuwin Current eddy region from the South Equatorial Current. The southern route to Australia is simpler via the Agulhas Return Current and the South Indian Ocean Current.

Comparison of statistical iceberg forecast models

Short-term iceberg drift prediction is challenging. Large uncertainties in the driving forces – current, wind and waves – usually prevent accurate forecasts. Recently several statistical iceberg forecast models have been proposed by the authors, which use iceberg position measurements to improve the short-term drift forecast. In this article these statistical forecast methods and models are briefly reviewed. An extensive comparison between the statistical models, in addition to a dynamic iceberg forecast model, is performed on several iceberg drift trajectories. Based on this comparison a new statistical forecast scheme is proposed that combines some of the advantages of the other methods.

Sawtooth mixing of alphas, knock-on D, and T ions, and its influence on NPA spectra in ITER plasma

Sawtooth mixing of fast ions is considered in ITER for plasma parameters close to the inductive 15 MA scenario. New mixing formula is presented for a drift trajectory averaged distribution function (DF) of fast passing and trapped ions. This formula generalizes Kadomtsev’s model for the case of non-circular magnetic surfaces, arbitrary aspect ratio, and significant deviations of charged particle drift trajectories from the magnetic flux surfaces, and can be applicable to a wide class of instabilities. Generalized formulae are implemented in the Fokker–Planck package three-dimensional (FPP-3D) code. FPP-3D is applied for calculation of the fast alpha-particle (He4), deuterium (D), and tritium (T) DFs. Simulations take into account D and T nuclear elastic scattering (NES or knock-on) by fast He4. Changes in the radial profile of He4 power deposition to the thermal plasma in the presence of plasma mixing are estimated.An effective tool for fast ion studies in ITER will be a system of neutral particle analyzers (NPA) since it allows direct measurements of the fast hydrogen ions’ DF inside the plasma. The main NPA goal on ITER is to measure the D/T fuel isotope ratio in the plasma core with the use of D and T atom spectra in the MeV range. The spectra and their sensitivity to sawtooth mixing are calculated. It was found that ITER NPA allows us to detect those oscillations. Simulations show the possibility of determining the D/T ratio with NPA regardless of whether sawtooth oscillations exist.

Global Observations of Horizontal Mixing from Argo Float and Surface Drifter Trajectories

AbstractMixing by mesoscale eddies in the ocean plays a major role in setting the distribution of oceanic tracers, with important implications for physical and biochemical systems at local to global scales. Roach et al. (2016; https://doi.org/10.1002/2015JC011440) demonstrated that a two‐particle analysis of Argo trajectories produces robust estimates of horizontal mixing in the Southern Ocean. Here we extend this analysis to produce global 1° × 1° maps of eddy diffusivity at the nominal Argo parking depth of 1,000 m. We also applied this methodology to estimate surface eddy diffusivities from Global Drifter Program (GDP) surface drifters. The global mean eddy diffusivity was 543 ± 155 m2/s at 1,000 m and 2637 ± 311 m2/s at the surface, with elevated diffusivities in regions of enhanced eddy kinetic energy, such as western boundary currents and along the Antarctic Circumpolar Current. The eddy kinetic energy at the equator is high at both the surface and depth, but the eddy diffusivity is only enhanced near the surface. At depth the eddy diffusivity is strongly suppressed due to the presence of mean flow. We used our observational estimates to test the validity of an eddy diffusivity parameterization that accounts for mixing suppression in the presence of zonal mean flows. Our results indicated that this parameterization generally agrees with the directly observed eddy diffusivities in the midlatitude and high‐latitude oceans.

Drift control for cornering maneuver of autonomous vehicles

Expert drivers have the ability to perform high side-slip angle maneuvers, like drifting, during racing to minimize lap time or avoid obstacles. Designing planning and control algorithms for autonomous drift maneuvers, however, is challenging because of the high lateral motion and nearly full saturation of rear tires. In this paper, the authors propose a complete path planning and motion control framework to plan and track a reference drift trajectory along a sharp bend in a track. The path planner divides the path horizon into three regions, finds a path using different planning sub-modules, and then concatenates the solutions to generate the reference trajectory. The controller then applies a mixed open-loop and closed-loop scheme to track the reference trajectory. We validate the planning and control algorithms in simulation using a high-fidelity model in Simulink/Carsim, and through experimentation using 1/10 scale Radio-Control (RC) vehicle.

Interaction of North Brazil Current rings with the Lesser Antilles Arc and Barbados Island: laboratory experiments and observations

The interaction of North Brazil Current (NBC) rings with the Lesser Antilles Arc (LAA) and the Barbados Island (BI) is addressed by experimental modeling and observations. Our results compare well with previous experimental results and numerical simulations. Several sizes, intensities and two different vorticity profiles (non-isolated and initially isolated vortices) were tested. Three regimes were found namely: (1) the vortex surrounds the BI and its translational motion (TM) stops North of BI; (2) the vortex passes through the corridor between the LAA and the BI by reducing its size; and (3) The vortex stopped at the entrance to the corridor South of the BI. Isolated vortices were prone to stopped North of the BI. Apparently the intensity in the outer vorticity ring has an influence on the fate of the NBC ring. Non-isolated vortices can also stop its TM North of the BI because when in \(\beta\) plane they develop an outer ring similar to the isolated vortices. From these results we conclude that intense and big NBC rings are likely to stop its TM North of the BI. Medium and moderate vortices stops its TM South of the BI and they reduce their size until they are able to pass through the corridor between the LAA and the BI. Mild vortices of all sizes stop South of the corridor, close to the BI and the LAA. Drifter trajectories and Sea Surface Height altimetry confirm the results.

Modeling Iceberg Drift in the Barents Sea from Field Data

The iceberg drift model is developed and used for simulating the iceberg drift trajectory in the Barents Sea. The model is forced by hydrometeorological characteristics obtained from ship observations. Original techniques for retrieving the sea-level slope gradient and surface velocity of currents are proposed, implemented, and validated using independent data. Thus, additional data were calculated from field data in order to use the iceberg drift model with the full set of external forces. This allowed improving the iceberg trajectory simulation and assessing the contribution of all forces that affect the iceberg drift. The iceberg drift calculations demonstrate that the drift characteristics are extremely sensitive to all external effects and the model parameters; therefore, the quality of input hydrometeorological data essentially affects the simulation of real iceberg trajectories.

Assessing the accuracy of satellite derived ocean currents by comparing observed and virtual buoys in the Greater Agulhas Region

In this study, we assess the accuracy of a combined geostrophic and Ekman current product (GlobCurrent) that estimates ocean currents at 15 m depth, by coupling it to a synthetic particle tracking tool and comparing the virtual trajectories to those of surface drifting buoys drogued at 15 m in the Greater Agulhas Current Region. The velocities from a total of 1041 drifters are compared and evaluated to the synthetic particle-derived velocities for the period 1993–2015. On average the GlobCurrent underestimates the velocity in the Greater Agulhas Current by approximately 27%. The underestimation ranges from 4 to 64% in different regions, with the smallest error found in the Agulhas retroflection region, and the highest in the Benguela Upwelling System. Furthermore, we compare the time taken for the separation between the virtual and real drifters to reach 35 km. The mean separation time was found to be 78 h, with the shortest time (35 h) found in the Agulhas Current and the longest time (116 h) located in the Agulhas Return Current. Deploying 10,000 virtual drifters in a 1° × 1° box within the southern Agulhas Current shows a convergence of trajectories towards the core of the current, while higher divergence is evident in the Agulhas retroflection. To evaluate the utility of this synthetic particle tracking tool coupled with GlobCurrent in open ocean search and rescue operations, two test cases are examined: (1) a capsized catamaran spotted south of Cape Recife and recovered 5 days later south of Cape Agulhas; and (2) a drifter trajectory in the same region. The comparison suggests that the GlobCurrent forced synthetic particle tracking tool is not appropriate for predicting the trajectory of a capsized catamaran that does not have the same drift characteristics as a surface drifting buoy drogued to 15 m.

Observation and prediction of flotsam trajectories in the California Current System based on surface drift of RAFOS floats

Surface drift of eighty-nine undrogued RAFOS floats in the California Current System (CCS) has been studied to describe patterns of flotsam drift, its seasonal variability and predictability limitations. The floats were launched in the California Undercurrent during 1992–2010 and were tracked by the ARGOS system when they surfaced at the end of their subsurface missions. The duration of surface trajectories varied from as short as 11 to as long as 280 days. The surface drift of these floats was typically equatorward in the California Current. However, some floats moved poleward into the Subpolar Gyre, and others drifted westward into the North Equatorial Current.Usually, observations of surface currents use drifters which are coupled to the surface layer by drogues located at 15 m depth. While drogued observations are useful for studies of circulation of the upper layer of the ocean, a more typical operational problem involves trying to find flotsam that has fallen off the deck of a ship or to predict the path of an abandoned vessel. To better understand the behavior of these surface drifting objects, observations of the surface drifts of RAFOS floats in the California Current system were used to compare the floats’ motions to wind-induced drift and to evaluate the drift prediction by three ocean models: Ocean Surface Current Simulator (OSCURS), Global Navy Coastal Model (gNCOM) and Hybrid Coordinate Ocean Model (HYCOM).Cross-Calibrated Multi-Platform Ocean Surface Wind Vector L3.0 First-Look Analysis wind data were used to analyze the surface drift of the RAFOS floats. The best correlation between float drift speed and wind speed was observed during summer and fall months, when the wind regime over the Northeast Pacific is dominated by the North Pacific High. The predominant winds associated with this regime are steady and contribute to the surface drift in the same direction as the large-scale geostrophic component of the California Current.Evaluation of the drift prediction by three ocean models was conducted by comparing observed drifter trajectories with model-simulated trajectories at 7-day time scales. The model-simulated trajectories were initially collocated with RAFOS positions and restarted every 15 days. The OSCURS model was the only one of three analyzed models which allowed for tuning based on the surface RAFOS trajectories. Two main parameters, Wind Angle Deflection (WAD) and Geostrophic Current Factor (GCF), were adjusted to minimize the separation between observed and modeled trajectories. At shorter time scales, from weeks to several months, the tuned OSCURS model outperformed gNCOM and HYCOM models in reproducing the observed surface drift. For seasonal and longer time scales, both gNCOM and HYCOM can reproduce the seasonal variability, flow direction, and eddy-like structures in the California Current system, which may be useful for long-term forecasts of the Northeast Pacific circulation.

Modeling the drift of European (Anguilla anguilla) and American (Anguilla rostrata) eel larvae during the year of spawning

The distribution of the leptocephalus larvae of European (Anguilla anguilla) and American (Anguilla rostrata) eels collected during recent Sargasso Sea surveys was used to model larval drift. The drift trajectories of individual larva were back-calculated to the estimated time of spawning, using current data from two global oceanographic assimilation models. The results of both models give the same overall result; widespread spawning extended in time from December to March. The drift was also calculated forwards for approximately 1 year. The forward drift modelling showed that most leptocephali remained in the area south of the Subtropical Frontal Zone. One conclusion is that the majority of leptocephali remain trapped and possibly die in the retention area. A small proportion of leptocephali are entrained into the Gulf Stream system. An implication is that the spawning success may be highly sensitive to oceanographic and climatic factors that alter the dispersion of leptocephali out from the retention area. An alternative interpretation is that the surveys were made too late after the peak spawning period and that the core spawning area was missed.

Nearshore drift dynamics of natural versus artificial seagrass wrack

Drifting macrophytes such as seagrass and macroalgae are commonly found washed ashore on sandy beaches but few studies have investigated the drift trajectories of macrophytes whilst near to the coast. This is the first study to investigate the surface drifting of small clumps of seagrass released at various distances from shore, across multiple days with contrasting wind and tidal conditions, in a large gulf in southern Australia. Natural and artificial radio-tagged seagrass units generally travelled in the same directions as tides but trajectories were variable across sampling days and when tagged units were released at different distances from shore. Natural and artificial units diverged from each other particularly on days when wind speeds increased but generally drifted in the same direction and ended up within close proximity to each other at the 6-h endpoint. During calm conditions, tagged seagrass units drifted with tides for 0.25–5 km and, during one sampling day when wind speeds increased, drifted for >5 km over the 6-h time period. Only tagged units that were released closest to shore stranded on sandy beaches within the six hours of observation, so it would be difficult to predict the eventual stranding location on shorelines for macrophytes released further offshore. This study provides evidence of the variability of macrophyte drift dynamics near to coastlines. Acknowledging this variability is essential for further understanding of the ecological significance of allochthonous material arriving at shorelines, which should be integrated into future research and management of sandy-beach ecosystems.

Prediction of Drifter Trajectory Using Evolutionary Computation

We used evolutionary computation to predict the trajectory of surface drifters. The data used to create the predictive model comprise the hourly position of the drifters, the flow and wind velocity at the location, and the location predicted by the MOHID model. In contrast to existing numerical models that use the Lagrangian method, we used an optimization algorithm to predict the trajectory. As the evaluation measure, a method that gives a better score as the Mean Absolute Error (MAE) when the difference between the predicted position in time and the actual position is lower and the Normalized Cumulative Lagrangian Separation (NCLS), which is widely used as a trajectory evaluation method of drifters, were used. The evolutionary methods Differential Evolution (DE), Particle Swarm Optimization (PSO), Covariance Matrix Adaptation Evolution Strategy (CMA-ES), and ensembles of the above were used, with the DE&PSO ensemble found to be the best prediction model. Considering our objective to find a parameter that minimizes the fitness function to identify the average of the difference between the predictive change and the actual change, this model yielded better results than the existing numerical model in three of the four cases used for the test data, at an average of 19.36% for MAE and 5.96% for NCLS. Thus, the model using the fitness function set in this study showed improved results in NCLS and thus shows that NCLS can be used sufficiently in the evaluation system.

Surface Connectivity and Interocean Exchanges From Drifter-Based Transition Matrices.

Global surface transport in the ocean can be represented by using the observed trajectories of drifters to calculate probability distribution functions. The oceanographic applications of the Markov Chain approach to modeling include tracking of floating debris and water masses, globally and on yearly‐to‐centennial time scales. Here we analyze the error inherent with mapping trajectories onto a grid and the consequences for ocean transport modeling and detection of accumulation structures. A sensitivity analysis of Markov Chain parameters is performed in an idealized Stommel gyre and western boundary current as well as with observed ocean drifters, complementing previous studies on widespread floating debris accumulation. Focusing on two key areas of interocean exchange—the Agulhas system and the North Atlantic intergyre transport barrier—we assess the capacity of the Markov Chain methodology to detect surface connectivity and dynamic transport barriers. Finally, we extend the methodology's functionality to separate the geostrophic and nongeostrophic contributions to interocean exchange in these key regions.

Record low sea-ice concentration in the central Arctic during summer 2010

The Arctic sea-ice extent has shown a declining trend over the past 30 years. Ice coverage reached historic minima in 2007 and again in 2012. This trend has recently been assessed to be unique over at least the last 1450 years. In the summer of 2010, a very low sea-ice concentration (SIC) appeared at high Arctic latitudes—even lower than that of surrounding pack ice at lower latitudes. This striking low ice concentration—referred to here as a record low ice concentration in the central Arctic (CARLIC)—is unique in our analysis period of 2003–15, and has not been previously reported in the literature. The CARLIC was not the result of ice melt, because sea ice was still quite thick based on in-situ ice thickness measurements. Instead, divergent ice drift appears to have been responsible for the CARLIC. A high correlation between SIC and wind stress curl suggests that the sea ice drift during the summer of 2010 responded strongly to the regional wind forcing. The drift trajectories of ice buoys exhibited a transpolar drift in the Atlantic sector and an eastward drift in the Pacific sector, which appeared to benefit the CARLIC in 2010. Under these conditions, more solar energy can penetrate into the open water, increasing melt through increased heat flux to the ocean. We speculate that this divergence of sea ice could occur more often in the coming decades, and impact on hemispheric SIC and feed back to the climate.

The effect of sawtooth oscillations on the alpha particle distribution and energy balance in the ITER plasma

The mixing of toroidal plasma under the conditions of sawtooth oscillations is considered using the Kadomtsev model. A new mixing formula for the averaged distribution function of fast transit and trapped particles is proposed in the methodology of a kinetic equation averaged over drift trajectories. The proposed formula generalizes the known results for the case of non-circular magnetic surfaces, an arbitrary aspect ratio, and charged particle drift trajectories significantly deviating from the magnetic surfaces. The formula is applicable for a sufficiently wide class of instabilities. The 3D kinetic equation is numerically solved using the FPP- 3D computation code for parameters close to the ITER inductive scenario. The alpha particle distribution function and the power introduced by alpha particles in plasma when sawtooth oscillations occur are calculated. It is shown that such oscillations may change the energy input of a thermonuclear reaction in certain areas by several times.

A new perspective on origin of the East Sea Intermediate Water: Observations of Argo floats

The East Sea Intermediate Water (ESIW), defined as the salinity minimum in the East Sea (hereafter ES) (Sea of Japan), is examined with respect to its overall characteristics and its low salinity origin using historical Argo float data from 1999 to 2015. Our findings suggest that the ESIW is formed in the western Japan Basin (40–42°N, 130–133°E), especially west of the North Korean front in North Korean waters, where strong negative surface wind stress curl resides in wintertime. The core ESIW near the formation site has temperatures of 3–4 °C and less than 33.98 psu salinity, warmer and fresher than that in the southern part of the ES. In order to trace the origin of the warmer and fresher water at the sea surface in winter, we analyzed the data in three different ways: (1) spatial distribution of surface water properties using monthly climatology from the Argo float data, (2) seasonal variation of heat and salt contents at the formation site, and (3) backtracking of surface drifter trajectories. Based on these analyses, it is likely that the warmer and fresher surface water properties found in the ESIW formation site are attributed to the low-salinity surface water advected from the southern part of the ES in autumn.

Systematic Evaluation of Low‐Frequency Hiss and Energetic Electron Injections

AbstractThe excitation of low‐frequency (LF) plasmaspheric hiss, over the frequency range from 20 Hz to 100 Hz, is systematically investigated by comparing the hiss wave properties with electron injections at energies from tens of keV to several hundreds of keV. Both particle and wave data from the Van Allen Probes during the period from September 2012 to June 2016 are used in the present study. Our results demonstrate that the intensity of LF hiss has a clear day‐night asymmetry, and increases with increasing geomagnetic activity, similar to the behavior of normal hiss (approximately hundred of hertz to several kilohertz). The occurrence rate of LF hiss in association with electron injections is up to 80% in the outer plasmasphere (L > 4) on the dayside, and the strong correlation extends to lower L shells for more active times. In contrast, at lower L shells (L < 3.5), LF hiss is seldom associated with electron injections. The LF hiss with Poynting flux directed away from the equator is dominant at higher magnetic latitudes and higher L shells, suggesting a local amplification of LF hiss in the outer plasmasphere. The averaged electron fluxes are larger at higher L shells, where significant LF hiss wave events are observed. Our study suggests the importance of electron injections and their drift trajectories toward the dayside plasmasphere in locally amplifying the LF hiss waves detected by the Van Allen Probes.

Validation of Oil Spill Transport and Fate Modeling in Arctic Ice

Reliability of oil spill modeling in Arctic waters for response planning and risk assessments depends on the accuracy of winds, currents, and ice data (cover and drift) used as input. We compared predicted transport in ice, using ice and ocean model results as input, with observed drifter trajectories in the Beaufort Sea and an experimental oil release in the Barents Sea. The ice models varied in ice rheology algorithms used (i.e., Elastic–Viscous–Plastic, presently used in climate models, versus a new Elasto-Brittle approach in pack ice) and the time averaging of their outputs, which were provided as input to oil spill models. Evaluations of model performance (skill) against drifters showed improvement using Elasto-Brittle instead of Elastic–Viscous–Plastic rheology. However, model skill was degraded by time-averaging of ocean and ice model vectors before input to the oil spill model. While the accuracy of individual oil model trajectories projected weeks to months into the future is expected to be low, in the event of a spill, forecasts could be updated frequently with satellite and other observations to improve reliability. Comparisons of modeled trajectories with drifters verified that use of the ice–ocean models for ensemble modeling as part of risk assessments is reliable.