Drifter Trajectories Research Articles

A Global Comparison of Marine Chlorophyll Variability Observed in Eulerian and Lagrangian Perspectives

AbstractOcean chlorophyll time series exhibit temporal variability on a range of timescales due to environmental change, ecological interactions, dispersal, and other factors. The differences in chlorophyll temporal variability observed at stationary locations (Eulerian perspective) or following water parcels (Lagrangian perspective) are poorly understood. Here we contrasted the temporal variability of ocean chlorophyll in these two observational perspectives, using global drifter trajectories and satellite chlorophyll to generate matched pairs of Eulerian‐Lagrangian time series. We found that for most ocean locations, chlorophyll variances measured in Eulerian and Lagrangian perspectives are not statistically different. In high latitude areas, the two perspectives may capture similar variability due to the large spatial scale of chlorophyll patches. In localized regions of the ocean, however, chlorophyll variability measured in these two perspectives may significantly differ. For example, in some western boundary currents, temporal chlorophyll variability in the Lagrangian perspective was greater than in the Eulerian perspective. In these cases, the observing platform travels rapidly across strong environmental gradients and constrained by the shelf topography, potentially leading to greater Lagrangian variability in chlorophyll. In contrast, we found that Eulerian chlorophyll variability exceeded Lagrangian variability in some key upwelling zones and boundary current extensions. In these cases, variability in the nutrient supply may generate intermittent chlorophyll anomalies in the Eulerian perspective, while the Lagrangian perspective sees the transport of such anomalies off‐shore. These findings aid with the interpretation of chlorophyll time series from different sampling methodologies, inform observational network design, and guide validation of marine ecosystem models.

Inferring Ocean Transport Statistics With Probabilistic Neural Networks

AbstractUsing a probabilistic neural network and Lagrangian observations from the Global Drifter Program, we model the single particle transition probability density function (pdf) of ocean surface drifters. The transition pdf is represented by a Gaussian mixture whose parameters (weights, means, and covariances) are continuous functions of latitude and longitude determined to maximize the likelihood of observed drifter trajectories. This provides a comprehensive description of drifter dynamics allowing for the simulation of drifter trajectories and the estimation of a wealth of dynamical statistics without the need to revisit the raw data. As examples, we compute global estimates of mean displacements over 4 days and lateral diffusivity. We use a probabilistic scoring rule to compare our model to commonly used transition matrix models. Our model outperforms others globally and in three specific regions. A drifter release experiment simulated using our model shows the emergence of concentrated clusters in the subtropical gyres, in agreement with previous studies on the formation of garbage patches. An advantage of the neural network model is that it provides a continuous‐in‐space representation and avoids the need to discretize space, overcoming the challenges of dealing with nonuniform data. Our approach, which embraces data‐driven probabilistic modeling, is applicable to many other problems in fluid dynamics and oceanography.

Analysis of the quasi-two-year variation of the Zapiola Anticyclone based on the reanalysis data and drifter trajectories

Zapiola anticyclone (ZA) is an anticyclonic circulation trapped by a seamount about 1200 m in height named Zapiola Rise (ZR). The ZA is adjacent to energetic flows of the South Atlantic Ocean, such as the Brazil/Malvinas Confluence (BMC) and the Antarctic Circumpolar Circulation (ACC). The interplay among the circulations makes the dynamic processes of the ZA complex and variable. Based on the reanalysis data and drifter observations, this study mainly aims to investigate the influence of a strong flow field outside of the seamount on variations of the ZA. Results show that the vorticity of the ZA has a significant quasi-two-year variation, which is an independent eigenmode in the basin. The advection of the potential vorticity over the bottom topography from the southern boundary plays the most significant role in controlling the vorticity variations of the ZA. Drifter trajectories also confirm the importance of the flow field in the south of the seamount. The statistical results of the drifter trajectories show that the transport process of the southern boundary flow field has the most significant effect on the dynamic processes of the ZA. It is worth mentioning that the results obtained using the dataset of the reanalysis are limited by its validity in the region.

Surface dispersion of coastal discharges in North America towards the Great Pacific Garbage Patch

This research addresses the surface dispersion of coastal discharges in North America that end up at the Great Pacific Garbage Patch. The evolution of the discharged concentration is calculated through statistical simulations using transition matrices and dispersion ellipses, both based on historical records of surface drifter trajectories. The discharge points are adjacent to urban areas distributed along the coast. The preferential routes, times of arrival, and relative contributions of each site to the accumulation area are quantified. A new statistical delimitation of the position, area and orientation of the garbage patch is proposed. Additional experiments suggest that the tracer retention during the summer is influenced by the low-level atmospheric anticyclone in the Northeastern Pacific, which enhances the Ekman drift and hence favours debris convergence. This effect is reduced when the anticyclone weakens in winter, thus decreasing the debris retention and promoting its westward dispersal by the trade winds.

Current shear and turbulence during a near-inertial wave

Surface currents and turbulent mixing were observed during a near-inertial wave (NIW) using an accousting doppler current profiler (ADCP) and satellite-tracked drifters. Drifter trajectories sampled at three depth levels show characteristics of an Ekman solution superposed with the NIW. Velocity and dissipation estimates from the ADCP reveal strong shear with a distinct constant flux layer in between the roughness length and a critical depth at 4m. Below, a shear free slab layer performing an inertial oscillation is observed. Dissipation, as estimated from the vertical beam of the ADCP, peaks in the wave-enhanced friction layer when the current opposes the wind and wave direction. Below the constant flux layer, maximum turbulence is observed when the NIW is in a phase that is in opposite direction to the time-averaged current. During this phase, currents at various depths rapidly realign in the entire boundary layer.

Sea Ice Drift from GPS Tracker Deployed in the Arctic Ocean

Sea ice is monitored on a regular basis by satellite observation; however, image-based drift tracking is imprecise as the time interval forming the image-pair is too large to capture the actual trajectory of ice drift. In this study, drift trajectory and speed of an Arctic sea ice floe measured by a GPS tracker for 3 months and the characteristics of the relating device and data, are introduced.

Surface Currents Along the Coast of the Chinese Mainland Observed by Coastal Drifters During April‐May 2019

Abstract We have analyzed the surface current characteristics along the coast of the Chinese mainland in autumn and winter by the trajectories of coastal drifters in a previous study (Yang et al., 2021). This paper is a follow-up study focusing on the surface current characteristics in spring. By the trajectories of coastal drifters in April‐May 2019, we continue to analyze the characteristics of surface currents. It is indicated that: (1) the surface currents were generally southwestward along the coasts of Zhejiang and Fujian, while they flowed northeastward in the offshore area in April 2019. (2) In May 2019, the surface current in the sea area north of 28°N had a small speed without a fixed direction nearshore, but had larger speed offshore. (3) The surface currents along the Zhejiang coast were variable in April‐May 2019, mainly corresponding to the change of wind field. (4) The coastal drifter is a useful instrument to observe circulation systems more precisely in nearshore but not deep ocean.

Persistence of Cold Wedges in the Somali Current System

AbstractThe Somali Current system in the western Arabian Sea reverses seasonally with the South Asian Monsoon and is associated with localized upwelling cells or cold wedges during the summer southwest monsoon. Drifter trajectories in boreal summer and fall 2014 provide rare observational evidence that the northward Somali Current and associated cold wedges can persist into the boreal fall intermonsoon period. The near‐surface circulation and sea surface temperatures further suggest that the wedge‐like surface signatures may intermittently be capped and then reappear at a later time. Our observations show that the northward Somali Current system rapidly decayed within 1 week after the onset of the winter northeast monsoon and the arrival of a cyclonic eddy at the coast in early November 2014. This eddy may not only have affected regional ocean‐atmosphere interactions but also biogeochemical processes and the marine ecosystem through the transport of water properties and locally induced upwelling.

Effects of Eulerian current, Stokes drift and wind while simulating surface drifter trajectories in the Baltic Sea

The simulation of Lagrangian drift is an important task in applications such as dispersion of pollutants, larvae and search and rescue activities. In this study, the Eulerian current, Stokes drift and wind effect on the simulation of observed drifters were analysed. The Lagrangian OceanParcels model was used, and the surface trajectories were assessed by comparison with 9 GPS drifter trajectories in the Gulf of Finland, Gulf of Riga and Lithuanian coast. The Normalised Cumulative Lagrangian Separation (NCLS) distance between the simulated and the satellite-tracked drifter trajectories, and the mean absolute error (MAE) were used as comparison metrics. The present study suggests the need to consider the Stokes drift and the wind factor in addition to the modelled Eulerian currents to obtain a better description of the trajectories of particles. By making these considerations, the OceanParcels model could adequately simulate particle trajectories in the sub-basins within the Baltic Sea. The realized model tests showed that motion of surface drifters are strongly controlled by the Stokes drift when the significant wave height is >1 m, whereas the wind component and the Eulerian currents are crucial when the significant wave height is <0.6 m or the wave (Stokes drift) directions do not match the wind direction.

Understanding the drift of Shackleton's Endurance during its last days before it sank in November 1915, using meteorological reanalysis data

Abstract. On 5 December 1914, Sir Ernest Shackleton and his crew set sail from South Georgia aboard the wooden barquentine vessel Endurance, beginning the Imperial Trans-Antarctic Expedition to cross the Antarctic continent. However, Shackleton and his crew never reached land because the vessel became beset in the sea ice of the Weddell Sea in January 1915. Endurance then drifted in the pack for 11 months, was crushed by the ice, and sank on 21 November 1915. Over many years, various predictions were made about the location of the wreck. These were based largely on navigational fixes taken by Captain Frank Worsley, the navigator of the Endurance, 3 d prior to and 1 d after the sinking of Endurance. On 5 March 2022, the Endurance22 expedition located the wreck some 9.4 km southeast of Worsley's estimated sinking position. In this paper, we describe the use of meteorological reanalysis data to reconstruct the likely ice drift trajectory of Endurance for the period between Worsley's final two fixes, at some point along which the vessel sank. Reconstructions are sensitive to choices of wind factor and turning angle, but allow an envelope of possible scenarios to be developed. A likely scenario yields a simulated sinking location some 3.5 km from the position at which the wreck finally was found, with a trajectory describing an excursion to the southeast and an anticlockwise turn to the northwest prior to sinking. Despite numerous sources of uncertainty, these results show the potential for such methods in marine archaeology.

A missing dusk-side loss process in the terrestrial electron ring current

The Earth’s magnetic field traps charged particles which are transported longitudinally around Earth, generating a near-circular current, known as the ring current. While the ring current has been measured on the ground and space for many decades, the enhancement of the ring current during geomagnetic storms is still not well understood, due to many processes contributing to its dynamics on different time scales. Here, we show that existing ring current models systematically overestimate electron flux observations of 10–50 keV on the nightside during storm onset. By analyzing electron drift trajectories, we show that this systematic overestimation of flux can be explained through a missing loss process which operates in the pre-midnight sector. Quantifying this loss reveals that the theoretical upper limit of loss has to be reached over a broad region of space in order to reproduce the observations. This missing loss may be attributed to inaccuracies in the parameterization of the loss due to chorus wave interactions, combined with the scattering by electrostatic electron cyclotron harmonic waves which is currently not included in ring current models.

Modeling the leeway drift characteristics of persons-in-water at a sea-area scale in the seas of China

Drowning events are becoming more common. It is important to accurately predict the drift trajectory of floating objects. This study proposes a sea-area-scale drift modeling method to determine the drift trajectory of persons-in-water in offshore China. The proposed method divided offshore China into 18 areas, and field experiments were carried out in the northern East China Sea (N_ES) and the Taiwan Strait (TS). First, a series of models were created, namely N_ES_I (N_ES upright), N_ES_II (N_ES facedown), TS_I (TS upright), TS_II (TS facedown), C_I (comprehensive upright), and C_II (comprehensive facedown). The leeway coefficients and jibing frequencies were then determined, and the positive crosswind speed (+CWL) probabilities under different marine environments were analyzed. A large volume of information from open-source channels was used to establish a database of historical drift cases. The driven model was validated using a combination of field experiments and historical drift information. The results indicate that the trajectories simulated using the sea-area-scale models proposed in this study are more consistent with the actual trajectories than those using the constant coefficients. The prediction areas are sensitive to the +CWL probability and the jibing frequencies. This study can help to improve the ability of the drift trajectory in China.

Cold ridge formation mechanisms on the Agulhas Bank (South Africa) as revealed by satellite-tracked drifters

The formation of the cold ridge, a seasonal mid-shelf upwelling feature on the Agulhas Bank, is investigated from a combination of satellite-tracked surface drifters, in situ current and temperature data, as well as satellite sea surface temperature (SST) and chlorophyll-a (Chl-a). The observational data showed coastal upwelling along the Tsitsikamma coast to be induced by the easterly wind in austral summer, with a coexistent westward flowing coastal current. Under this wind and current regime, surface drifters moved west and onto the mid-shelf with the cold water, thereafter following the southward-curved 100 m isobath. Satellite observations in March–April 2008 showed increased south-westward advection of wind-driven upwelled water during an Agulhas Current intrusion that resulted in the formation of a classically shaped cold ridge. Also of interest was the passing of a Natal Pulse along the south-eastern shelf edge of the Agulhas Bank in July 2007. This generated a large cyclonic eddy in the Agulhas Bight with an attendant warm water plume that swept over the adjacent shelf. Two idle drifters positioned on the mid-shelf at the time, immediately west of the Natal Pulse, were rapidly advected offshore along the curved 100 m isobath, confirming the long-thought association of the cold ridge with an Agulhas Bight cyclone. A moored current meter adjacent to the Tsitsikamma coast (1.5 km offshore) showed the effect of the Natal Pulse on the nearshore. Drifter trajectories show accelerated south-westward flow on the offshore side of the cold ridge and recirculation towards the east on its shoreward side. These findings strongly support the Westward Transport Hypothesis where squid paralarvae are advected westward from the inshore breeding grounds, to the food maxima on the interior of the central Agulhas Bank, to improve their chances of survival and can therefore have important implications for the management of the South African chokka squid fishery.

Lagrange tracking-based long-term drift trajectory prediction method for Autonomous Underwater Vehicle.

Autonomous Underwater Vehicle (AUV) works autonomously in complex marine environments. After a severe accident, an AUV will lose its power and rely on its small buoyancy to ascend at a slow speed. If the reserved buoyancy is insufficient, when reaching the thermocline, the buoyancy will rapidly decrease to zero. Consequently, the AUV will experience prolonged lateral drift within the thermocline. This study focuses on developing a prediction method for the drift trajectory of an AUV after a long-term power loss accident. The aim is to forecast the potential resurfacing location, providing technical support for surface search and salvage operations of the disabled AUV. To the best of our knowledge, currently, there is no mature and effective method for predicting long-term AUV underwater drift trajectories. In response to this issue, based on real AUV catastrophes, this paper studies the prediction of long-term AUV underwater drift trajectories in the cases of power loss. We propose a three-dimensional trajectory prediction method based on the Lagrange tracking approach. This method takes the AUV's longitudinal velocity, the time taken to reach different depths, and ocean current data at various depths into account. The reason for the AUV's failure to ascend to sea surface lies that the remaining buoyancy is too small to overcome the thermocline. As a result, AUV drifts long time within the thermocline. To address this issue, a method for estimating thermocline currents is proposed, which can be used to predict the lateral drift trajectory of the AUV within the thermocline. Simulation is conducted to compare the results obtained by the proposed method and that in a real accident. The results demonstrate that the proposed approach exhibits small directional and positional errors. This validates the effectiveness of the proposed method.

Modeling the energetic electron fluxes in the inner radiation belt based on a drift-source model

The Macau Science Satellite-1 (MSS-1), designed by the Macao University of Science and Technology and the National Space Science Center (NSSC) of China, is equipped to detect the fine structure of the magnetic field over the South Atlantic Anomaly (SAA) region, monitoring geomagnetic field variations, and obtaining the energetic electron spectrum distributions in the Earth’s inner radiation belt. In this study, we simulate the distributions of trapped, quasi-trapped, and untrapped electrons along the orbit of MSS-1 based on a drift-source model. The simulation results show that the particle detector with 90° looking direction can observe trapped electrons in the SAA region, untrapped electrons in the regions conjugated with the SAA region at the north hemisphere, and quasi-trapped electrons in all other regions. In contrast, the detectors with &lt;60° looking directions can measure only untrapped electrons. Generally, quasi-trapped electron fluxes accumulate along the drift trajectory and are due primarily to CRAND, until reaching the SAA region where quasi-trapped electrons are all lost into the atmosphere.

Re-examining flow pathways over the Chukchi Sea continental shelf

Velocity records from 8 mooring sites (2010–2019) and trajectories of drogued drifters (2012–2020) are used to evaluate patterns of flow on the Chukchi Sea continental shelf, with an emphasis on the latter three years (2016–2019). Together, these data provided insight into the temporal and spatial variability of the currents over this shelf. These data extend previous observations by five years and include three previously unoccupied sites, two of which span the Central Channel. Bathymetry directs a significant portion of the northward flow in Central Channel eastward across the Chukchi shelf where it joins the coastal flow prior to exiting Barrow Canyon and entering the Beaufort Sea. In addition, shelf-wide volume transport estimated from three mooring sites located off Icy Cape is modified from earlier analysis and extended in time. The resulting transport is highly correlated with that flowing through Central Channel, with similar magnitude. Icy Cape transport varies seasonally with variations in atmospheric forcing, as well as inter-annually, with an annual low of 0.24 Sv (September 2011–August 2012) to a annual high of 0.64 Sv (September 2017–August 2018), and a 9-year average of 0.43 Sv, or approximately 40% of the flow through Bering Strait. This nearly decade-long Icy Cape volume transport record also exhibits an increasing, although not significant, trend of ∼0.03 Sv/year.

Tracking of the huge Antarctic iceberg A-76

ABSTRACT This paper presents a processing scheme whose aim is to provide the basis for a fast time-series analysis of iceberg drifting patterns. The study focuses on the drifting of iceberg A-76 and makes use of SAR images which were collected by the Sentinel-1 satellites, from May 2021 to June 2022. The processing scheme is composed of the following steps: a) image pre-processing by morphological theory; b) contextual analysis for performing the segmentation task; and c) regularization of the derived label field. The implemented processing scheme incorporates a further step for measuring the drift trajectory and a geometric characterization of the iceberg’s shape.

Circulation structure and dynamic characteristics of Western Tropical Indian Ocean associated with monsoon transitions

The circulation structure in the western tropical Indian Ocean (WTIO) was investigated based on surface drifters and Simple Ocean Data Assimilation (SODA), with particular attention to the characteristics of the circulation structure associated with seasonal monsoon transitions. Surface drifter trajectories manifest three distinct seasonally dependent patterns of western boundary currents in the WTIO. The northward Somali Current (SC) flows across the equator to ∼10 °N in the boreal summer (June to August; JJA). The northward SC turns offshore near the equator during monsoon transitions. The southward SC feeds the South Equatorial Countercurrent together with the northward East African Coast Current in the boreal winter (December to February; DJF). While during the monsoon transition (MAM and SON), drifters prefer to turn eastward and feed the strong equatorial currents. Based on satellite observations, the results of a balanced diagnostic model demonstrate that the geostrophic component Uζ and the wind-driven componentUτ predominate the mean state and the seasonal variations of circulation in the WTIO. The vertical motion shows that upwelling primarily occurs along the coasts of Somali and Oman and is induced by offshore Ekman transport. Ekman transport and associated vertical motions constitute meridional overturning circulation. The SODA results confirm the existence of an equatorial roll in the boreal summer and winter. The results elucidate the dynamic relationships between 3-D circulation and monsoon transitions in the WTIO.

Determining the drift characteristics of open lifeboats based on large-scale drift experiments

Lifeboat is one of the most important life-saving equipment for escaping at sea when a ship is abandoned in an extreme emergency. An accurate drift model can help rescuers find the drift position of lifeboat in the shortest time, thus improving the efficiency of marine search and rescue (SAR) at sea and ensuring the safety of wrecked people. The purpose of this paper is to investigate the drift characteristics and to develop an accurate drift prediction model for the open lifeboat. First, large-scale drift experiments were conducted to analyze the drift characteristics with three 6.5-meter-long real-size open lifeboats in the South China Sea. Next, three drift prediction models of the lifeboats were developed using the least squares method based on the drift experimental data. Finally, the drift prediction models of the lifeboats were compared and evaluated using the Lagrangian method and Monte Carlo technique, respectively. Results indicate that the probability of positive crosswind leeway (CWL) of the open lifeboat is 47.5%. The jibing frequency is 6% per hour, and the maximum leeway divergence angle is 45°. These drift characteristics are very important for the prediction of the open lifeboat drift trajectory. The comparison results of three drift models show that the improved drift model is more accurate than the other two drift models for predicting drift trajectories of the open lifeboat, which can be directly applied to maritime search and rescue operations in the South China Sea.

Hydrodynamic and Particle Drift Modeling as a Support System for Maritime Search and Rescue (SAR) Emergencies: Application to the C-212 Aircraft Accident on 2 September, 2011, in the Juan Fernández Archipelago, Chile

Search and rescue (SAR) refers to every operation aiming to find someone presumed lost, sick, or injured in remote or hard-to-access areas. This study presents the design of an operational system that supports maritime SAR emergencies by combining information from global hydrodynamic models (GHM) and a local hydrodynamic model (LHM) implemented in FVCOM. The output of these hydrodynamic models is used as input in a multiple particle drift estimator (MPDE) to estimate the trajectories of the floating elements derived from accidents in the ocean. The MPDE also includes trajectory estimates using the empirical LEEWAY formulation. The modeling system is validated with data collected during a SAR emergency that occurred on 2 September 2011, where a C-212 aircraft from the Chilean Air Force destined to the Juan Fernández Archipelago crashed in the ocean between the islands of Santa Clara and Robinson Crusoe. Trajectories were assessed in terms of the commonly used NCLS (normalized cumulative Lagrangian separation) performance indicator and a modified version, NCLSmod, which considers both the movement and orientation of the trajectories. The LHM was executed in three scenarios: forced only with tide, forced with tide and wind combined, and forced only with wind. The performance of the different models varied in response to the ocean–atmosphere conditions and their local variations at the time of the accident. In times of calm wind, models with tidal influence performed better, while wind-forced models performed better when winds were greater than 7 km h−1. The use of FVCOM (LHM) solved the coastal circulation and accounted for bathymetric effects in the Juan Fernández Archipelago area. This resulted in an improved variability and distribution of the modeled trajectories compared to the observed drifter trajectories. This work is the first study related to cases of maritime SAR emergencies in Chile, and provides a fast tool to estimate search areas based on an ensemble of particle drift and trajectory forecasts using multiple publicly available data sources.