Surface Drifters Research Articles

Extracting statistically significant eddy signals from large Lagrangian datasets using wavelet ridge analysis, with application to the Gulf of Mexico

Abstract. A method for objectively extracting the displacement signals associated with coherent eddies from Lagrangian trajectories is presented, refined, and applied to a large dataset of 3770 surface drifters from the Gulf of Mexico. The method, wavelet ridge analysis, is a general method for the analysis of modulated oscillations, here modified to be more suitable to the eddy-detection problem. A means for formally assessing statistical significance is introduced, addressing the issue of false positives arising by chance from an unstructured turbulent background and opening the door to confident application of the method to very large datasets. Significance is measured through a frequency-dependent comparison with a stochastic dataset having statistical and spectral properties that match the original, but lacking organized oscillations due to eddies or waves. The application to the Gulf of Mexico reveals major asymmetries between cyclones and anticyclones, with anticyclones dominating at radii larger than about 50 km, but an unexpectedly rich population of highly nonlinear cyclones dominating at smaller radii. Both the method and the Gulf of Mexico eddy dataset are made freely available to the community for noncommercial use in future research.

On the non-geostrophic appearance of some mean surface velocity observations in the Gulf of Mexico

AbstractA previous study of currents in the Gulf of Mexico by the author used long-term means from three independent data sources. Ship-drift results are in good agreement with surface drifters, but these two do not agree with satellite sea-surface heights (SSH). The agreement between the first two suggested the possibility that there could be errors in the SSH or that the mean surface flow is not in geostrophic balance. The present results, using the addition of a fourth long-term mean from hydrographic data, which agrees with the SSH, resolves the issue. The lack of agreement between different long-term means is from inadequate coverage in space and time in data from ship drifts and drifters.

Reconstruction of Diffusion Coefficients and Power Exponents from Single Lagrangian Trajectories

Using discrete wavelets, a novel technique is developed to estimate turbulent diffusion coefficients and power exponents from single Lagrangian particle trajectories. The technique differs from the classical approach (Davis (1991)’s technique) because averaging over a statistical ensemble of the mean square displacement (<X2>) is replaced by averaging along a single Lagrangian trajectory X(t) = {X(t), Y(t)}. Metzler et al. (2014) have demonstrated that for an ergodic (for example, normal diffusion) flow, the mean square displacement is <X2> = limT→∞τX2(T,s), where τX2 (T, s) = 1/(T − s) ∫0T−s(X(t+Δt) − X(t))2 dt, T and s are observational and lag times but for weak non-ergodic (such as super-diffusion and sub-diffusion) flows <X2> = limT→∞≪τX2(T,s)≫, where ≪…≫ is some additional averaging. Numerical calculations for surface drifters in the Black Sea and isobaric RAFOS floats deployed at mid depths in the California Current system demonstrated that the reconstructed diffusion coefficients were smaller than those calculated by Davis (1991)’s technique. This difference is caused by the choice of the Lagrangian mean. The technique proposed here is applied to the analysis of Lagrangian motions in the Black Sea (horizontal diffusion coefficients varied from 105 to 106 cm2/s) and for the sub-diffusion of two RAFOS floats in the California Current system where power exponents varied from 0.65 to 0.72. RAFOS float motions were found to be strongly non-ergodic and non-Gaussian.

A gridded surface current product for the Gulf of Mexico from consolidated drifter measurements

Abstract. A large set of historical surface drifter data from the Gulf of Mexico – 3770 trajectories spanning 28 years and more than a dozen data sources – are collected, uniformly processed and quality controlled, and assimilated into a spatially and temporally gridded dataset called GulfFlow. This dataset is available in two versions, with 1/4∘ or 1/12∘ spatial resolution respectively, both of which have overlapping monthly temporal bins with semimonthly spacing and which extend from the years 1992 through 2020. Together these form a significant resource for studying the circulation and variability in this important region. The uniformly processed historical drifter data from all publicly available sources, interpolated to hourly resolution, are also distributed in a separate product called GulfDriftersOpen. Forming a mean surface current map by directly bin-averaging the hourly drifter data is found to lead to severe artifacts, a consequence of the extremely inhomogeneous temporal distribution of the drifters. Averaging instead the already monthly-averaged data in GulfFlow avoids these problems, resulting in the highest-resolution map of the mean Gulf of Mexico surface currents yet produced. The consolidated drifter dataset is freely available at https://doi.org/10.5281/zenodo.3985916 (Lilly and Pérez-Brunius, 2021a), while the gridded products are available for noncommercial use only (for reasons discussed herein) at https://doi.org/10.5281/zenodo.3978793 (Lilly and Pérez-Brunius, 2021b).

Relative Dispersion on the Inner Shelf: Evidence of a Batchelor Regime

AbstractOceanographic relative dispersion (based on drifter separations r) has been extensively studied, mostly finding either Richardson–Obukhov () or enstrophy cascade [] scaling. Relative perturbation dispersion (based on perturbation separation r − r0, where r0 is the initial separation) has a Batchelor scaling () for times less than the r0-dependent Batchelor time. Batchelor scaling has received little oceanographic attention. GPS-equipped surface drifters were repeatedly deployed on the Inner Shelf off of Pt. Sal, California, in water depths ≤ 40 m. From 12 releases of ≈18 drifters per release, perturbation and regular relative dispersion over ≈4 h are calculated for 250 ≤ r0 ≤ 1500 m for each release and the entire experiment. The perturbation dispersion is consistent with Batchelor scaling for the first 1000–3000 s with larger r0 yielding stronger dispersion and larger Batchelor times. At longer times, and scale-dependent diffusivities begin to suggest Richardson–Obukhov scaling. This applies to both experiment averaged and individual releases. For individual releases, nonlinear internal waves can modulate dispersion. Batchelor scaling is not evident in as the correlations between initial and later separations are significant at short time scaling as ~t. Thus, previous studies investigating are potentially aliased by initial separation effects not present in the perturbation dispersion . As the underlying turbulent velocity wavenumber spectra is inferred from the dispersion power law time dependence, analysis of both and is critical.

Observation and model resolution implications to ocean prediction

We address ocean modeling capability that has grown exponentially while ocean observation growth has not maintained pace, a situation leading to seemingly degraded forecast skill when model resolution is increased. Skill in predicting ocean instabilities such as mesoscale eddies requires satellite and in situ observations continually correcting numerical model conditions. Observations constrain positions of larger ocean model features, while smaller features are unconstrained. By means of an Observation System Simulation Experiment (OSSE), we show that time–space observation coverage controls the separation of constrained and unconstrained feature scales. Using 1000 independent surface drifters, we show constrained scales have deterministic prediction skill and unconstrained scales predict areas of higher expected errors. The results are shown to be consistent with ensemble forecasts. Separating constrained and unconstrained features, and using information within each appropriately, allows us to manage the present gap between observation and model resolution.

The turbulent dispersion of surface drifters by water waves: experimental study

Dispersion of a passive tracer by water waves is of significant importance for many scientific and technological problems including bio-diversity of marine life, ecological impact of anthropogenic incidents, planning of rescue operation and global oceanic transport. Formally, turbulent dispersion of a passive tracer by surface waves is a Brownian motion caused by a prescribed noise, viz., random fluctuations of the wave field. From this perspective, it is similar to the conventional dispersion by tracer particles by turbulent flows initially described in the seminal work of Richardson, Taylor and Obukhov. The additional challenges of this problem are imposed by the complexity of the underlining wave field—different dispersion relations and correlation structure, directionality and its spread, wave breaking—and this complexity necessitates further theoretical and experimental research. The aim of the present study is experimental validation of scaling relations for the mean drift and mean variance of tracer particles predicted by the wave turbulence theory. We report results of a set of targeted experiments in a large three-dimensional wave tank where the positions of the tracer particles—modelled as surface drifters—were tracked down with optical cameras. The experimental data are analysed and discussed in the light of the weak turbulence theory.

Dispersion of Surface Drifters in the Tropical Atlantic

The Tropical Atlantic Ocean has recently been the source of enormous amounts of floating Sargassum macroalgae that have started to inundate shorelines in the Caribbean, the western coast of Africa and northern Brazil. It is still unclear, however, how the surface currents carry the Sargassum, largely restricted to the upper meter of the ocean, and whether observed surface drifter trajectories and hydrodynamical ocean models can be used to simulate its pathways. Here, we analyze a dataset of two types of surface drifters (38 in total), purposely deployed in the Tropical Atlantic Ocean in July, 2019. Twenty of the surface drifters were undrogued and reached only ∼8 cm into the water, while the other 18 were standard Surface Velocity Program (SVP) drifters that all had a drogue centered around 15 m depth. We show that the undrogued drifters separate more slowly than the drogued SVP drifters, likely because of the suppressed turbulence due to convergence in wind rows, which was stronger right at the surface than at 15 m depth. Undrogued drifters were also more likely to enter the Caribbean Sea. We also show that the novel Surface and Merged Ocean Currents (SMOC) product from the Copernicus Marine Environmental Service (CMEMS) does not clearly simulate one type of drifter better than the other, highlighting the need for further improvements in assimilated hydrodynamic models in the region, for a better understanding and forecasting of Sargassum drift in the Tropical Atlantic.

Bathymetric observations of an extreme tidal flow: Approaches to the Gulf of Corryvreckan, western Scotland, UK

The growing interest in tidal energy has led to a need to better understand the interaction between energetic tidal flows and submarine bathymetry. The Gulf of Corryvreckan (GoC) is a highly energetic tidal channel, ∼3.2 km long and ∼1.1 km wide with a maximum depth of 220 m occurring between the islands of Scarba and Jura, western Scotland, UK. A high-resolution bathymetric survey of the region, groundtruthed by sediment samples, has enabled the bathymetry to be related to Lagrangian flow data from GPS-tracked surface drifter buoys. The maximum recorded drifter velocity was 4.75m/s within the GoC during the ebb (east-flowing) tide. Exiting the GoC into open water to the west on the flood tide, the ‘Great Race’ decelerates and subsequently flows in a curve to the north. East and west of the GoC, sedimentary bedforms lie on the flanks of the main flow paths. The northern flank of the Great Race aligns with a 6 km elongate bedform (‘banner bank’) extending from the GoC. East of the GoC, complex tidal bottom flows are suspected to result in uncommon sediment morphology, including suspected sediment wave interference patterns expressed by the mobile bedforms. The novelty of these findings indicates the importance of further research into sediment pathways in energetic tidal zones. A greater understanding of how sediment transport may be modified during offshore construction is an important consideration for offshore installations and the feasibility of marine renewables.

Observational study of super typhoon Meranti (2016) using satellite, surface drifter, Argo float and reanalysis data

The present work describes the basic features of super typhoon Meranti (2016) by multiple data sources. We mainly focus on the upper ocean response to Meranti using multiplatform satellites, in situ surface drifter and Argo floats, and compare the results with the widely used idealized wind vortex model and reanalysis datasets. The pre-existing meso-scale eddy provided a favor underlying surface boundary condition and also modulated the upper ocean response to Meranti. Results show that the maximum sea surface cooling was 2.0°C after Meranti. The satellite surface wind failed to capture the core structure of Meranti as the idealized wind vortex model deduced. According to the observation of sea surface drifters, the near-inertial currents were significantly enhanced during the passage of Meranti. The temperature and salinity profiles from Argo floats revealed both the mixed-layer extension and subsurface upwelling induced by Meranti. The comparison results show that the sea surface temperature and surface wind in the reanalysis datasets differs from those in remote sensing system. Sea surface cooling is similar in both satellite and in situ observation, and sea surface salinity response has a lower correlation with the precipitation rate.

A Compact GPS Surface Drifter with LoRa Telemetry and Self-Contained Tracking System

AbstractShallow marine environments are characterized by complex and variable physical processes that have significance for biological and human communities. However, the global disparity in scientific resources limits examination of the coastal ocean in some regions, so the development of universal and accessible technologies is needed. We combined a compact, GPS-equipped drifter design with long-range (LoRa) radio telemetry and a receiver system housed on a research vessel to develop a low-cost, easily assembled system of drifters. LoRa transmission of real-time location data while deployed was used to increase recoverability, minimizing the initial and long-term costs of constructing and maintaining the operation of this drifter system. Bench and field testing confirmed the performance of the system, which was optimized for LoRa transmission, which is compromised when occurring near the sea surface. The swift currents, complex coastline, and varied sea states present in our study area provided a rigorous testing environment for the drifter system. We established the working limits of battery life and LoRa data transmission within our configuration of transmitters and receiver, evaluated the accuracy of the GPS receivers, and determined the most robust variant of the drifter body design. This study offers proof of concept for a radio-tracked drifter system that is ready for deployment to track nearshore surface currents. Both the drifter system and the data telemetry protocol can be adapted to other investigations of the marine environment by integrating new sensors, modifying the configuration or deployment procedure, or applying LoRa radios to transfer data from fixed sensing platforms.

Dynamical features of near-inertial motions in global ocean based on the GDP dataset from 2000 to 2019

Based on the latest oceanic surface drifter dataset from the global drifter program during 2000–2019, this study investigated the global variation of relative frequency shift (RFS), near-inertial energy (NIE) and inverse excess bandwidth (IEB) of near-inertial motions, and analyzed their relations with oceanic mesoscale dynamics, relative vorticity and strain. Compared with previous works, we have some new findings in this study: (1) the RFS was high with negative values in some regions in which we found a significant blue shift of the RFS in the equatorward of 30°N (S) and from 50°N to 60°N in the Pacific, and a red shift in the western boundary currents and their extension regions, the North Atlantic and the Antarctic Circumpolar Current regions; (2) more peak values of the NIE were found in global regions like the South Indian Ocean, the Luzon Strait and some areas of the South Ocean; (3) the global distribution of the IEB were characterized by clear zonal bands and affected by vorticity and wind field; (4) the RFS was elevated as the absolute value of the gradient of vorticity increased, the IEB did not depend on the gradient of vorticity, and the eddy kinetic energy (EKE) weakened with the decrease of the absolute value of RFS; (5) the NIE decreased with increasing absolute value of the relative vorticity and the gradient of vorticity, but it increased with increasing strain and EKE when EKE was larger than 0.003 2 m2/s2.

Subtidal surface circulation in lower Cook Inlet and Kachemak Bay, Alaska

Nearly 90 Lagrangian surface drifters were deployed and often recovered and re-deployed in lower Cook Inlet and Kachemak Bay from 2003 through 2007 and from 2012 through 2017. Velocity vectors calculated from the hourly position data by central finite-differences were low-passed filtered to create maps of the mean, sub-tidal surface circulation. The circulation maps, while generally consistent with the results of Burbank (1977) and Muench et al. (1978), provide new detail showing anti-cyclonic circulation around Kalgin Island, a single cyclonic circulation cell in outer Kachemak Bay, and two circulation cells, one cyclonic and the other anti-cyclonic, in inner Kachemak Bay.

An Assessment of Global Ocean Barotropic Tide Models Using Geodetic Mission Altimetry and Surface Drifters

AbstractThe accuracy of three data-constrained barotropic ocean tide models is assessed by comparison with data from geodetic mission altimetry and ocean surface drifters, data sources chosen for their independence from the observational data used to develop the tide models. Because these data sources do not provide conventional time series at single locations suitable for harmonic analysis, model performance is evaluated using variance reduction statistics. The results distinguish between shallow and deep-water evaluations of the GOT410, TPXO9A, and FES2014 models; however, a hallmark of the comparisons is strong geographic variability that is not well summarized by global performance statistics. The models exhibit significant regionally coherent differences in performance that should be considered when choosing a model for a particular application. Quantitatively, the differences in explained SSH variance between the models in shallow water are only 1%–2% of the root-mean-square (RMS) tidal signal of about 50 cm, but the differences are larger at high latitudes, more than 10% of 30-cm RMS. Differences with respect to tidal currents variance are strongly influenced by small scales in shallow water and are not well represented by global averages; therefore, maps of model differences are provided. In deep water, the performance of the models is practically indistinguishable from one another using the present data. The foregoing statements apply to the eight dominant astronomical tides M2, S2, N2, K2, K1, O1, P1, and Q1. Variance reduction statistics for smaller tides are generally not accurate enough to differentiate the models’ performance.

Detecting flow features in scarce trajectory data using networks derived from symbolic itineraries: an application to surface drifters in the North Atlantic

Abstract. The basin-wide surface transport of tracers such as heat, nutrients and plastic in the North Atlantic Ocean is organized into large-scale flow structures such as the Western Boundary Current and the Subtropical and Subpolar gyres. Being able to identify these features from drifter data is important for studying tracer dispersal but also for detecting changes in the large-scale surface flow due to climate change. We propose a new and conceptually simple method to detect groups of trajectories with similar dynamical behaviour from drifter data using network theory and normalized cut spectral clustering. Our network is constructed from conditional bin-drifter probability distributions and naturally handles drifter trajectories with data gaps and different lifetimes. The eigenvalue problem of the respective Laplacian can be replaced by a singular value decomposition of a related sparse data matrix. The construction of this matrix scales with O(NM+Nτ), where N is the number of particles, M the number of bins and τ the number of time steps. The concept behind our network construction is rooted in a particle's symbolic itinerary derived from its trajectory and a state space partition, which we incorporate in its most basic form by replacing a particle's itinerary by a probability distribution over symbols. We represent these distributions as the links of a bipartite graph, connecting particles and symbols. We apply our method to the periodically driven double-gyre flow and successfully identify well-known features. Exploiting the duality between particles and symbols defined by the bipartite graph, we demonstrate how a direct low-dimensional coarse definition of the clustering problem can still lead to relatively accurate results for the most dominant structures and resolve features down to scales much below the coarse graining scale. Our method also performs well in detecting structures with incomplete trajectory data, which we demonstrate for the double-gyre flow by randomly removing data points. We finally apply our method to a set of ocean drifter trajectories and present the first network-based clustering of the North Atlantic surface transport based on surface drifters, successfully detecting well-known regions such as the Subpolar and Subtropical gyres, the Western Boundary Current region and the Caribbean Sea.

Observations of surface drift and effects induced by wind and surface waves in the Baltic Sea for the period 2011–2018

This study examines one of the longest collection of in-situ surface drifters, deployed at different seasons for the period of 2011–2018 in the Gulf of Finland, Baltic Sea. Overall 38 drifters deployed in batches of three were designed to capture the surface drift in the uppermost 2 m layer of the sea. The properties of drifter trajectories and the effects of wind and waves on the surface drift are analysed both qualitatively and quantitatively. The overall transport is asymmetric and the prevailing direction is out of the gulf against predominant winds. The most frequent drifter speed was 0.05–0.15 m/s (49% of occurrences) and wave height less than 0.5 m (46% of occurrences). Larger drifter speeds generally occurred during periods of stronger winds and higher waves. Wave heights >0.5 m and surface drift >0.15 m/s accounted for 21% of occurrences. In some occasions relatively high drifter speeds (>0.15 m/s) occurred under low wind and in almost calm sea (wave height of <0.5 m). These occasions accounted for 7% and 14% of occurrences, respectively. Rapid motions of drifters were apparently governed by other processes (e.g., eddies, fronts, upwellings, downwellings) during such episodes. For 68% of occurrences the surface current speed is 1.5% of the wind speed with an error of less than ±0.1 m/s. These results demonstrate that whilst wind and surface waves largely govern the surface drift on >2/3 of the occasions, underlying circulation may often dominate the motions in the uppermost layer in the study area.

Surface water exchanges in the Luzon Strait as inferred from Lagrangian coherent structures

This study presents a Lagrangian view of upper water exchanges across the Luzon Strait based on the finite-time Lyapunov exponents (FTLE) fields computed from the surface geostrophic current. The Lagrangian coherent structures (LCSs) extracted from the FTLE fields well identify the typical flow patterns and eddy activities around the Luzon Strait. In addition, they reveal the intricate transport paths and fluid domains, which are validated by the tracks of satellite-tracked surface drifters and cannot be visually recognized in the velocity maps. The FTLE fields indicate that there are mainly four types of transport patterns near the Luzon Strait; among them, the Kuroshio northward-flowing “leaping” pattern and the clockwise rotating “looping” pattern occur more frequently than the “leaking” pattern of the direct Kuroshio branch into the SCS and the “outflowing” pattern from the SCS to the Pacific. The eddy shedding events of the Kuroshio at the Luzon Strait are further analyzed, and the importance of considering LCSs in estimating transport by eddies is highlighted. The anticyclonic eddy (ACE) shedding cases reveal that ACEs mainly originate from the looping paths of Kuroshio and thus could effectively trap the Kuroshio water before eddy detachments. LCSs provide useful information to predict the positions of the upstream waters that finally enter the ACEs. In contrast, LCS snapshots indicate that during the formation of cyclonic eddies (CEs), most CEs are not connected with the pathways of Kuroshio water. Hence, the contribution of CEs to the surface water exchanges from the Pacific into the SCS is tiny.

Submesoscale Kinematic Properties in Summer and Winter Surface Flows in the Northern Gulf of Mexico

AbstractStatistical properties of near‐surface horizontal velocity gradients are obtained from four drifter experiments conducted in the Gulf of Mexico during Summer 2012 and Winter 2016. The data density provided by the near‐simultaneous deployments of 90‐326 surface drifters in each allows direct, drifter‐based estimates of the scale dependence of velocity gradients at separation scales ranging from 200 m to 5 km. The robustness of these estimates, derived from uniquley sampled, nearly equilateral triplets, is confirmed by comparisons with estimates produced from larger drifter clusters, and with estimates based on concurrent Eulerian X‐band radar observations. The winter launches were deployed above a ∼80 m deep mixed layer, one in a region with nearly homogeneous horizontal density structure, the other in a region of strong surface density gradients associated with filaments of fresh Mississippi River water. The summer launches occurred in a shallow (10m) mixed layer, one launched across a mesoscale frontal jet separating regions of horizontally homogeneous density and the other, similar to the corresponding winter launch, also in a region filamented by shallow lenses of cold, fresh water. Seasonal differences are observed, with larger velocity fluctuations and greater variance in divergence and vorticity, especially at the smallest scales, in winter. Differences between same‐season launches are, however, as large as seasonal differences. In both seasons, observations sampling regions directly impacted by fresh water fluxes show strongly skewed vorticity distributions, with cyclonic vorticity dominating strain. For the other launches, one in each season, strain dominated minimally skewed vorticity.

Impact of the Monthly Variability of the Trent River on the Hydrodynamical Conditions of the Bay of Quinte, Ontario: A Case Study 2016–2019

The spatial and temporal (monthly) variability of river discharge has a significant effect on circulation and transport pathways within shallow embayments whose dynamics are largely controlled by wind and riverine inputs. This study illustrates the effects of the monthly variation in Trent River discharge on simulated particle transport and settling destination in the Bay of Quinte, Lake Ontario for the years 2016–2019. Observations of Lagrangian surface drifter data were used to derive Trent River discharge forcing for a three-dimensional hydrodynamic numerical model of the Bay of Quinte. Peak monthly flushing was up to three times as much as the lowest monthly flushing in any year, with the Trent River responsible for up to 95% of the flushing in low runoff years. Particle transport simulations showed that particles could be trapped along shorelines, which extended residence times, and Trent River releases suggest that researchers should look for delayed peaks in Total Phosphorous (TP) load measurements in observations between Trenton and Belleville as particles move downstream. Particles with constant settling velocities originating from the Trent River did not move downstream past Big Bay, and particles from the Napanee River were the primary source for Longreach.

Pathways and Water Mass Transformation Along and Across the Mohn‐Knipovich Ridge in the Nordic Seas

AbstractAtlantic Water takes various pathways through the Nordic Seas, and its transformation to denser waters forms a crucial connection to the lower limb of the Atlantic Meridional Overturning Circulation. Circulation maps often schematize two distinct pathways of Atlantic Water: one following the Norwegian Atlantic Slope Current along the continental slope of Norway and one following the Norwegian Atlantic Front Current along the Mohn and Knipovich Ridges. In this paper, the connectivity between the northward flow along these ridges is investigated. Analyzing trajectories of surface drifters and ARGO floats, we find that only 8% of the floats that travel near the mid‐ocean ridges take the frontal pathway to the north. Indeed, by tracing numerical particles in a realistic numerical simulation, part of the water mass traveling along the Mohn Ridge follows the 2,500 m isobath eastward and joins the slope current, instead of flowing north along the Knipovich Ridge. Furthermore, north of 74°N, frequent exchange between the slope current and the front current is observed. Therefore, the slope current and front current are less isolated than often schematized. Additionally, the observational data set reveals substantial cross‐ridge exchange; 31% of the floats that travel within 60 km from the mid‐ocean ridges cross it. Results from numerical simulations indicate that the cross‐ridge exchange leads to cooling and freshening of the Atlantic Water along the front. Deployments of floats near the mid‐ocean ridges are needed to investigate the pathway of Atlantic Water and its exchange across the ridge in more detail.