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Our unmanned aerial system (UAS) current mapping is based on optical video data of the sea surface. We use three-dimensional fast Fourier transform and least squares fitting to measure the surface waves’ phase velocities and the currents via the linear dispersion relationship. Our UAS is a low-cost off-the-shelf quadcopter with inaccurate camera position and attitude measurements, which may cause spurious currents as large as the signal. We present a novel wave-based UAS heading and position correction, improving the image rectification accuracy by a factor of ~3.5 and the current measurements’ temporal repeatability by factors of 1.8–4.8. This validation study maps the currents at high spatiotemporal resolution (5 m and 4 s) across the ~700-m-wide tidally dominated Bear Cut channel in Miami, Florida. The UAS currents are compared to flotsam tracks, obtained through automated UAS video object detection and tracking, drifter tracks, and acoustic Doppler current profiler measurements. The root-mean-square errors of the cross- and along-channel currents are better than 0.03 m s−1 for the flotsam comparison and better than 0.06 m s−1 for the drifter comparison; the latter revealed a 0.06 m s−1 along-wind bias due to wind- and wave-driven vertical current shear. UAS current mapping could be used to monitor river discharge, buoyant pollutants, or submesoscale fronts and eddies; the proposed wave-based heading and position correction enables its use in areas without ground control points.

The Gulf of Mexico is a place where the environment and the economy both coexist and contend. It is a resilient large marine ecosystem that has changed in response to many drivers and pressures that we are only now beginning to fully understand. Coastlines of the states that border the Gulf comprise about half of the US southern seaboard, and those states are capped by the vast Midwest. The Gulf drains most of North America and is both an economic keystone and an unintended waste receptacle. It is a renowned resource for seafood markets, recreational fishing, and beach destinations and an international maritime highway fueled by vast, but limited, hydrocarbon reserves. Today, more is known about the Gulf than was imagined possible only a few years ago. That gain in knowledge was driven by one of the greatest environmental disasters of this country’s history, the Deepwater Horizon oil spill. The multitude of response actions and subsequent funded research significantly contributed to expanding our knowledge and, perhaps most importantly, to guiding the work needed to restore the damage from that oil spill. Funding for further work should not wait for the next major disaster, which will be too late; progress must be maintained to ensure that the Gulf continues to be resilient.

Physical transport processes such as the circulation and mixing of waters largely determine the spatial distribution of materials in the ocean. They also establish the physical environment within which biogeochemical and other processes transform materials, including naturally occurring nutrients and human-made contaminants that may sustain or harm the region’s living resources. Thus, understanding and modeling the transport and distribution of materials provides a crucial substrate for determining the effects of biological, geological, and chemical processes. The wide range of scales in which these physical processes operate includes microscale droplets and bubbles; small-scale turbulence in buoyant plumes and the near-surface “mixed” layer; submesoscale fronts, convergent and divergent flows, and small eddies; larger mesoscale quasi-geostrophic eddies; and the overall large-scale circulation of the Gulf of Mexico and its interaction with the Atlantic Ocean and the Caribbean Sea; along with air-sea interaction on longer timescales. The circulation and mixing processes that operate near the Gulf of Mexico coasts, where most human activities occur, are strongly affected by wind- and river-induced currents and are further modified by the area’s complex topography. Gulf of Mexico physical processes are also characterized by strong linkages between coastal/shelf and deeper offshore waters that determine connectivity to the basin’s interior. This physical connectivity influences the transport of materials among different coastal areas within the Gulf of Mexico and can extend to adjacent basins. Major advances enabled by the Gulf of Mexico Research Initiative in the observation, understanding, and modeling of all of these aspects of the Gulf’s physical environment are summarized in this article, and key priorities for future work are also identified.

Understanding the size of oil droplets released from a jet in crossflow is crucial for estimating the trajectory of hydrocarbons and the rates of oil biodegradation/dissolution in the water column. We present experimental results of an oil jet with a jet-to-crossflow velocity ratio of 9.3. The oil was released from a vertical pipe 25 mm in diameter with a Reynolds number of 25 000. We measured the size of oil droplets near the top and bottom boundaries of the plume using shadowgraph cameras and we also filmed the whole plume. In parallel, we developed a multifluid large eddy simulation model to simulate the plume and coupled it with our VDROP population balance model to compute the local droplet size. We accounted for the slip velocity of oil droplets in the momentum equation and in the volume fraction equation of oil through the local, mass-weighted average droplet rise velocity. The top and bottom boundaries of the plume were captured well in the simulation. Larger droplets shaped the upper boundary of the plume, and the mean droplet size increased with elevation across the plume, most likely due to the individual rise velocity of droplets. At the same elevation across the plume, the droplet size was smaller at the centre axis as compared with the side boundaries of the plume due to the formation of the counter-rotating vortex pair, which induced upward velocity at the centre axis and downward velocity near the sides of the plume.

We present an analysis of ocean surface dispersion characteristics, on 1–100-m scales, obtained by optically tracking a release of bamboo plates for 2 h in the northern Gulf of Mexico. Under sustained 5–6 m s−1 winds, energetic Langmuir cells are clearly delineated in the spatially dense plate observations. Within 10 min of release, the plates collect in windrows with 15-m spacing aligned with the wind. Windrow spacing grows, through windrow merger, to 40 m after 20 min and then expands at a slower rate to 50 m. The presence of Langmuir cells produces strong horizontal anisotropy and scale dependence in all surface dispersion statistics computed from the plate observations. Relative dispersion in the crosswind direction initially dominates but eventually saturates, while downwind dispersion exhibits continual growth consistent with contributions from both turbulent fluctuations and organized mean shear. Longitudinal velocity differences in the crosswind direction indicate mean convergence at scales below the Langmuir cell diameter and mean divergence at larger scales. Although the second-order structure function measured by contemporaneous GPS-tracked surface drifters drogued at ~0.5 m shows persistent r2/3 power law scaling down to 100–200-m separation scales, the second-order structure function for the very near surface plates observations has considerably higher energy and significantly shallower slope at scales below 100 m. This is consistent with contemporaneous data from undrogued surface drifters and previously published model results indicating shallowing spectra in the presence of direct wind-wave forcing mechanisms.

The extensive drifter deployment during the Lagrangian Submesoscale Experiment (LASER) provided observations of the surface velocity field in the northern Gulf of Mexico with high resolution in space and time. Here, we estimate the submesoscale velocity field sampled by those drifters using a procedure that statistically interpolates these data both spatially and temporally. Because the spacing of the drifters evolves with the flow, causing the resolution that they provide to vary in space and time, it is important to be able to characterize where and when the estimated velocity field is more or less accurate, which we do by providing fields of interpolation errors. Our interpolation uses a squared-exponential covariance function characterizing correlations in latitude, longitude, and time. Two novelties in our approach are 1) the use of two scales of variation per dimension in the covariance function and 2) allowing the data to determine these scales along with the appropriate amplitude of observational noise at these scales. We present the evolution of the reconstructed velocity field along with maps of relative vorticity, horizontal divergence, and lateral strain rate. The reconstructed velocity field exhibits horizontal length scales of 0.4–3.5 km and time scales of 0.6–3 h, and features with convergence up to 8 times the planetary vorticity f, lateral strain rate up to 10f, and relative vorticity up to 13f. Our results point to the existence of a vigorous and substantial ageostrophic circulation in the submesoscale range.

In the aftermath of the Deepwater Horizon event, GoMRI-funded research consortia carried out several field campaigns in the northern Gulf of Mexico with the objectives of understanding physical processes that influence transport of oil in the ocean and evaluating the accuracy of current-generation ocean models. A variety of new instruments were created to achieve unprecedented levels of dense and overlapping datasets that span five orders of magnitude of spatial and temporal scales. The observational programs: GLAD (DeSoto Canyon, Summer 2012), SCOPE (Destin inner shelf, Winter 2013 14), LASER (DeSoto Canyon, Winter 2016) and SPLASH (Louisiana shelf, Spring 2017) were designed to capture transport by ocean currents that are not presently well resolved by operational models. The overarching objective of these experiments was to collect data from a variety of sensors (drifting, aerial and ship-board) to document the circulation and near-surface variability of fronts, where much of the surface oil tends to be concentrated. Two state-of-the-art models were also run in real-time during all the experiments; a multiply-nested Navy Coastal Ocean Model with horizontal resolutions ranging from 1 km in the outer nest down to 100 m, as well as a fully coupled atmosphere-wave-ocean model. The purpose of this submission is to summarize the advances made in both understanding and modeling the near-surface transport in the Gulf of Mexico.

Horizontal and vertical motions associated with mesoscale (10–100 km) and submesoscale (1–10 km) features, such as fronts, meanders, eddies, and filaments, play a critical role in redistributing physical and biogeochemical properties in the ocean. This study makes use of a multiplatform data set of 82 drifters, a Lagrangian float, and profile timeseries of temperature and salinity, obtained in a ∼1-m/s semipermanent frontal jet in the Alboran Sea as part of CALYPSO (Coherent Lagrangian Pathways from the Surface Ocean to Interior). Drifters drogued at ∼1-m and 15-m depth capture the mesoscale and submesoscale circulation aligning along the perimeter of fronts due to horizontal shear. Clusters of drifters are used to estimate the kinematic properties, such as vorticity and divergence, of the flow by fitting a bivariate plane to the horizontal drifter velocities. Clusters with submesoscale length scales indicate normalized vorticity ζ/f > 1 with Coriolis frequency f and normalized divergence of urn:x-wiley:21699275:media:jgrc24446:jgrc24446-math-0001(1) occurring in patches along the front, with error variance around 10%. By computing divergence from drifter clusters at two different depths, we estimate minimum vertical velocity of urn:x-wiley:21699275:media:jgrc24446:jgrc24446-math-0002(−100 m day−1) in the upper 10 m of the water column. These results are at least twice as large as previous estimates of vertical velocity in the region. Location, magnitude, and timing of the convergence are consistent with behavior of a Lagrangian float subducting in the center of a drifter cluster. These results improve our understanding of frontal subduction and quantify convergence and vertical velocity using Lagrangian tools., This research was supported by the Office of Naval Research (ONR) Departmental Research Initiative CALYPSO under program officers Terri Paluszkiewicz and Scott Harper.

This article belongs to the Special Issue Observing the Flow of Ocean Currents and Circulation Using Remote Sensing., An Algerian Eddy, anticyclonic vortex generated by the instability of the Algerian Current in the southwestern Mediterranean Sea, is studied using data provided by drifters (surface currents), Argo floats (temperature and salinity profiles), environmental satellites (absolute dynamic topography maps and ocean color images) and operational oceanography products. The eddy was generated in May 2018 and lasted as an isolated vortex until November 2018. Its morphology and kinematics are described in June–July 2018 when drifters were trapped in its core. During that period, the eddy was slowly moving to the NE (~2 km/day), with an overall diameter of about 200 km (slowly growing with time) and maximal surface swirl velocity of ~50 cm/s at a radius of ~50 km. Geostrophic currents derived from satellite altimetry data compare well with low-pass filtered drifter velocities, with only a slight overestimation, which is expected as its maximum vorticity corresponds to a small Rossby number of ~0.6. Satellite ocean color images and some drifters show that the eddy has an elliptical spiral structure. The looping tracks of the drifters trapped in the eddy were analyzed using two statistical methods: least-squares ellipse fitting and wavelet ridge analysis, revealing a typical eccentricity of about 0.5, a wide range of inclination and a rotation period between 3 and 10 days. Clusters of drifters on the northeastern limb of the eddy were also considered to estimate divergence and vorticity. The results indicate convergence (divergence) and downwelling (upwelling) at scales of 20–50 km near the northeastern (northwestern) edge of the eddy, in agreement with the quasi-geostrophic theory. Vertically, the eddy extends mostly down to 250 m depth, with a warm, low-salinity and low-density signature and with geostrophic currents near 50 cm/s in the top layer (down to ~80 m) reducing to less than 10 cm/s near 250 m. Near the surface, colder water is advected into it., This research was mainly funded by the US Office of Naval Research (ONR) as part of the CALYPSO Departmental Research Initiative through grants N00014-18-1-2418 and N00014-18-1-2138. LC and the SVP drifters were funded by ONR grants N00014-17-1-2517 and N00014-19-1-269. D.T., A.P. and S.R. were funded by ONR grant N00014-16-1-3130.

The dynamics of the near-surface ocean currents result from the nonlinear interaction of simultaneous mechanisms at different scales. Of these, the submesoscale range (a few hundred meters to 10 km, hours to a few days) remains particularly challenging to observe directly, due to the high variability in both time and space. In this study, the scale-dependence of kinematic properties (divergence, vorticity and strain) in the submesoscale range, as well as their response to atmospheric forcing, is investigated in two distinct geographic regions: the Ligurian (NW-Mediterranean) Sea and the Northern Gulf of Mexico. The two applications are characterized by different dynamics, and the estimates of kinematic properties are derived from distinctly different observational approaches: in situ GPS drifters observations and remote HF radar data. The Ligurian Sea application is based on HF radar measurements obtained for the JERICO-NEXT (Joint European Research Infrastructure network for Coastal Observatory – Novel European eXpertise for coastal observaTories) and IMPACT (Port Impact on Protected Marine Areas: Cooperative Cross-Border Actions) projects. Surface current measurements span 40 km off the coast with 1.5 km resolution, available every hour. The velocity fields are used to estimate the kinematic properties with an Eulerian approach, which allows the identification of structures such as eddies and jets of the order of a few km. We focus in particular on the response of the submesoscales to an extreme wind event that was captured by the observations. The deformation of the spatial structures suggests nonlinear interactions with the wind forcing, and the kinematic properties' magnitudes are almost doubled (exceeding the Coriolis parameter, f). In the Gulf of Mexico, velocity observations are available from a series of massive, nearly simultaneous drifter releases conducted by CARTHE (Consortium for Advanced Research of Transport of Hydrocarbons in the Environment). Drifter triplets are analysed to estimate the kinematic properties of the flow at scales between 100 m and 5 km over a time scale of a day. Results show that the mean kinematic properties increase in magnitude with decreasing scales, with winter values generally higher than summer ones. For winter flows, vorticity and divergence distributions have more substantial tails of values multiple times the Coriolis paramater f at scales up to 2 km, while in the summer such large values are restricted to smaller scales (100-500 m). The Lagrangian estimates of kinematic properties obtained in the Gulf of Mexico were also compared to Eulerian estimates from concurrent X-band radar measurements, showing good correlation and validating the comparison across observational methods. Moreover, the scale-dependence of the kinematic properties from drifter triplets was found to be consistent with turbulence scaling laws evaluated as two-particle statistics. We conclude that the kinematic properties metric provides a robust complementary methodology to characterize submesoscales and can be used both with Lagrangian and Eulerian observations.

Advances in drifter technology applied to oil spill studies from 1970 to the present are summarized here. Initially, drifters designed for oil spill response were intended to remotely track trajectories of accidental spills and help guide responders. Most recently, inexpensive biodegradable drifters were developed for massive deployments, making it possible to significantly improve numerical transport models and to investigate, via observations, the processes leading to dispersion and accumulation of surface pollutants across multiple scales. Over the past 50 years, drifters have benefited from constant improvements in electronics for accurate and frequent location and data transmission, as well as progress in material sciences to reduce fabrication costs and minimize the environmental impact of sacrificial instruments. The large amount of in-situ data provided by drifters, covering a broad area, is crucial to validate the numerical models and remote sensing products that are becoming more important in guiding response and policy decisions.

Formed in the aftermath of the Deepwater Horizon event, the largest accidental marine oil spill, the Consortium for Advanced Research on Transport of Hydrocarbons in the Environment (CARTHE) focused on understanding the physical processes controlling the transport of material from a deep blowout all the way to the coast. Even though CARTHE was initially a modeling-oriented team, it progressively became more focused on observations in order to collect the data needed for model evaluation. A number of new technological advances needed to be made to collect the necessary data. This article reviews most of these, with special focus on surface sampling, where much of the oil is located during oil spills, as well as the measurement of near-field droplet size distribution.

The Lagrangian Submesoscale Experiment (LASER) involved the deployment of ~1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F. G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR–drifter pairs, the root-mean-square error for the current speed is 4 cm and that for the current direction is 12°. The MR samples currents at a greater effective depth than the CARTHE drifters (1–5 m vs ~0.4 m). The mean MR–drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15°). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP.

The Lagrangian Submesoscale Experiment (LASER) was designed to study surface flows during winter conditions in the northern Gulf of Mexico. More than 1000 mostly biodegradable drifters were launched. The drifters consisted of a surface floater extending 5 cm below the surface, containing the satellite tracking system, and a drogue extending 60 cm below the surface, hanging beneath the floater on a flexible tether. On some floats, the drogue separated from the floater during storms. This paper describes methods to detect drogue loss based on two properties that distinguish drogued from undrogued drifters. First, undrogued drifters often flip over, pointing their satellite antenna downward and thus intermittently reducing the frequency of GPS fixes. Second, undrogued drifters respond to wind forcing more than drogued drifters. A multistage analysis is used: first, two properties are used to create a preliminary drifter classification; then, the motion of each unclassified drifter is compared to that of its classified neighbors in an iterative process for nearly all of the drifters. The algorithm classified drifters with a known drogue status with an accuracy of virtually 100%. Drogue loss times were estimated with a precision of less than 0.5 and 3 h for 60% and 85% of the drifters, respectively. An estimated 40% of the drifters lost their drogues in the first 7 weeks, with drogue loss coinciding with storm events, particularly those with steep waves. Once the drogued and undrogued drifters are classified, they can be used to quantify the differences in material dispersion at different depths.

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km 2 converged into a 60 × 60 m region within a week, a factor of more than 10 5 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s −1 and 0.01 ms −1 , respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material.

Plastics and spilled oil pose a critical threat to marine life and human health. As a result of wind forcing and wave motions, theoretical and laboratory studies predict very strong velocity variation with depth over the upper few centimeters of the water column, an observational blind spot in the real ocean. Here we present the first-ever ocean measurements of the current vector profile defined to within 1 cm of the free surface. In our illustrative example, the current magnitude averaged over the upper 1 cm of the ocean is shown to be nearly four times the average over the upper 10 m, even for mild forcing. Our findings indicate that this shear will rapidly separate pieces of marine debris which vary in size or buoyancy, making consideration of these dynamics essential to an improved understanding of the pathways along which marine plastics and oil are transported.

Targeted observations of submesoscale currents are necessary to improve science’s understanding of oceanic mixing, but these dynamics occur at spatiotemporal scales that are currently challenging to detect. Prior studies have recently shown that the submesoscale surface velocity field can be measured by tracking hundreds of surface drifters released in tight arrays. This strategy requires drifter positioning to be accurate, frequent, and to last for several weeks. However, because of the large numbers involved, drifters must be low-cost, compact, easy to handle, and also made of materials harmless to the environment. Therefore, the novel Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifter was designed following these criteria to facilitate massive sampling of near-surface currents during the Lagrangian Submesoscale Experiment (LASER). The drifting characteristics were determined under a wide range of currents, waves, and wind conditions in laboratory settings. Results showed that the drifter accurately follows the currents in the upper 0.60 m, that it presents minimal wave rectification issues, and that its wind-induced slip velocity is less than 0.5% of the neutral wind speed at 10 m. In experiments conducted in both coastal and deep ocean conditions under wind speeds up to 10 m s−1, the trajectories of the traditional Coastal Ocean Dynamics Experiment (CODE) and the CARTHE drifters were nearly identical. Following these tests, 1100 units were produced and deployed during the LASER campaign, successfully tracking submesoscale and mesoscale features in the Gulf of Mexico. It is hoped that this drifter will enable high-density sampling near metropolitan areas subject to stress by the overpopulation, such as lakes, rivers, estuaries, and environmentally sensitive areas, such as the Arctic.

The 2010 Deepwater Horizon (DwH) accident in the Gulf of Mexico has renewed oceanographic interest in point source buoyant convection. The present paper applies modern numerical techniques to study this problem, focussing specifically on the DwH event. The gas/oil/seawater nature of the problem requires a ‘multiphase’ approach, which is relatively unfamiliar in physical oceanography, although applications are becoming more common. The model is cast in an Eulerian framework and includes feedbacks between the convection and the environment, unlike past oil/gas plume simulations that adopt a semi-passive, Lagrangian approach. Fully three dimensional (3d) simulations are too computationally demanding for practical multi-day use, so a two-dimensional (2d) radially symmetric model is developed from the equations and calibrated to the 3d results. Both the 2d and 3d solutions show the somewhat unexpected result that oil/bubble plumes modelled after the DwH event are strongly affected by rotation and exert a considerable dynamic feedback on the ambient. These effects are not typically included in classical oil/gas plume models.

The effects of system rotation on the turbulent dynamics of bubble plumes evolving in stratified environments are numerically investigated by considering variations in both the system rotation rate and the gas-phase slip velocity. The turbulent dispersion of a passive scalar injected at the source of a buoyant plume is strongly altered by the rotation of the system and the nature of the buoyancy at the source. When the plume is driven by the density defect associated with the presence of slipping gas bubbles, the location of the main lateral intrusion decreases with respect to the single-phase case with identical inlet volume, momentum, and buoyancy fluxes. Enhanced downdrafts of carrier phase fluid result in increased turbulent mixing and short-circuiting of detraining plume water that elevate near-field effluent concentrations. Similarly, rotation fundamentally alters dynamic balances within the plume leading to the encroachment of the trapping height on the source and an increase in turbulent dispersion in the near field. System rotation, even at modest Rossby numbers, produces a sustained, robust, anticyclonic precession of the plume core. The effects of rotation and the presence of bubbles are cumulative. The vertical encroachment of the primary intrusion and the overall dispersion of effluent are greatest at smallest Rossby numbers and largest slip velocities. The main characteristic feature in rotating single-phase plumes, namely the robust anticyclonic precession, persists in bubble plumes. Analysis of the momentum budgets reveal that the mechanism responsible for the organized precession, i.e., the establishment of an unstable vertical hydrostatic equilibrium related to radial cyclostrophic balance, does not differ from the single-phase case.

Estimation of near-surface current is essential to the estimation of upper-ocean material transport. Wind forcing and wave motions are dominant in the near-surface layer [within O(0.01) m of the surface], where the highly sheared flows can differ greatly from those at depth. This study presents a new method for remotely measuring the directional wind and wave drift current profile near to the surface (between 0.01 and 0.001 m for the laboratory and between 0.1 and 0.001 m for the field). This work follows the spectral analysis of high spatial (0.002 m) and temporal resolution (60 Hz) wave slope images, allowing for the evaluation of near-surface current characteristics without having to rely on instruments that may disturb the flow. Observations gathered in the 15 m × 1 m × 1 m wind-wave flume at the University of Miami’s Surge-Structure-Atmosphere Interaction (SUSTAIN) facility show that currents retrieved via this method agree well with the drift velocity of camera-tracked dye. Application of this method to data collected in the mouth of the Columbia River (MCR) indicates the presence of a near-surface current component that departs considerably from the tidal flow and may be steered by the wind stress. These observations demonstrate that wind speed–based parameterizations alone may not be sufficient to estimate wind drift and to hold implications for the way in which surface material (e.g., debris or spilled oil) transport is estimated when atmospheric stress is of relatively high magnitude or is steered off the mean wind direction.

Flow splitting occurs when part of a gravity current becomes neutrally buoyant and separates from the bottom-trapped plume as an interflow. This phenomenon has been previously observed in laboratory experiments, small-scale water bodies (e.g., lakes) and numerical studies of small-scale systems. Here, the potential for flow splitting in oceanic gravity currents is investigated using high-resolution (Δx = Δz = 5 m) two-dimensional numerical simulations of gravity flows into linearly stratified environments. The model is configured to solve the non-hydrostatic Boussinesq equations without rotation. A set of experiments is conducted by varying the initial buoyancy number B 0 = Q 0 N 3 / g ′ 2 (where Q0 is the volume flux of the dense water flow per unit width, N is the ambient stratification and g′ is the reduced gravity), the bottom slope (α) and the turbulent Prandtl number (Pr). Regardless of α or Pr, when B0 ≤ 0.002 the outflow always reaches the deep ocean forming an underflow. Similarly, when B0 ≥ 0.13 the outflow always equilibrates at intermediate depths, forming an interflow. However, when B0 ∼ 0.016, flow splitting always occurs when Pr ≥ 10, while interflows always occur for Pr = 1. An important characteristic of simulations that result in flow splitting is the development of Holmboe-like interfacial instabilities and flow transition from a supercritical condition, where the Froude number (Fr) is greater than one, to a slower and more uniform subcritical condition (Fr

Motivated by the Deepwater Horizon oil spill, the Surfzone and Coastal Oil Pathways Experiment obtained Acoustic Doppler Current Profiler (ADCP) Eulerian and GPS-drifter based Lagrangian “surface” (

This study evaluates shipboard marine X‐band radar (MR) near‐surface current and bathymetry measurements under shallow water conditions. The retrieval algorithm is based on the surface wave signal within three‐dimensional wave number frequency MR backscatter intensity variance spectra. The MR data were collected during a research cruise that investigated submesoscale processes and their impact on oil spill transport in the Louisiana Bight. The MR currents and bathymetry are validated using measurements from 500 GPS‐equipped surface drifters, a shipboard acoustic Doppler current profiler, and a shipboard single‐beam echo sounder. Earlier results from the same experiment but using a different set of sensors indicate strong upper ocean vertical current shear over a 3.5 hr period, despite only mild wind forcing. Here, the MR currents are derived as a function of wave number, providing a measure of vertical shear for the full duration of the cruise. Strong vertical shear is frequently observed, with a maximum difference of 0.42 m/s between the MR high (effective depth of urn:x-wiley:jgrc:media:jgrc23817:jgrc23817-math-00010.9 m) and low ( urn:x-wiley:jgrc:media:jgrc23817:jgrc23817-math-00022.4 m) wave number bins. Treating the drifter and echo sounder measurements as truth, the accuracies of the MR near‐surface currents and bathymetry are 0.04–0.07 m/s and 1.2 m (or 7% of the mean water depth). However, it is shown that urn:x-wiley:jgrc:media:jgrc23817:jgrc23817-math-000350% of the MR high wave number and drifter current differences is due to vertical shear. The shallow water MR near‐surface current accuracy thus matches findings from a previous deep water validation where vertical shear was much weaker.

Following the 2010 Deepwater Horizon accident of a massive blow-out in the Gulf of Mexico, scientists from government, industry, and academia collaborated to advance oil spill modeling and share best practices in model algorithms, parameterizations, and application protocols. This synergy was greatly enhanced by research funded under the Gulf of Mexico Research Initiative (GoMRI), a 10-year enterprise that allowed unprecedented collection of observations and data products, novel experiments, and international collaborations that focused on the Gulf of Mexico, but resulted in the generation of scientific findings and tools of broader value. Operational oil spill modeling greatly benefited from research during the GoMRI decade. This paper provides a comprehensive synthesis of the related scientific advances, remaining challenges, and future outlook. Two main modeling components are discussed: Ocean circulation and oil spill models, to provide details on all attributes that contribute to the success and limitations of the integrated oil spill forecasts. These forecasts are discussed in tandem with uncertainty factors and methods to mitigate them. The paper focuses on operational aspects of oil spill modeling and forecasting, including examples of international operational center practices, observational needs, communication protocols, and promising new methodologies.

Much of the vertical transport near the surface of the ocean, which plays a critical role in the transport of dissolved nutrients and gases, is thought to be associated with ageostrophic submesoscale phenomena. Vertical velocities are challenging not only to model accurately, but also to measure because of how difficult they are to locate in the surface waters of the ocean. Using unique massive drifter releases during the Lagrangian Submesoscale Experiment (LASER) campaign in the Gulf of Mexico and the Coherent Lagrangian Pathways from the Surface Ocean to the Interior (CALYPSO) experiment in the Mediterranean Sea, we investigate the generation of submesoscale structures along two different mesoscale fronts. We use a novel method to project Lagrangian trajectories to Eulerian velocity fields, in order to calculate horizontal velocity gradients at the surface, which are used as a proxy for vertical transport. The velocity reconstruction uses a squared-exponential covariance function, which characterizes velocity correlations in horizontal space and time, and determines the scales of variation using the data itself. SST and towed CTD measurements support the findings revealed by the drifter data. Due to the production of a submesoscale instability eddy in the Gulf of Mexico, convergence magnitudes of up to &sim, 20 times the planetary vorticity, f, are observed, the value of which is almost 3 times larger than that found in the mesoscale dominated Western Mediterranean Sea.

Statistical properties of near‐surface horizontal velocity gradients are obtained from four drifter experiments conducted in the Gulf 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.

LASER data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data. gulfresearchinitiative.org under the following DOIs: 10.7266/N7S75DRP (shipboard meteorological data), 10.7266/n7-93j3-mn56 (shipboard ADCP), 10.7266/N7N01550 (radar), 10.7266/N7H130FC (MVP), and 10.7266/N7W0940J (drifters). NDBC buoy data is available at https://www. ndbc.noaa.gov/station_page.php? station=42887. The article of record as published may be found at https://doi.org/10.1029/2019JC015236 Submesoscale oceanic fronts (SFs), which typically occur on a spatial scale of 0.1–10 km, may have a large influence on the atmospheric surface layer (ASL). However, due to their short temporal-spatial scales, evaluating their direct impact on this layer remains challenging and characterizing the nature of SF-ASL interaction has not been done in the field. To address this, a study of the air-sea response to SFs was conducted using observations collected during the Lagrangian Submesoscale Experiment, which took place in the northern Gulf of Mexico. This manuscript focuses on the meteorological measurements made from a pair of masts installed on the bow of the R/V Walton Smith. This work represents one of the first observation-based investigations into the potential influence that SFs have on the ASL. Contemporaneous measurements from an X-band marine radar, moving vessel profiler, and Lagrangian drifters were also used to analyze the SF dynamics. Systematic surface wind velocity changes over several cross-frontal transects were observed, a process previously associated with mesoscale fronts. A comparison between the eddy covariance and parameterized (COARE 3.5) air-sea fluxes revealed that the directly observed heat flux was 1.5 times larger than the bulk value in the vicinity of the SFs. This suggests that the hydrodynamic processes near the front enhance the local exchange of sensible and latent heat. Given the prevalence of SF over the global upper ocean, these findings suggest that these features may have a widely distributed and cumulative impact on air-sea interactions. Gulf of Mexico Research Initiative (Grant SA-1515 CARTHE)

The dynamics of crude oil and different surface ocean drifters were compared to study the physical processes that govern the transport and landfall of marine oil spills. In a wave-tank experiment, drifters with drogue did not follow oil slicks. However, patches of undrogued drifters and thin bamboo plates did spread at the same rate and in the same direction as the crude oil slicks. Then, the trajectories of the Deepwater Horizon oil spill and 1300 drifters released near the spill source were investigated. Undrogued drifters were transported twice as fast as drogued drifters across the isobaths. 25% of the undrogued drifters landed, versus about 5% of the drogued ones, for the most part, on the same coastline locations where oil was found after Deepwater Horizon. Results highlight the importance of near surface gradients in controlling the cross-shelf transport and landing of surface material on the Gulf of Mexico's northern shores.

Very-near-surface ocean currents are dominated by wind and wave forcing and have large impacts on the transport of buoyant materials in the ocean. Surface currents, however, are under-resolved in most operational ocean models due to the difficultly of measuring ocean currents close to, or directly at, the air–sea interface with many modern instrumentations. Here, observations of ocean currents at two depths within the first meter of the surface are made utilizing trajectory data from both drogued and undrogued Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters, which have draft depths of 60 and 5 cm, respectively. Trajectory data of dense, colocated drogued and undrogued drifters were collected during the Lagrangian Submesoscale Experiment (LASER) that took place from January to March of 2016 in the northern Gulf of Mexico. Examination of the drifter data reveals that the drifter velocities become strongly wind- and wave-driven during periods of high wind, with the pre-existing regional circulation having a smaller, but non-negligible, influence on the total drifter velocities. During these high wind events, we deconstruct the total drifter velocities of each drifter type into their wind- and wave-driven components after subtracting an estimate for the regional circulation, which pre-exists each wind event. In order to capture the regional circulation in the absence of strong wind and wave forcing, a Lagrangian variational method is used to create hourly velocity field estimates for both drifter types separately, during the hours preceding each high wind event. Synoptic wind and wave output data from the Unified Wave INterface-Coupled Model (UWIN-CM), a fully coupled atmosphere, wave and ocean circulation model, are used for analysis. The wind-driven component of the drifter velocities exhibits a rotation to the right with depth between the velocities measured by undrogued and drogued drifters. We find that the average wind-driven velocity of undrogued drifters (drogued drifters) is ∼3.4 %–6.0 % (∼2.3 %–4.1 %) of the wind speed and is deflected ∼5–55∘ (∼30–85∘) to the right of the wind, reaching higher deflection angles at higher wind speeds. Results provide new insight on the vertical shear present in wind-driven surface currents under high winds, which have vital implications for any surface transport problem.

The interaction of cold-vertically stratified (CVS) Mississippi River water with warm-horizontally stratified (WHS) Gulf of Mexico water resulted in a front that affected the oceanic surface layer. Our cross-frontal observations demonstrated two vertical layers. The cross-frontal deep layer (9–30 m) averaged a temperature dissipation rate (TD) varied by a factor of 1000 and was larger on the CVS side. The near-surface layer (0–9 m) averaged TD did not vary significantly across the front. The deep layer frontal asymmetry coincided with depths where the Thorpe scale was large. The situation was similar for the layer averaged turbulent kinetic energy dissipation rate (TKED). Within both layers, the averaged-TKED values were 10–30 times larger on the CVS side. The surface turbulent heat flux was up to 4 times larger on the WHS side. The observed asymmetric response of the turbulence across the front could play a significant role in the ocean-atmosphere climate system.

Internal gravity waves that are generated in the open ocean have a universal frequency spectrum, called Garrett–Munk spectrum. By initializing internal waves that satisfy the Garrett–Munk spectrum in a non-hydrostatic numerical model, we investigate the material dispersion produced by these internal waves. Three numerical experiments are designed: Exp.-1 uses a linearly stratified fluid, Exp.-2 has an upper mixed layer, and Exp.-3 incorporates a circular front into the upper mixed layer. Resorting to neutrally buoyant particles, we investigate the dispersion in terms of metrics of the relative dispersion and finite-scale Lyapunov exponent (FSLE). Exp.-1 shows that the dispersion regime produced by these internal waves is between ballistic and diffusive based on relative dispersion, and is however ballistic according to FSLE. The maximum FSLE at scales of 100 m is about 5 day $$^{-1}$$ , which is comparable to that calculated using ocean drifters. Exp.-2 demonstrates that internal waves can generate flows and material dispersion in an upper mixed layer. However, when mixed layer eddies are present, as in Exp.-3, the dispersion in the mixed layer is controlled by the eddies. In addition, we show that inertial oscillations do not affect the relative dispersion, but impact FSLE at scales of inertial oscillations.

Understanding material distribution at the ocean’s surface is important for a number of applications, in particular for buoyant pollutants such as oil spills. The main tools to estimate surface flows are satellite altimeters, as well as data-assimilative ocean general circulation models (OGCMs). Current-generation altimeter products rely on the geostrophic approximation to derive surface currents. Recent modeling and experimental work revealed existence of ageostrophic submesoscale motions within the upper ocean boundary layer. The next frontier is how submesoscales influence transport pathways in the upper ocean, which is a multi-scale problem involving the interaction of submesoscale and mesoscale coherent structures. Here we focus on a mesoscale eddy that exhibits submesoscale fluctuations along its rim. The high-resolution OCGM fields are then treated with two filters. A Lanczos filter is applied to velocity fields to remove the kinetic energy over the submesoscales. Then a Gaussian filter is used for the modeled sea surface height to simulate a geostrophic velocity field that would be available from gridded satellite altimeter data. Lagrangian Coherent Structures (LCS) are then generated from full-resolution and filtered fields to compare Lagrangian characteristics corresponding to different realizations of the surface velocity fields. It is found that while mesoscale currents exert a general control over the pathways of the tracer initially launched in the mesoscale eddy, there is a leak across the mesoscale transport barriers, induced by submesoscale motions. This leak is quantified as 20% of the tracer when using the submesoscale filter over one month of advection, while it increases to 50% using the geostrophic velocity field. We conclude that LCS computed from mesoscale surface velocity fields can be considered as a good first-order proxy, but the leakage of material across them in the presence of submesoscales can be significant.

We numerically investigate the effects of rotation on the turbulent dynamics of thermally driven buoyant plumes in stratified environments at the large Rossby numbers characteristic of deep oceanic releases. When compared to nonrotating environments, rotating plumes are distinguished by a significant decrease in vertical buoyancy and momentum fluxes leading to lower and thicker neutrally buoyant intrusion layers. The primary dynamic effect of background rotation is the concentration of entraining fluid into a strong cyclonic flow at the base of the plume resulting in cyclogeostrophic balance in the radial momentum equation. The structure of this cyclogeostrophic balance moving upward from the well head is associated with a net adverse vertical pressure gradient producing an inverted hydrostatic balance in the mean vertical momentum budgets. The present simulations reveal that the primary response to the adverse pressure gradient is an off-axis deflection of the plume that evolves into a robust, organized anticyclonic radial precession about the buoyancy source. The off-axis evolution is responsible for the weaker inertial overshoots, the increased thickness of lateral intrusion layers, and the overall decrease in the vertical extent of rotating plumes at intermediate Rossby numbers compared to the nonrotating case. For inlet buoyancy forcings and environmental Rossby numbers consistent with those expected in deepwater blowout plumes, the speed of the organized precession is found to be as large as typical oceanic cross-flow speeds.

The Gulf of Mexico ecosystem is a hotspot for biological diversity and supports a number of industries, from tourism to fishery production to oil and gas exploration, that serve as the economic backbone of Gulf coast states. The Gulf is a natural hydrocarbon basin, rich with stores of oil and gas that lie in reservoirs deep beneath the seafloor. The natural seepage of hydrocarbons across the Gulf system is extensive and, thus, the system׳s biological components experience ephemeral, if not, frequent, hydrocarbon exposure. In contrast to natural seepage, which is diffuse and variable over space and time, the 2010 Macondo oil well blowout, represented an intense, focused hydrocarbon infusion to the Gulf׳s deepwaters. The Macondo blowout drove rapid shifts in microbial populations and activity, revealed unexpected phenomena, such as deepwater hydrocarbon plumes and marine “oil snow” sedimentation, and impacted the Gulf׳s pelagic and benthic ecosystems. Understanding the distribution and fate of Macondo oil was limited to some degree by an insufficient ability to predict the physical movement of water in the Gulf. In other words, the available physical oceanographic models lacked critical components. In the past six years, much has been learned about the physical oceanography of the Gulf, providing transformative knowledge that will improve the ability to predict the movement of water and the hydrocarbons they carry in future blowout scenarios. Similarly, much has been learned about the processing and fate of Macondo hydrocarbons. Here, we provide an overview of the distribution, fate and impacts of Macondo hydrocarbons and offer suggestions for future research to push the field of oil spill response research forward.