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Ocean boundary current systems are key components of the climate system, are home to highly productive ecosystems, and have numerous societal impacts. Establishment of a global network of boundary current observing systems is a critical part of ongoing development of the Global Ocean Observing System. The characteristics of boundary current systems are reviewed, focusing on scientific and societal motivations for sustained observing. Techniques currently used to observe boundary current systems are reviewed, followed by a census of the current state of boundary current observing systems globally. The next steps in the development of boundary current observing systems are considered, leading to several specific recommendations.

Complex two-dimensional nearshore current patterns are generated by feedbacks between sub-aqueous morphology and momentum imparted on the water column by breaking waves, winds, and tides. These non-stationary features, such as rip currents and circulation cells, respond to changing environmental conditions and underlying morphology. However, using fixed instruments to observe nearshore currents is limiting due to the high costs and logistics necessary to achieve adequate spatial sampling resolution. A new technique for processing surf-zone imagery, WAMFlow, quantifies fluid velocities to reveal complex, multi-scale (10 s–1000 s meters) nearshore surface circulation patterns. We apply the concept of a wave-averaged movie (WAM) to measure surf-zone circulation patterns on spatial scales of kilometers in the alongshore and 100 s of meters in the cross-shore. The approach uses a rolling average of 2 Hz optical imagery, removing the dominant optical clutter of incident waves, to leave the residual foam or water turbidity features carried by the flow. These residual features are tracked as quasi-passive tracers in space and time using optical flow, which solves for u and v as a function of image intensity gradients in x, y, and t. Surf zone drifters were deployed over multiple days with varying nearshore circulations to validate the optically derived flow patterns. Root mean square error are reduced to 0.1 m per second after filtering based on image attributes. The optically derived patterns captured longshore currents, rip currents, and gyres within the surf zone. Quantifying nearshore circulation patterns using low-cost image platforms and open-source computer vision algorithms presents the potential to further our understanding of fundamental surf zone dynamics.

The Lagrangian method—where current location and intensity are determined by tracking the movement of flow along its path—is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice‐free oceans and begin to construct basin‐scale, and eventually global‐scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC. Plain Language Summary: Measuring ocean currents by following their paths—rather than by observing them at a fixed location—is the oldest method for exploring ocean currents. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to track freely drifting surface buoys continuously throughout the ice‐free oceans and begin to construct global maps of the surface currents. At about the same time, the development of methods using sound to track floats below the surface led to important new discoveries regarding the currents bar below the surface. Since then, these techniques have been used to explore the location and strength of currents, and how they change in time, throughout the world's oceans, but especially in the Atlantic Ocean. Computer simulations of the oceans are also used to calculate the pathways of virtual surface drifters and subsurface floats. In this review, studies that use these flow‐following methods to measure the pathways of the north‐south shallow and deep currents that make up the Atlantic Meridional Overturning Circulation (also sometimes referred to as the Great Ocean Conveyor) have been gathered together to illustrate aspects of the conveyor system that are uniquely captured by this technique. These include the importance of large recirculations, or "waiting areas," where water circulates around and around before moving onto the next segment of the conveyor. Also, these techniques for observing ocean currents illustrate how disconnected some parts of the conveyor are, and how swirling, translating pools of water ~100 km in diameter, and not continuous currents, help to move water along the conveyor's path. Vast areas of the deep ocean remain where there are no direct observations of the pathways of the conveyor. Key Points: Lagrangian methods are critical to describing the three‐dimensional structure of the Atlantic Meridional Overturning CirculationFluid parcel trajectories reveal the lack of meridional connectivity in both the shallow and deep limbs of the overturning circulationHigh spatial‐temporal resolution trajectories identify regions where coherent eddies contribute to the overturning circulation [ABSTRACT FROM AUTHOR]

During the Lagrangian submesoscale experiment (LASER), 1,000 drifters were launched to sample the surface ocean flow in the northern Gulf of Mexico. Due to half a dozen strong winter storms, about 40% of the drifters lost their drogue. This unintended situation facilitated documentation of both near‐surface (5 cm) and deeper (60 cm) flows. These depths are relevant to transport of oil spills, as well as marine debris, such as microplastics, a rapidly growing environmental problem. Here, we improve the surface Lagrangian current prediction by combining a state‐of‐the‐art ocean forecast model with wind and wave data. The ocean surface velocities are obtained from the Navy Coordinate Ocean Model at 1‐km horizontal resolution, while the wind and wave fields are from the Unified Wave INterface Coupled Model coupled atmosphere‐wave‐ocean model. Two Lagrangian parameterizations are tested: one is based on Ekman dynamics, and the other directly on the surface winds. LASER data set is then used to assess the performance of these formulations, as a function of wind/wave conditions, as well as geographic region. It is found that incorporation of wind and wave data into the ocean circulation model can lead to major prediction improvement, by reducing the average 2‐day separation from the modeled and real LASER trajectories by a factor ranging from 1.4 to 4.9. This is a significant improvement for applications, where a rapid deployment of assets is needed, such as oil spill response, or other tracking problems. Key Points: The surface wind contributes significantly to the upper 60 cm transport in the Gulf of MexicoThe transport in the upper 5 cm differs from the upper 60 cm, due to the wave contribution [ABSTRACT FROM AUTHOR]

Porpoise click detectors (C-PODs) and icListenHF hydrophones that record with a high sample rate were deployed on a drifter in fast tidal currents in Minas Passage, Bay of Fundy, on six days during June 2017. Harbour porpoise vocalizations were detected within the icListenHF records by using the Coda algorithm and subsequent processing which included a detailed manual review of each click in each click train. The proportion of harbour porpoise detection positive minutes (DPM) was only 0.04 C-PODs compared to 0.19 for the icListenHF hydrophones. Nevertheless, both methods are an incomplete measure of porpoise vocalizations. DPM obtained by the C-PODs had a 65% likelihood of also being classified a DPM from the analysis of icListenHF measurements. Both methods measure some fraction of porpoise vocalization activity so long-term environmental monitoring with either C-PODs or icListenHF hydrophones should serve to discover any substantial change in patterns of porpoise echolocation activity. Nearby active acoustic devices were associated with 38% of the false-positive DPM obtained by C-PODs but Coda did not register such signals as harbour porpoise vocalizations. It is convenient to deploy C-PODs on bottom-moored subsurface-floats in order to monitor porpoise in fast currents at sites where in-stream tidal turbines are tested. However, C-POD performance degrades when non-target noise quickly fills the memory buffer, resulting in lost detection time. The drifter measurements demonstrated increases in memory loss as current speed increases above 1.5 m/s. Additional lost time might be caused when a C-POD is moored using an unstable tethered subsurface float. The type of mooring may matter because an icListenHF hydrophone attached to a stable bottom platform gave very similar DPM results to those on the drifter. [ABSTRACT FROM AUTHOR]

Wind power on oceanic near‐inertial oscillations is thought to play an important role in furnishing the diapycnal mixing in the ocean. Yet their global distribution and magnitude have not been quantified based on observations. In this study, we use hourly ocean current records derived from surface drifters to compute the global near‐inertial wind power during 1993–2016, with a combination of surface wind measurements obtained from satellites. The climatological near‐inertial wind power integrated between 60°S and 60°N is estimated to be 0.3–0.6 TW. The strongest energy flux occurs in the 30–60° latitude band during the winter season as a result of energetic storm activities. Ocean current imprint on wind stress has a significant impact on the estimated global near‐inertial wind power from drifters. Neglecting this imprint overestimates the near‐inertial wind power locally by 25–120% and its global mean value by 60%. Plain Language Summary: Wind stress fluctuations in the near‐inertial band can resonantly force near‐inertial oscillations (NIOs) in the surface mixed layer. These wind‐generated NIOs radiate downwards into the ocean interior, transfer their energy to small scales, and eventually break, providing energy to furnish the diapycnal turbulent mixing in the thermocline and deep ocean. In the past two decades, the global near‐inertial wind power has been extensively studied based on numerical models. However, the estimated values differ significantly (0.3–1.5 TW) among studies. Such a great range throws doubts on the validity of the model simulated near‐inertial wind power. It also causes uncertainties in the contribution of NIOs to diapycnal turbulent mixing and their parametrizations in ocean general circulation models. Therefore, it is essential to obtain a benchmark for global near‐inertial wind power based on observations. In this study, we evaluate the global near‐inertial wind power based on drifters and satellite‐measured winds. The climatological near‐inertial wind power integrated between 60°S and 60°N is estimated to be 0.3–0.6 TW. It is found that the ocean current imprint on wind stress has a significant impact on the estimated near‐inertial wind power. Neglecting this imprint leads to overestimation of near‐inertial wind power globally by 60% and locally by 25–120%. Key Points: The climatological near‐inertial wind power integrated between 60 degrees south and 60 degrees north is estimated to be 0.3‐0.6 TWNeglecting ocean current imprint on wind stress overestimates the near‐inertial wind power [ABSTRACT FROM AUTHOR]

Abstract We have assessed the use of satellite derived currents to calculate oceanic surface drift, as an alternative to using currents from ocean models. Predicted trajectories are compared to observed trajectories of two types of drifting buoys, which are subject to different degree of wind and wave forcing, in addition to the ocean surface current. This variable degree of ocean-, wind- and wave forcing is highly relevant for the practical problem of predicting the drift of e.g. oil to aid cleanup operations, and the drift of floating objects to aid search and rescue operations. The calculated trajectories are evaluated in terms of separation distance, and a calculated skill score. For the submerged drifters (CODE/DAVIS), we find that a high resolution non-assimilated ocean model gives the best results, whereas a coarser scale assimilated model performs similarly to satellite derived surface currents from the GlobCurrent project (geostrophic + Ekman components). For the wind-exposed drifters (iSphere), the wind-drift contribution is found to be dominating, and there is only a small impact of using any of the available information about ocean currents. The wind drift contribution to the iSphere drift is estimated to be 3% of the wind if Stokes drift is included in the calculations, and 4% if the Stokes drift is not included. Including Stokes drift in the calculations gives improvement for the submerged drifters, but shows no improvement for the wind exposed drifters. Highlights • Satellite derived currents are found to be useful for ocean trajectory forecasting. • Ocean currents represent the largest uncertainty in ocean trajectory forecasting. • Ocean surface drift may be fairly well predicted from wind only. [ABSTRACT FROM AUTHOR]

Drifters deployed in close proximity collectively provide a unique observational data set with which to separate mesoscale and submesoscale flows. In this paper we provide a principled approach for doing so by fitting observed velocities to a local Taylor expansion of the velocity flow field. We demonstrate how to estimate mesoscale and submesoscale quantities that evolve slowly over time, as well as their associated statistical uncertainty. We show that in practice the mesoscale component of our model can explain much first and second-moment variability in drifter velocities, especially at low frequencies. This results in much lower and more meaningful measures of submesoscale diffusivity, which would otherwise be contaminated by unresolved mesoscale flow. We quantify these effects theoretically via computing Lagrangian frequency spectra, and demonstrate the usefulness of our methodology through simulations as well as with real observations from the LatMix deployment of drifters. The outcome of this method is a full Lagrangian decomposition of each drifter trajectory into three components that represent the background, mesoscale, and submesoscale flow.

The Northern Thermaikos Gulf is a semi-enclosed coastal region of the Aegean Sea, characterized by anthropogenic and natural stresses such as intense industrial and agricultural activities, urban outflows, and several river discharges facing severe pollution events. The main motivation of this study is to investigate the prevailing oceanographic conditions of Northern Thermaikos Gulf, which are associated with the quality of the semi-enclosed basin's water masses. The hydrography and the hydrodynamic circulation patterns are revisited, based on the findings of a multi-platform observational study, conducted during a recent annual cycle, from June 2021 until May 2022. The observational findings are supported by a three-dimensional high-resolution hydrodynamic model (Deltf3D-Thermaikos) with updated freshwater input from all important land sources. The physical connectivity between the sub-basins of Thermaikos, the renewal of its northern coastal areas and the seawater quality are strongly related to the variability of wind-induced circulation. Northerly and southerly winds affect the spreading of the nutrient-rich riverine waters, discharged at the west coast of the gulf. The prevailing northerly winds contribute to the southward removal of the polluted sea surface and riverine waters, enhancing the cyclonic circulation around the gulf, allowing the inflow of clearer Aegean Sea Waters along the eastern coasts of the gulf. Northerlies also promote a connectivity pathway between the environmentally stressed Thessaloniki Bay (urban seafront) and the commonly less polluted southeastern coasts of the central basin. Southerlies mainly confine riverine waters in the western and northern coastal regions, weaken the renewal ability of the enclosed basins, and impose the formation of anticyclonic, mesoscale, circulation eddies in the outer and central areas. [Display omitted] • Integrated observational-modeling study to investigate the hydrography of Thermaikos Gulf. • Wind-induced circulation controls physical the physical connectivity, the renewal and the seawater quality of the Gulf. • Intensification of northerlies enhance the renewal of the inner parts. • Southerly winds mainly confine riverine waters in the western and northern coastal regions. [ABSTRACT FROM AUTHOR]

In this work we present the exchange flows between a semi-enclosed Bay and the adjacent ocean as measured by a real-time coastal observing system in Baja California, Mexico. Several sources of information as numerical modeling, currents profile data, high frequency radar observations and drifters tracking, all together show an interesting three-dimensional structure to the exchange flows. The northern entrance (relatively shallow, ∼40 m at its deepest portion) showed an estuarine-like exchange pattern, with a net outflow at the shallow parts and inflow at the deepest, suggesting that the frictional effect is more important than the earth's rotation, as typically observed in relatively small bodies of water like estuaries and coastal lagoons. On the other hand, the southern entrance (with a ∼400 m deep canyon) is more complex, with recurrent anticyclonic exchange pattern with inflows at the western limit and outflows at the east. We suggest that the vertical structure of this circulation may be sustained by the deep canyon which lowers the thermocline, but further investigation is needed. [ABSTRACT FROM AUTHOR]

Abstract We identify a newly described current, the Atlantic Inflow Current (AIC), a persistent pathway of Atlantic Water from the European Slope onto the Malin Shelf. Using drifters and gliders we examine the vertical and horizontal structure of the AIC and use Lagrangian statistics to quantify lateral mixing along its path. We estimate this current to have a transport of approximately 0.2 Sv, advecting 2.8 TW of heat onto the shelf (referenced to 7 °C) and 2.4 kT/day of the limiting nutrient, nitrate. This nutrient-rich AIC joins the Irish Coastal Current, continuing into the Minch and the outer Hebridean Shelf before ultimately entering the North Sea. The biological consequences of the influx of water masses and nutrients onto the shelf can range from altering primary production, and subsequent food web dynamics, to recruitment of fish larvae from oceanic water. A better understanding of the current dynamics described here is crucial for the assessment of shelf sea primary production and its impacts on carbon draw-down as well as recruitment for commercially important fisheries. Highlights • Slope water has been tracked on the European Shelf using drifters and gliders. • The deflection onto the shelf is not captured in models. • The slope water has a higher nitrate concentration that the shelf water, and supplies nutrients to the shelf. [ABSTRACT FROM AUTHOR]

Abstract The present work aims to detect Lagrangian transport barriers in the Gulf of Trieste by means of Lyapunov-exponent approach and tensorlines of the Cauchy-Green tensor. Lagrangian Coherent Structures (LCSs) are calculated employing 2D surface velocity fields measured by the coastal radars of the TOSCA EU research project (Tracking Oil Spills & Coastal Awareness Network). Moreover, surface drifters were deployed during the project. Comparisons between Eulerian velocity of HF-radar fields and Lagrangian velocity of drifters are carried out alongside single-particle tracking reliability. In particular, the possible influence of the data gaps in the HF-radar fields have been carefully considered. LCSs have proven to be robust against the quality of the starting HF-radar fields, leading to helpful insights in drifter positions. Indeed, after 24-hour integration the observed position of the drifter is approximately 1.5 km far from the nearest LCS, while a standard approach based on single-particle computations leads to larger errors (up to 5–7 km). However, such result must be properly interpreted taking into account the elongated nature of LCSs. A comparison between two common diagnostic tools of Lagrangian barriers is performed: Finite-Time and Finite-Size Lyapunov Exponent fields are compared in order to assess whether the patterns detected by the two measures are comparable. Finally, a joint analysis between LCSs and single-particle tracking is carried out and the results suggest that it would be desirable to couple these two approaches in real applications. Highlights • A comparison between Finite-Time and Finite-Size Lyapunov Exponents is carried out. • Lagrangian Coherent Structures are detected in a Mediterranean coastal area: the Gulf of Trieste. • Lagrangian Coherent Structures and observed/simulated drifter trajectories are compared. • A joint search approach between Lagrangian Coherent Structures and single particle tracking is proposed. [ABSTRACT FROM AUTHOR]

This study uses drifter‐based observations to investigate the role of wind and waves on spreading and mixing in the Fraser River plume. Local winter wind patterns commonly result in two distinct forcing conditions, moderate winds from the southeast (SE) and strong winds from the northwest (NW). We examine how these patterns influence the spreading and mixing dynamics of the plume. Under SE winds, the plume thins, spreads, and turns to the right (north) upon exiting the river mouth. Mixing is initially intense in the region of maximum spreading, but it is short‐lived. Under NW winds, which oppose the rightward tendency of the plume, the plume remains thicker, narrower, and flows directly across the Strait with a lateral front on its northern side. Mixing is initially lower than under SE forcing but persists further across the Strait. A Lagrangian stream‐normal momentum balance shows that wind and interfacial stress under NW conditions compress the sea surface height anomaly formed by the river discharge and guide the flow across the Strait. This reconfiguration changes spreading and mixing dynamics of the plume; plume spreading, which drives intense mixing under SE winds, is shut down under NW winds, and mixing rates are consequently much lower. Despite the initially lower mixing rates, the region of active mixing extends further under NW winds, resulting in higher net mixing. These results highlight that the wind, which is often a primary cause of increased plume mixing, can also significantly influence mixing by changing the geometry of the plume. Plain Language Summary: Rivers transport sediment, pollutants, and nutrients from inland regions to coastal seas. Where rivers meet the ocean, freshwater flows over the ambient salty seawater, forming a river plume. The quantities that the river transports into the ocean are mixed into the seawater along with the freshwater, and so it is vital to understand this mixing process. While ocean surface waves might be the most striking visual feature of the coastal ocean, at the Fraser River mouth, south of Vancouver, Canada, we find that wind is a much more important influence on river plume mixing than waves. Wind can influence mixing by changing the geometry of the plume to either favor or discourage intense mixing occurring in the system. This is a result of the wind encouraging or discouraging plume spreading, which is the primary cause of mixing close to the river mouth. Ocean surface waves, despite being a visually striking feature of this system, do not play a large role in mixing the Fraser River plume. Thus, in order to correctly predict river plume mixing, we must take into account wind conditions near the river mouth, while waves are less important. Key Points: Local wind and ambient current forcing combine to alter the plume momentum balance, resulting in less spreading and mixing under NW windsThis plume reconfiguration results in larger net mixing under NW winds, when low local mixing rates act over a larger mixing regionThe plume is largely unaffected by the estimated wave‐driven effects on its dynamics and mixing [ABSTRACT FROM AUTHOR]

Velocities of surface drifters are analyzed to study tidal currents throughout the Mediterranean Sea, with main focus on the semi-diurnal tide. Harmonic analysis on 15-day long trajectory segments reveals semi-diurnal tidal amplitudes larger than 5 cm/s in the Alboran Sea, the northern Adriatic Sea, the Sicily Channel and the Gulf of Gabès. Elsewhere the tidal currents are weak (< 2 cm/s) except for a few isolated locations where the semi-diurnal currents are significant, such as in the eastern Aegean Sea. S2 is similar to M2, but the amplitudes are generally smaller. In general, the drifter-inferred results confirm previous (mostly coastal) observations and numerical simulations of the Mediterranean tidal currents. In addition, they provide novel information on the tidal currents in the open sea. It was found that substantial K1 signals prevail in the Alboran Sea and some regions of the Sicily Channel and the Adriatic Sea. However, the results for the diurnal constituents should be interpreted with caution due to the possible contamination by leakage of near-inertial energy in the tidal bands and by currents forced by the sea breeze. [ABSTRACT FROM AUTHOR]

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

Numerical drifters are tracked in a hydrodynamic simulation of circulation over the Texas-Louisiana shelf to analyze patterns in cross-shelf transport of materials. While the important forcing mechanisms in the region (wind, river, and deep eddies) and associated flow patterns are known, the resultant material transport is less well understood. The primary metric used in the calculations is the percent of drifters released within a region that cross the 100 m isobath. Results of the analysis indicate that, averaged over the eleven years of the simulation, there are two regions on the shelf – over the Texas shelf during winter, and over the Louisiana shelf in summer – with increased seasonal probability for offshore transport. Among the two other distinct regions, the big bend region in Texas has increased probability for onshore transport, and the Mississippi Delta region has an increase in offshore transport, for both seasons. Some of these regions of offshore transport have marked interannual variability. This interannual variability is correlated to interannual changes in forcing conditions. Winter transport off of the Texas shelf is correlated with winter mean wind direction, with more northerly winds enhancing offshore transport; summer transport off the Louisiana shelf is correlated with Mississippi River discharge. [ABSTRACT FROM AUTHOR]

Abstract: Position and velocity data are analyzed from a release of surface ocean drifters in the Arctic Ocean's Beaufort Sea in ice‐free conditions. Position information is returned at sufficiently high frequency to allow for the investigation of surface‐ocean flows ranging from around 0.5 km in lateral scale (submesoscale, SM, flows) to flows that are tens of kilometers in horizontal extent. Lagrangian statistics from the drifter release are analyzed in conjunction with Eulerian (ship‐based) measurements of surface ocean temperature and salinity. Results show dynamics that are largely consistent with flows at similar scales in the midlatitude oceans. Horizontal wavenumber k spectra of density in the surface ocean scale as k − 2, consistent with energetic SM flows. Lagrangian drifters indicate local dispersion in the surface ocean layer at horizontal scales smaller than 10 km, which confirms the presence of active submesoscale dynamics. Features at these scales give rise to lateral diffusivities (in the range 1–103 m2 s−1) of similar range to values inferred in the midlatitudes. Velocity structure functions present an energy‐cascade inertial range at submesoscales with indication of a transition to a forward energy cascade at scales smaller than 1 km confirming the transition to 3‐D turbulence. The active SM flow‐field drives enhanced lateral and vertical fluxes in the Arctic Ocean mixed layer, which has first‐order implications to the transport of heat, sea‐ice floes, nutrients, and contaminants. [ABSTRACT FROM AUTHOR]

This paper presents an acoustic archival tag capable of tracking small marine animals. It is also a technology that can be ported to other platforms, such as the next-generation acoustic and Argo floats as well as gliders. Tracking is achieved by standard RAFOS triangulation using the arrival times of unique sound signals emitted by moored sources. At the core of the tag is a custom microchip that controls all system operations. It incorporates the critical acoustic arrival time detector, a thermal sensor, and a pressure sensor interface. All the electronic components are housed inside a cylindrical hydrophone of 25.4-mm length and 10.7-mm diameter. The collected data are archived in nonvolatile memory chips with a total capacity of 4 Mb, sufficient storage to record position, temperature, and pressure on an hourly basis for 2 years. The tag consumes 4-5 μW in standby mode and between 60 and 90 μWwhile the sound arrival time detector is in operation. The power is provided by two button cell silver-oxide batteries, which enable an active tag lifetime of approximately 2 years. [ABSTRACT FROM AUTHOR]

The surface circulation and the thermohaline properties of the water masses of the eastern Levantine Sea (Mediterranean Sea) were monitored with mobile autonomous systems (surface drifters and gliders) during the period September 2016−August 2017. The drifters provided data for more than a year and revealed complex circulation features at scales ranging from the basin scale to the sub-mesoscale. Three drifters were captured in a semi-permanent gyre (Cyprus Eddy) allowing a quantitative study of its kinematics. During the experiment, three gliders were operated, in two different periods: September to December 2016 and February to March 2017. The autonomous instruments crossed the prevailing sub-basin structures several times. The collected in-situ observations were analyzed and interpreted in concert with remote sensing products (sea surface temperature and altimetry). The evolution of some of the prevailing features confirmed the complexity of the circulation of the basin. The Cyprus Eddy is the most persistent anticyclone, moving its geographical position and sometimes merging with the North Shikmona Eddy in a bigger structure. The gliders sampled this wide anticyclonic feature revealing its vertical structure in the two different periods. In fall, in stratified conditions, a high salinity core is evident below the thermocline. The isopycnals are characterized by an upward bending over the high salinity lens and a downward bending below it, typical of an anticyclonic modewater eddy. In winter, the core disappears following the vertical mixing that, homogenizes the upper Cyprus Eddy water down to 300 m.

Nowadays, drifters are used for a wide range of applications for researching and exploring the sea. However, the power constraint makes it difficult for their sampling intervals to be smaller, meaning that drifters cannot transmit more accurate measurement data to satellites. Furthermore, due to the power constraint, a modern Surface Velocity Program (SVP) drifter lives an average of 400 days before ceasing transmission. To overcome the power constraint of SVP drifters, this article proposes an adaptively counter-rotating wave energy converter (ACWEC) to supply power for drifters. The ACWEC has the advantages of convenient modular integration, simple conversion process, and minimal affection by the crucial sea environment. This article details the design concept and working principle, and the interaction between the wave energy converter (WEC) and wave is presented based on plane wave theory. To verify the feasibility of the WEC, the research team carried out a series of experiments in a wave tank with regular and irregular waves. Through experiments, it was found that the power and efficiency of the ACWEC are greatly influenced by parameters such as wave height and wave frequency. The maximum output power of the small scale WEC in a wave tank is 6.36 W, which allows drifters to detect ocean data more frequently and continuously.

Whereas numerous numerical modelling have been conducted to gain further insights about the hydrodynamic characteristics of tidal stream energy sites, the long-term transport of water particles has been routinely ignored. However, a refined evaluation of circulation pathways is fundamental to assess water-quality changes induced by tidal energy converters. The present study investigates the tide-induced Lagrangian circulation in north-western coastal waters of Brittany (France, western Europe), including numerous islands, shoals and straits with strong potential for turbine farm implementation. Particular attention is dedicated to the hydrodynamic impact of tidal stream power extraction from a series of horizontal-axis turbines. An original approach, based on the coupling of the Lagrangian barycentric method with predictions from a model at high spatial resolution, is considered to assess circulation pathways in a single synoptic view. Spatial results reveal a complex Lagrangian circulation highly controlled by bathymetry and islands geometry. The residual flow in the Fromveur Strait, separating Ushant island from Molène archipelago, displays thus a strong asymmetry, between a prominent north-eastern pathway with residual currents up to 0.45 m s - 1 and a southward circulation. The positions of cyclonic and anti-cyclonic recirculations, both upstream and downstream the Strait, are furthermore precisely defined. The north eddy is found closely located to the sand bank of the Four. The extraction of tidal stream power impacts mainly the residual Lagrangian circulation along the current stream emerging from the Strait, with a tendency for surrounding eddies to get closer to the tidal stream energy site. The displacements of the south and north recirculations may thus exceed 1.5 km with possible implications on the evolution of surrounding seabed features. [ABSTRACT FROM AUTHOR]

A South Atlantic ring is studied through remote sensing altimetry, hydrographic stations, and drifters' trajectories. The ring's core was characterized by warmer and saltier Indian Ocean waters. At the time of the cruise, the ring's signature extended radially out to 124 km and vertically down to 2000 m, and its core absolute dynamic topography (ADT) exceeded the surrounding Atlantic Ocean waters in 0.4 m. The geostrophic velocities were anticyclonic with maximum speeds about 35 cm s−1 at 100 m and reaching negligible values near 4500 m. The rotational transport inside the ring was 33 Sv in the thermocline and intermediate layers. The drifters' data distinguish a 30-km core revolving as a solid body with periodicity near 5 days and a transitional band that revolves with constant tangential velocity, resembling a Rankine vortex. The ADT data identify the ring's track, showing that it was shed by the Agulhas Current retroflection in November 2009 and propagated northwest rapidly during the first 2 months (mean speed of about 10 cm s−1) but slowed down substantially (3-4 cm s−1) between March and July 2010, when it was last detected. The altimetry data also outlines the evolution of the ring's core ADT, radius, vorticity, and, through a simple calibration with the cruise data, rotational transport. In particular, the ring surface and vertical-mean vorticity decay with time scales of 373 and 230 days, respectively, indicating that most of the property anomalies contained by the ring are diffused out to the subtropical gyre before it reaches the western boundary current system. [ABSTRACT FROM AUTHOR]

Increased spatial and temporal resolution of recent observations and modeling have pointed out the importance of small scale structures (in the range of 1–50 km) for the mixing processes in the ocean. Based on high-frequency drifter measurements, we show here that the near-inertial currents (NICs) can contribute significantly to the surface kinetic energy in the Brazil Current (BC) region and, therefore, should be properly taken into account in the studies of transport and mixing processes. To characterize these submesoscale features, we examine the current response to the wind forcing in the Brazilian ocean margin between 24°S and 36°S using 3-hourly sampled trajectories of satellite-tracked drifters. Our results indicate a preference for anti-cyclonic circular motions, with a rotating period close to the local inertial period, consistent with near-inertial motions in the Southern Hemisphere (SH). Wind stress time series, from three months of wind measurements, along with synoptic weather charts, are used to relate the observed NICs to the atmospheric forcing. During SH spring, NICs occur in 4.7−15 day bursts and account for 15–45% of the total surface current variance. This intermittency is related to atmospheric cold frontal passages, low pressure systems, and sea breeze/land breeze circulations. The predominance of NICs south of 28°S appears to be related to the increased Effective Inertial Frequency (EIF), which is the inertial frequency changed by the sub-inertial background flow. [ABSTRACT FROM AUTHOR]

Near-real time sea surface current information is needed for ocean operations. On a global scale, only satellites can provide such measurements. This can be done with data from infrared radiometers, available on several satellites, thus giving several images a day. This work analyses the accuracy of such an estimation of surface current fields retrieved with the maximum cross correlation (MCC) method, here used to track patterns of Advanced Very High Resolution Radiometer (AVHRR) brightness temperature between 224 pairs of consecutive images taken between January and December 2015 in the western Mediterranean Sea. Comparison with in-situ drifters shows that relatively small patterns, moving at a slow speed, tracked between images separated by less than four hours give the best agreement. The agreement was strongest in summer, and consistent with low wind, non-eddying situations. When compared to a daily reanalysis field, the averaged satellite-retrieved fields showed good agreement, but not the in-situ drifter data. Drifter data should hence be used to complement satellite-retrieved currents rather than to validate them, since they may measure different components of the surface currents. [ABSTRACT FROM AUTHOR]

Numerical models of ocean circulation often depend on parameters that must be tuned to match either results from laboratory experiments or field observations. This study demonstrates that an initial, suboptimal estimate of a parameter in a model of a small bay can be improved by assimilating observations of trajectories of passive drifters. The parameter of interest is the Manning’s n coefficient of friction in a small inlet of the bay, which had been tuned to match velocity observations from 2011. In 2013, the geometry of the inlet had changed, and the friction parameter was no longer optimal. Results from synthetic experiments demonstrate that assimilation of drifter trajectories improves the estimate of n , both when the drifters are located in the same region as the parameter of interest and when the drifters are located in a different region of the bay. Real drifter trajectories from field experiments in 2013 also are assimilated, and results are compared with velocity observations. When the real drifters are located away from the region of interest, the results depend on the time interval (with respect to the full available trajectories) over which assimilation is performed. When the drifters are in the same region as the parameter of interest, the value of n estimated with assimilation yields improved estimates of velocity throughout the bay. [ABSTRACT FROM AUTHOR]

Discussion about the Great War at sea tends to centre on capital ships and Jutland. Far less has been written about the crucial role played by fishermen, fishing vessels and their coastal communities. Yet armed fishing vessels and their fishermen crews were continually on the maritime front line. They formed the backbone of the Auxiliary Patrol, working principally on anti-U-boat patrols or minesweeping. Around 3000 fishing vessels were requisitioned whilst more than 39,000 fishermen joined the Trawler Section of the Royal Naval Reserve. The cultural, social and attitudinal gulf between working fishermen and many RN officers was enormous. Those vessels left fishing also played a vital role in maintaining the food supplies; many were sunk whilst working. Around 672 fishing vessels were lost through enemy action whilst either fishing or on Admiralty service. After the war, their substantial role was gradually forgotten, now hardly recognised. This paper examines their contribution. [ABSTRACT FROM PUBLISHER]

Five men in a paddle boat were lost in the northern Marmara Sea, Turkey, on 17 August 2014. One of the bodies and some parts of the paddle boat were recovered 100 km south of the incident after seven days. The other four bodies were never recovered. An ocean circulation and a Lagrangian drift model were used together to predict the possible recovery site of the four bodies after they were lost in the Marmara Sea. Based on the circulation model output and atmospheric conditions at the time of the incident, it is concluded that there are two possible fates for the missing bodies. They either travelled west and reached the western coast of the Marmara Sea or, most probably, they drifted further to the south and left the Marmara Sea by passing through the Dardanelles Strait. [ABSTRACT FROM PUBLISHER]

As rampant modernization profoundly reshapes the economy, culture and ideologies of post-reform China, Han Chinese have encountered directly the malaises of modernity, and are gradually losing faith in modernity’s promises of reason and progress. Numerous small towns and cities, consequently, have become anchors of alternative identities and lifestyles. This article examines the group of ‘drifters in Lhasa’, namely Han Chinese who have migrated to and settled in Lhasa, Tibet, in pursuit of slow-paced lifestyles, communal belonging and assumedly more authentic social relations. Drawing literature on the critiques of modernity and modern subject’s invocation of othered places and peoples as a means of self-criticism, this article argues that the constructed Tibetanness offers the drifters critical cultural resources to counteract ordering endeavours of Chinese modernity. At the same time, however, the modern subject in search of otherness tends to reproduce the modern modality of cognition which results in the objectivation of the other. The article investigates the effects of objectivation from two perspectives, namely the rendering-silent of the others’ voices and interests; and re-inscription of boundaries of difference between the self and the other, against broader contexts of socio-economic transformations in Tibet. [ABSTRACT FROM PUBLISHER]

Validation of oil spill forecasting systems suffers from a lack of data due to the scarcity of oil slick in situ and satellite observations. Drifters (surface drifting buoys) are often considered as proxy for oil spill to overcome this problem. However, they can have different designs and consequently behave in a different way at sea, making it not straightforward to use them for oil spill model validation purposes and to account for surface currents, waves and wind when modelling them. Stemming from the need to validate the MEDESS4MS (Mediterranean Decision Support System for Marine Safety) multi-model oil spill prediction system, which allows access to several ocean, wave and meteorological operational model forecasts, an exercise at sea was carried out to collect a consistent dataset of oil slick satellite observations, in situ data and trajectories of different type of drifters. The exercise, called MEDESS4MS Serious Game 1 (SG1), took place in the Elba Island region (Western Mediterranean Sea) during May 2014. Satellite images covering the MEDESS4MS SG1 exercise area were acquired every day and, in the case an oil spill was observed from satellite, vessels of the Italian Coast Guard (ITCG) were sent in situ to confirm the presence of the pollution. During the exercise one oil slick was found in situ and drifters, with different water-following characteristics, were effectively deployed into the oil slick and then monitored in the following days. Although it was not possible to compare the oil slick and drifter trajectories due to a lack of satellite observations of the same oil slick in the following days, the oil slick observations in situ and drifters trajectories were used to evaluate the quality of MEDESS4MS multi-model currents, waves and winds by using the MEDSLIK-II oil spill model. The response of the drifters to surface ocean currents, different Stokes drift parameterizations and wind drag has been examined. We found that the surface ocean currents mainly drive the transport of completely submerged drifters. The accuracy of the simulations increases with higher resolution currents and with addition of the Stokes drift, which is better estimated when provided by wave models. The wind drag improves the modelling of drifter trajectories only in the case of partially emerged drifters, otherwise it leads to an incorrect reproduction of the drifters׳ direction, which is particularly evident in high speed wind conditions. [ABSTRACT FROM AUTHOR]

The strong wintertime westward current in the northwestern Pacific between 18°N and 23°N was investigated using data from satellite-tracked drifters. The decadal strengthening of the negative wind stress curl and trade wind in the subtropical area north of Hawaii and west of 150°E drives the decadal variability of this wintertime current. The Sverdrup transport computed from a high-resolution (1° × 1°) wind stress reanalysis product reproduces the Wintertime Subtropical Current (WSTC) well, and shows that its Sverdrup transport increases in excess of 15 Sv (1 Sv = 106 m3 s–1) from the early 1970s to the 1990s. The annual variation of the WSTC transport is well correlated with the North Pacific Index, which represents the strength of the Aleutian Low before the 1988/89 climate shift, but the correlation with teleconnection patterns or climate indices becomes weak after the 1988/89 shift. The annual fluctuations of WSTC transport are found to be reflected in the springtime Kuroshio transport south of Japan.

Circulation within a body of water controls not only fluid transport, but just as importantly, chemical, biological, and sedimentological constituents. Transport and dispersal of these constituents is of major concern when it comes to the proper management of rivers, bays, lakes, and nearshore communities, however, most students fail to realize the complexities related to the forcing of this flow field or its variability over time. Through the use of very basic tools, students having access to a small research vessel would be able to map surface and deeper circulation of a small region within a given water body (for ease, “lake” will be used from here on even though it implies any region of interest). Meteorological data will be used to look at the effects of wind forcing while bathymetric information can provide aspects of topographic control. The acquisition of these data during repeated cruises will lead to a better understanding of mean circulation and its variability.

Hydrodynamic dispersion has a significant impact on the mass transport of sediments and contaminants within coastal waters. In this study apparent horizontal dispersion in a tidally-dominated shallow estuary was investigated using field observations and a numerical model. A cluster of four Lagrangian drifters was released in two shallow regions inside Moreton Bay, Australia: between two small islands and in an open water area. During a 16-h tracking period, the drifters generally showed similar behaviour, initially moving with the dominant current and remaining together before spreading apart at the change of tide. Two dispersion regimes were identified, a slow dispersion during the earlier stage and a rapid dispersion during the latter stage of deployment. Such change in regime typically occurred during the succeeding ebb or flow tides, which may be attributable to residual eddies breaking down during reversal of tidal direction. In addition, a power function of the squared separation distance over the apparent dispersion coefficient produced an R 2 exceeding 0.7, indicating a significant relationship between them. By applying a three-dimensional hydrodynamic model, the trajectories of artificial particles spreading in the bay were simulated, which allowed the calculation of dispersion coefficients throughout the entire bay. The study results demonstrate that the tidal effects on dispersion were dependent on the effect of tidal excursion and residual current. The tide was found to be the most dominant driver of dispersion in the bay when unobstructed by land; however, bathymetric and shoreline characteristics were also significant localised drivers of dispersion between the two islands as a result of island wake. [ABSTRACT FROM AUTHOR]

The gains from implementing high-resolution versus less costly low-resolution models to describe coastal circulation are not always clear, often lacking statistical evaluation. Here we construct a hierarchy of ocean–atmosphere models operating at multiple scales within a 1 × 1° domain of the Belizean Barrier Reef (BBR). The various components of the atmosphere–ocean models are evaluated with in situ observations of surface drifters, wind and sea surface temperature. First, we compare the dispersion and velocity of 55 surface drifters released in the field in summer 2013 to the dispersion and velocity of simulated drifters under alternative model configurations. Increasing the resolution of the ocean model (from 1/12° to 1/100°, from 1 day to 1 h) and atmosphere model forcing (from 1/2° to 1/100°, from 6 h to 1 h), and incorporating tidal forcing incrementally reduces discrepancy between simulated and observed velocities and dispersion. Next, in trying to understand why the high-resolution models improve prediction, we find that resolving both the diurnal sea-breeze and semi-diurnal tides is key to improving the Lagrangian statistics and transport predictions along the BBR. Notably, the model with the highest ocean–atmosphere resolution and with tidal forcing generates a higher number of looping trajectories and sub-mesoscale coherent structures that are otherwise unresolved. Finally, simulations conducted with this model from June to August of 2013 show an intensification of the velocity fields throughout the summer and reveal a mesoscale anticyclonic circulation around Glovers Reef, and sub-mesoscale cyclonic eddies formed in the vicinity of Columbus Island. This study provides a general framework to assess the best surface transport prediction from alternative ocean–atmosphere models using metrics derived from high frequency drifters’ data and meteorological stations. [ABSTRACT FROM AUTHOR]

A massive drifter deployment in the Gulf of Naples (Southern Tyrrhenian Sea, Western Mediterranean Sea) over a ten-day multidisciplinary in situ experiment in Summer 2012 provided sea-truth data for validating the performance of a high-frequency (HF) radar network. The buoys were frequently retrieved and relaunched to ensure an optimal coverage of the domain. The total velocity of the drifters, together with the associated zonal and meridional components, was compared with the HF radar surface current estimates. Divergence between virtual and real drifter trajectories, and also the simulated movement of clouds of particles give useful insights for scenarios of search and rescue. All comparisons were performed, considering both ideal and antenna radiation pattern-corrected fields. The results of the investigation testify the high precision of HF radars and confirm the necessity of periodically verifying the antenna pattern to ensure the optimal functionality of these systems. [ABSTRACT FROM PUBLISHER]