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Ocean eddies affect large‐scale circulation and induce a kinetic energy cascade through their non‐linear interactions. However, since global observations of eddy dynamics come from satellite altimetry maps that smooth eddies and distort their geometry, the strength of this cascade is underestimated. Here, we use deep learning to improve observational estimates of global surface geostrophic currents and explore the implications for the cascade. By synthesizing multi‐modal satellite observations of sea surface height (SSH) and temperature, we achieve up to a 30% improvement in spatial resolution over the community‐standard SSH product. This reveals numerous strongly interacting eddies that were previously obscured by smoothing. In many regions, these newly resolved eddies lead to nearly an order‐of‐magnitude increase in the upscale kinetic energy cascade that peaks in spring and is strong enough to drive the seasonality of large mesoscale eddies. Our study suggests that deep learning can be a powerful paradigm for satellite oceanography. Plain Language Summary: We developed a deep learning method to estimate global maps of surface ocean currents from satellite observations with significantly improved resolution and accuracy compared to existing methods. These maps dramatically improve our ability to observe eddy dynamics and the impact of eddies on the transfer of energy between scales in the ocean. Our study suggests that deep learning can be a powerful paradigm for satellite oceanography. Key Points: We develop the first deep learning global estimates of surface ocean currents from multi‐modal satellite observationsOur deep learning method is able to map surface currents with state‐of‐the‐art resolution and accuracyThe diagnosed kinetic energy cascade is an order of magnitude higher compared to conventional altimetry products [ABSTRACT FROM AUTHOR]

The High-Frequency (HF) radar network has become an effective method for detecting coastal currents. In this study, we confirmed the effectiveness of the HF radar measurements by comparing with the Acoustic Doppler Current Profiler (ADCP) and explore the possibility of assimilating radar data into a regional coastal ocean model. A regional high-resolution model with resolution of 10 m was first built in the Pearl River Estuary (PRE). However, analysis of the Hovmöller diagrams from the model simulations in this study indicated a significant deficiency in representing Near-Inertial Oscillations (NIOs) in the PRE, particularly in the east–west direction, despite including wind fields in the input data, during the week from 3 to 8 August 2022. To overcome the model deficiency, we conducted a set of assimilation experiments and performed sensitivity analyses. The results of sensitivity experiments indicate that the model exhibits a sufficient capacity to replicate NIOs after assimilation, lasting approximately 5–6 days. To further analyze the reasons for the decay in the magnitude of the NIOs, data from the three ADCP stations were compared with model results by applying the momentum equation. The assimilated vertical diffusion term outperforms the unassimilated model in representing NIOs. These findings highlight the importance of the vertical diffusion term for simulating NIOs and the data assimilation in improving the model's representation of physical processes. [ABSTRACT FROM AUTHOR]

This study derives the climatology of surface currents in the Arabian Gulf using the current velocities obtained from the Copernicus Marine Service (CMEMS) for the period 1993–2019. It reveals distinct temporal and spatial variability in the surface current speeds induced by the variability in surface winds, bathymetry and the changes in the lateral gradients in density. The mean speed of the Iranian Coastal Current (ICC) during summer reaches up to 0.33 m·s−1 along the coast of Iran, while the mean speed of Arabian Coastal Current (ACC) reaches up to 0.26 m·s−1 along the coast of Saudi Arabia. We found the occurrence of 2 major and 1 minor cyclonic eddies in the annual, seasonal and monthly climatology, while these eddies are more prevalent during summer. The major cyclonic eddy in the central Gulf develops in May and persists till November with varying patterns, and decays in December. The climatological mean current speeds are higher during summer compared to winter, due to the seasonal changes in thickness of the surface layer by the stratification/destratification processes. The highest mean current speeds along the coast of Qatar are found in June and the lowest in winter months. The highest annual, monthly and seasonal mean current speeds are observed along the north and northeast coast of Qatar, while the lowest are observed along the west coast and southeast coast of Qatar. Interannual variability in surface current speeds is evident, with notable links with the El Niño–Southern Oscillations (ENSO) and Indian Ocean Dipole (IOD). The annual mean current speeds show positive trends, of the order of 0.06–0.14 cm·s−1·year−1 in the offshore regions and 0.05–0.24 cm·s−1·year−1 in the nearshore regions, wherein the highest positive trend is observed off Ras Laffan and the lowest off Dukhan. [ABSTRACT FROM AUTHOR]

Abstract Ocean eddies affect large‐scale circulation and induce a kinetic energy cascade through their non‐linear interactions. However, since global observations of eddy dynamics come from satellite altimetry maps that smooth eddies and distort their geometry, the strength of this cascade is underestimated. Here, we use deep learning to improve observational estimates of global surface geostrophic currents and explore the implications for the cascade. By synthesizing multi‐modal satellite observations of sea surface height (SSH) and temperature, we achieve up to a 30% improvement in spatial resolution over the community‐standard SSH product. This reveals numerous strongly interacting eddies that were previously obscured by smoothing. In many regions, these newly resolved eddies lead to nearly an order‐of‐magnitude increase in the upscale kinetic energy cascade that peaks in spring and is strong enough to drive the seasonality of large mesoscale eddies. Our study suggests that deep learning can be a powerful paradigm for satellite oceanography.

The ocean circulation is typically constrained in operational analysis and forecasting systems through the assimilation of sea level anomaly (SLA) retrievals from satellite altimetry. This approach has limited benefits in the Arctic Ocean and surrounding seas due to data gaps caused by sea ice coverage. Moreover, assimilation of SLA in seasonally ice-free regions may be negatively affected by the quality of the Mean Sea Surface (MSS) used to derive the SLA. Here, we use the Regional Ice Ocean Prediction System (RIOPS) to investigate the impact of assimilating Absolute Dynamic Topography (ADT) fields on the circulation in the Arctic Ocean. This approach avoids the use of a MSS and additionally provides information on sea level in ice covered regions using measurements across leads (openings) in the sea ice. RIOPS uses a coupled ice-ocean model on a 3-4 km grid-resolution pan-Arctic domain together with a multi-variate reduced-order Kalman Filter. The system assimilates satellite altimetry and sea surface temperature together with in situ profile observations. The background error is modified to match the spectral characteristics of the ADT fields, which contain less energy at small scales than traditional SLA due to filtering applied to reduce noise originating in the geoid product used. A series of four-year reanalyses demonstrate significant reductions in innovation statistics with important impacts across the Arctic Ocean. Results suggest that the assimilation of ADT can improve circulation and sea ice drift in the Arctic Ocean, and intensify volume transports through key Arctic gateways and resulting exchanges with the Atlantic Ocean. A reanalysis with a modified Mean Dynamic Topography (MDT) is able to reproduce many of the benefits of the ADT but does not capture the enhanced transports. Assimilation of SLA observations from leads in the sea ice appears to degrade several circulation features; however, these results may be sensitive to errors in MDT. This study highlights the large uncertainties that exist in present operational ocean forecasting systems for the Arctic Ocean due to the relative paucity and reduced quality of observations compared to icefree areas of the Global Ocean. Moreover, this underscores the need for dedicated and focused efforts to address this critical gap in the Global Ocean Observing System. [ABSTRACT FROM AUTHOR]

Real-time reconstruction of ocean surface currents is a challenge due to the complex, non-linear dynamics of the ocean, the small number of in situ measurements, and the spatio-temporal heterogeneity of satellite altimetry observations. To address this challenge, we introduce HIRES-CURRENTS-Net, an operational real-time convolutional neural network (CNN) model for daily ocean current reconstruction. This study focuses on the Mediterranean Sea, a region where operational models have great difficulty predicting surface currents. Notably, our model showcases higher accuracy compared to commonly used alternative methods. HIRES-CURRENTS-Net integrates high-resolution measurements from the infrared or visible spectrum—high resolution Sea Surface Temperature (SST) or chlorophyll (CHL) images—in addition to the low-resolution Sea Surface Height (SSH) maps derived from satellite altimeters. In the first stage, we apply a transfer learning method which uses a high-resolution numerical model to pre-train our CNN model on simulated SSH and SST data with synthetic clouds. The observation of System Simulation Experiments (OSSEs) offers us a sufficient training dataset with reference surface currents at very high resolution, and a model trained on this data can then be applied to real data. In the second stage, to enhance the real-time operational performance of our model over previous methods, we fine-tune the CNN model on real satellite data using a novel pseudo-labeling strategy. We validate HIRES-CURRENTS-Net on real data from drifters and demonstrate that our data-driven approach proves effective for real-time sea surface current reconstruction with potential operational applications such as ship routing. [ABSTRACT FROM AUTHOR]

The High-Frequency (HF) radar network has become an effective method for detecting coastal currents. In this study, we confirmed the effectiveness of the HF radar measurements by comparing with the Acoustic Doppler Current Profiler (ADCP) and explore the possibility of assimilating radar data into a regional coastal ocean model. A regional high-resolution model with resolution of 10 m was first built in the Pearl River Estuary (PRE). However, analysis of the Hovmöller diagrams from the model simulations in this study indicated a significant deficiency in representing Near-Inertial Oscillations (NIOs) in the PRE, particularly in the east–west direction, despite including wind fields in the input data, during the week from 3 to 8 August 2022. To overcome the model deficiency, we conducted a set of assimilation experiments and performed sensitivity analyses. The results of sensitivity experiments indicate that the model exhibits a sufficient capacity to replicate NIOs after assimilation, lasting approximately 5–6 days. To further analyze the reasons for the decay in the magnitude of the NIOs, data from the three ADCP stations were compared with model results by applying the momentum equation. The assimilated vertical diffusion term outperforms the unassimilated model in representing NIOs. These findings highlight the importance of the vertical diffusion term for simulating NIOs and the data assimilation in improving the model’s representation of physical processes.

The processes of ocean dynamics are complex near the Pearl River Estuary and are not clear due to a lack of abundant observations. The spatial characteristics of the spring sea surface currents in the adjacent waters of the Pearl River Estuary were analyzed using the current data observed by a three-station high-frequency surface wave radar system (HFSWRS). Compared with the two-station HFSWRS, the deviation of current velocity and direction observed by the three-station HFSWRS from the underway measurements decreased by 42.86% and 38.30%, respectively. The analyzed results show that the M2 tidal current is the dominant current among all the tidal constituents, followed by K1, with angles of inclination ranging from 130° to 150°. The tidal flow is dominated by northwest–southeast back-and-forth flow. In the southern part of the observed area, which is far from the coastline, the tidal current ellipses exhibit a circular pattern. The prevalent tidal current type in this region is irregularly semi-diurnal, and the shallow water constituents also have a significant effect. The tidal energy in the adjacent waters of the Pearl River Estuary is affected by potential energy flux and kinetic energy flux. As the water depth and currents velocity increase in the southeast direction, the tidal energy flux increases. In the nearshore zone, the direction of tidal energy flux varies along the coastline. The changes in the residual current within the observed area are correlated with the sea surface wind field. Based on the high-precision sea surface current observed by the three-station HFSWRS, the characteristics of the ocean dynamic processes near the Pearl River Estuary are analyzed comprehensively, which provides important reference and confidence for the application of the developing new radar observing network with about 10 radar stations near the Pearl River Estuary. [ABSTRACT FROM AUTHOR]

The performance of a high-frequency (HF) radar network situated within the Pearl River Estuary from 17 July to 13 August 2022 is described via a comparison with seven acoustic Doppler current profilers (ADCPs). The radar network consists of six OSMAR-S100 compact HF radars, with a transmitting frequency of 13–16 MHz and a direction-finding technique. Both the radial currents and vector velocities showed good agreement with the ADCP results (coefficient of determination r2: 0.42–0.78; RMS difference of radials: 11–21.6 c m   s − 1 ; bearing offset Δ θ : − 4.8 ° – 16.1 ° ; complex correlation coefficient γ : 0.62–0.96; and phase angle α : −24.3 ° – 17.8 ° ). For these radars, the Δ θ values are not constant but vary with azimuthal angles. The relative positions between the HF radar and ADCPs, as well as factors such as the presence of island terrain obstructing the signal, significantly influence the errors. The results of spectral analysis also demonstrate a high level of consistency and the capability of HF radar to capture diurnal and semidiurnal tidal frequencies. The tidal characteristics and the Empirical Orthogonal Function (EOF) results measured by the HF radars also resemble the ADCPs and align with the characteristics of the estuarine current field. [ABSTRACT FROM AUTHOR]

The ocean circulation is typically constrained in operational analysis and forecasting systems through the assimilation of sea level anomaly (SLA) retrievals from satellite altimetry. This approach has limited benefits in the Arctic Ocean and surrounding seas due to data gaps caused by sea ice coverage. Moreover, assimilation of SLA in seasonally ice-free regions may be negatively affected by the quality of the Mean Sea Surface (MSS) used to derive the SLA. Here, we use the Regional Ice Ocean Prediction System (RIOPS) to investigate the impact of assimilating Absolute Dynamic Topography (ADT) fields on the circulation in the Arctic Ocean. This approach avoids the use of a MSS and additionally provides information on sea level in ice covered regions using measurements across leads (openings) in the sea ice. RIOPS uses a coupled ice-ocean model on a 3-4 km grid-resolution pan-Arctic domain together with a multi-variate reduced-order Kalman Filter. The system assimilates satellite altimetry and sea surface temperature together with in situ profile observations. The background error is modified to match the spectral characteristics of the ADT fields, which contain less energy at small scales than traditional SLA due to filtering applied to reduce noise originating in the geoid product used. A series of four-year reanalyses demonstrate significant reductions in innovation statistics with important impacts across the Arctic Ocean. Results suggest that the assimilation of ADT can improve circulation and sea ice drift in the Arctic Ocean, and intensify volume transports through key Arctic gateways and resulting exchanges with the Atlantic Ocean. A reanalysis with a modified Mean Dynamic Topography (MDT) is able to reproduce many of the benefits of the ADT but does not capture the enhanced transports. Assimilation of SLA observations from leads in the sea ice appears to degrade several circulation features; however, these results may be sensitive to errors in MDT. This study highlights the large uncertainties that exist in present operational ocean forecasting systems for the Arctic Ocean due to the relative paucity and reduced quality of observations compared to ice-free areas of the Global Ocean. Moreover, this underscores the need for dedicated and focused efforts to address this critical gap in the Global Ocean Observing System.

Research on coastal ocean circulation patterns over long time periods is significant for various marine endeavors: environmental protection, coastal engineering construction, and marine renewable energy extraction. Based on sea surface current data remotely observed using a shore-based high frequency radar (HFR) system for one year (2016), spatiotemporal characteristics of surface flow fields of sea surface flow fields along the west coast of Ireland are studied using harmonic analysis, rotary spectral analysis and representative flow fields over different seasons and the whole year. Coastal surface currents in the study area are strongly affected by tidal dynamics of the M2 constituent, showing significant characteristics of regular semidiurnal tide, such as M2 and S2. The energy spectrum distribution indicates that the tidal constituents M2 and S2 are the dominant periodic energy constituents in a counterclockwise spectrum, which mainly presents rotating flow; the representative diurnal tidal constituents is the constituent K1, and its energy spectrum distribution is mainly clockwise. A comparison between probable maximum current velocity (PMCV) and measured maximum current velocity (MMCV) is presented. It shows that although tidal current characteristics in the study area are significant, the main driving force of the currents at the time of the maximum currents is wind energy. These results provide new insights into a region of huge societal potential at early stages of sustainable economic exploitation where few data currently exist. [ABSTRACT FROM AUTHOR]

The western Arctic marginal seas undergo large seasonal variation, but are very challenging to observe directly due to sea ice and shallow depths. Deployments of several saildrone uncrewed surface vehicles in the summers of 2018 and 2019 provided unique opportunities to validate the satellite‐derived near surface currents, Ocean Surface Current Analysis Real‐time (OSCAR), in the western Arctic marginal seas against in situ upper ocean current measurements. Overall, OSCAR current is biased low (by 5.3 cm/s) with significant noise. Higher vector correlation estimated by the cosine similarity and speed differences often occur where stronger currents (often topography‐steered) are observed. Such differences reveal that the data set resolvability depends on spatial and temporal resolutions, smoothing, and latitudes, suggesting that OSCAR is able to depict the major current systems but significantly underestimates their strength. Poorer vector correlation occurs at weaker current regimes (<10 cm/s), over the shallow Hanna Shoal, near fresher water due to ice melt and river discharge. The latter two water class regimes highlight the importance of salinity contribution to the buoyancy force which is neglected in the OSCAR formulation. Plain Language Summary: It is challenging to make direct measurements in the western Arctic marginal seas, which undergo large seasonal swings, because of the sea ice and shallow depths. We validate the satellite‐derived surface current product, Ocean Surface Current Analysis Real‐time (OSCAR), against observations from saildrone uncrewed surface vehicles in the western Arctic marginal seas cruising in summers of 2018 and 2019. Overall, OSCAR current is biased low and noisy. Higher current direction relation and speed differences often occur where stronger currents are observed. Such differences suggest that the reliability of OSCAR depends on several factors. It also suggests that OSCAR is able to depict the major current systems but significantly underestimates their strength. Poorer current direction relation occurs at weaker current regimes, over the shallow Hanna Shoal, near fresher water due to ice melt and river discharge. The latter two regimes highlight the importance of salinity information, which however, is neglected in the OSCAR model. Key Points: This study validates Ocean Surface Current Analysis Real‐time (OSCAR) satellite derived surface currents in the western Arctic, including the very shallow waters, by saildronesHigh vector correlation but larger vector difference between OSCAR and saildrone currents often occur in strong topography guided currentsLow vector correlation occurs at weaker currents, over the shallow Hanna Shoal, and near fresher waters due to ice melt and river discharge [ABSTRACT FROM AUTHOR]

Near Inertial Oscillations (NIOs) are ocean oscillations forced by intermittent winds. They are most energetic at mid-latitudes, particularly in regions with atmospheric storm tracks. Wind-driven, large-scale NIOs are quickly scattered by ocean mesoscale eddies (with sizes ranging from 100 to 400 km), causing a significant portion of the NIO energy to propagate into the subsurface ocean interior. This kinetic energy pathway illustrates that the wind energy input to NIO is critical for maintaining deep ocean stratification and thus closing the total energy budget, as emphasised by numerous modelling studies. However, this wind energy input to NIO remains poorly observed on a global scale. A remote sensing approach that observes winds and ocean currents co-located in time and space with high resolution is necessary to capture the intermittent air-sea coupling. The current satellite observations do not meet these requirements. This study assesses the potential of a new satellite mission concept, Ocean DYnamics and Surface Exchange with the Atmosphere (OSYSEA), to recover wind-forced NIOs from co-located winds and currents. To do this, we use an Observation System Simulation Experiment (OSSE) based on hourly observations of ocean surface currents and surface winds from five surface moorings covering latitudes from 15 ° to 50 ° . ODYSEA wind and current observations are expected to have a spatial resolution of 10 km with about a 12 h sampling frequency in mid-latitudes. Results show that NIOs can be recovered with high accuracy using the ODYSEA spatial and temporal resolution, but only if observations are made over a wide area of 1800 km. A narrower swath (1000 km) may lead to significant aliasing. [ABSTRACT FROM AUTHOR]

Real-time reconstruction of ocean surface currents is a challenge due to the complex, non-linear dynamics of the ocean, the small number of in situ measurements, and the spatio-temporal heterogeneity of satellite altimetry observations. To address this challenge, we introduce HIRES-CURRENTS-Net, an operational real-time convolutional neural network (CNN) model for daily ocean current reconstruction. This study focuses on the Mediterranean Sea, a region where operational models have great difficulty predicting surface currents. Notably, our model showcases higher accuracy compared to commonly used alternative methods. HIRES-CURRENTS-Net integrates high-resolution measurements from the infrared or visible spectrum—high resolution Sea Surface Temperature (SST) or chlorophyll (CHL) images—in addition to the low-resolution Sea Surface Height (SSH) maps derived from satellite altimeters. In the first stage, we apply a transfer learning method which uses a high-resolution numerical model to pre-train our CNN model on simulated SSH and SST data with synthetic clouds. The observation of System Simulation Experiments (OSSEs) offers us a sufficient training dataset with reference surface currents at very high resolution, and a model trained on this data can then be applied to real data. In the second stage, to enhance the real-time operational performance of our model over previous methods, we fine-tune the CNN model on real satellite data using a novel pseudo-labeling strategy. We validate HIRES-CURRENTS-Net on real data from drifters and demonstrate that our data-driven approach proves effective for real-time sea surface current reconstruction with potential operational applications such as ship routing.

Abstract The western Arctic marginal seas undergo large seasonal variation, but are very challenging to observe directly due to sea ice and shallow depths. Deployments of several saildrone uncrewed surface vehicles in the summers of 2018 and 2019 provided unique opportunities to validate the satellite‐derived near surface currents, Ocean Surface Current Analysis Real‐time (OSCAR), in the western Arctic marginal seas against in situ upper ocean current measurements. Overall, OSCAR current is biased low (by 5.3 cm/s) with significant noise. Higher vector correlation estimated by the cosine similarity and speed differences often occur where stronger currents (often topography‐steered) are observed. Such differences reveal that the data set resolvability depends on spatial and temporal resolutions, smoothing, and latitudes, suggesting that OSCAR is able to depict the major current systems but significantly underestimates their strength. Poorer vector correlation occurs at weaker current regimes (

The kinetic energy transfer between the atmosphere and oceans, called wind work, affects ocean dynamics, including near-inertial oscillations and internal gravity waves, mesoscale eddies, and large-scale zonal jets. For the most part, the recent numerical estimates of global wind work amplitude are almost five times larger than those reported 10 years ago. This large increase is explained by the impact of the broad range of spatial and temporal scales covered by winds and currents, the smallest of which has only recently been uncovered by increasingly high-resolution modeling efforts. However, existing satellite observations do not fully sample this broad range of scales. The present study assesses the capabilities of ODYSEA, a conceptual satellite mission to estimate the amplitude of wind work in the global ocean. To this end, we use an ODYSEA measurement simulator fed by the outputs of a km scale coupled ocean–atmosphere model to estimate wind work globally. The results indicate that compared with numerical truth estimates, the ODYSEA instrument performs well globally, except for latitudes north of 40 ∘ N during summer due to unresolved storm evolution. This performance is explained by the wide-swath properties of ODYSEA (a 1700 km wide swath with 5 km posting for winds and surface currents), its twice-a-day (daily) coverage at mid-latitudes (low latitudes), and the insensitivity of the wind work to uncorrelated errors in the estimated wind and current. [ABSTRACT FROM AUTHOR]

An operational ocean Ensemble Prediction System (EPS) for the coastal seas off Northern Norway is evaluated by comparing with high-frequency radar current speed estimates. The EPS is composed of 24 members for which the ocean current is not perturbed nor constrained but forced with an atmosphere ensemble. The ocean ensemble spread stems from (i) accumulated differences in wind-forcing history and (ii) constraints of sea surface temperature by data assimilation. The intention of the ensemble is to reflect the actual uncertainty in initial conditions, which are largely unknown in terms of mesoscale circulation. We find a low but pronounced predictive skill in surface currents along with a good statistic skill. Additionally, current speeds show deterioration of the validation metrics over the forecast range. Further, high-resolution wind forcing seems to provide better forecast skill in currents compared to lower resolution forcing. In general, the ensemble exhibits the ability to predict forecast uncertainty.

The processes of ocean dynamics are complex near the Pearl River Estuary and are not clear due to a lack of abundant observations. The spatial characteristics of the spring sea surface currents in the adjacent waters of the Pearl River Estuary were analyzed using the current data observed by a three-station high-frequency surface wave radar system (HFSWRS). Compared with the two-station HFSWRS, the deviation of current velocity and direction observed by the three-station HFSWRS from the underway measurements decreased by 42.86% and 38.30%, respectively. The analyzed results show that the M2 tidal current is the dominant current among all the tidal constituents, followed by K1, with angles of inclination ranging from 130° to 150°. The tidal flow is dominated by northwest–southeast back-and-forth flow. In the southern part of the observed area, which is far from the coastline, the tidal current ellipses exhibit a circular pattern. The prevalent tidal current type in this region is irregularly semi-diurnal, and the shallow water constituents also have a significant effect. The tidal energy in the adjacent waters of the Pearl River Estuary is affected by potential energy flux and kinetic energy flux. As the water depth and currents velocity increase in the southeast direction, the tidal energy flux increases. In the nearshore zone, the direction of tidal energy flux varies along the coastline. The changes in the residual current within the observed area are correlated with the sea surface wind field. Based on the high-precision sea surface current observed by the three-station HFSWRS, the characteristics of the ocean dynamic processes near the Pearl River Estuary are analyzed comprehensively, which provides important reference and confidence for the application of the developing new radar observing network with about 10 radar stations near the Pearl River Estuary.

The performance of a high-frequency (HF) radar network situated within the Pearl River Estuary from 17 July to 13 August 2022 is described via a comparison with seven acoustic Doppler current profilers (ADCPs). The radar network consists of six OSMAR-S100 compact HF radars, with a transmitting frequency of 13–16 MHz and a direction-finding technique. Both the radial currents and vector velocities showed good agreement with the ADCP results (coefficient of determination r2: 0.42–0.78; RMS difference of radials: 11–21.6 cm s−1; bearing offset Δθ: −4.8°–16.1°; complex correlation coefficient γ: 0.62–0.96; and phase angle α: −24.3°–17.8°). For these radars, the Δθ values are not constant but vary with azimuthal angles. The relative positions between the HF radar and ADCPs, as well as factors such as the presence of island terrain obstructing the signal, significantly influence the errors. The results of spectral analysis also demonstrate a high level of consistency and the capability of HF radar to capture diurnal and semidiurnal tidal frequencies. The tidal characteristics and the Empirical Orthogonal Function (EOF) results measured by the HF radars also resemble the ADCPs and align with the characteristics of the estuarine current field.

This paper reviews physical mechanisms, observation techniques and modelling approaches dealing with surface currents on short time scales (hours to days) relevant for operational oceanography. Key motivations for this article include fundamental difficulties in reliable measurements and the persistent lack of a widely held consensus on the definition of surface currents. These problems are augmented by the fact that various methods to observe and model ocean currents yield very different representations of a surface current. We distinguish between four applicable definitions for surface currents; (i) the interfacial surface current, (ii) the direct wind-driven surface current, (iii) the surface boundary layer current, and (iv) an effective drift current. Finally, we discuss challenges in synthesising various data sources of surface currents - i.e. observational and modelling – and take a view on the predictability of surface currents concluding with arguments that parts of the surface circulation exhibit predictability useful in an operational context. [ABSTRACT FROM AUTHOR]

Aim: Our main aim was to identify the distribution of, and potential mechanisms underpinning, hybrid‐rich zones – regions with a disproportionate number of unique interspecific hybrids. We investigated whether coral reef fish hybrids coincided with factors such as phylogenetic relatedness, biogeographic barriers, species richness, geographic isolation, endemism, and oceanic currents. Location: Global. Time period: Contemporary. Major taxa studied: Coral reef fishes. Methods: We conducted a literature review and mapping to assess the taxonomic and global prevalence of hybridisation in coral reef fishes. We then fit Generalised additive models using a full‐subsets and Bayesian framework to assess which variables are associated with hybrid‐rich zones. Results: We found 143 unique interspecific coral reef fish hybrids involving 204 species – which accounts for approximately 7% of coral reef fish species, indicating that hybridisation is as common in the sea as it is on land. Characteristic coral reef fish families were not homogeneously represented in our dataset, with particularly colourful groups standing out. Mapping our dataset revealed that coral reef fish hybrids are found worldwide, though some ecoregions (e.g., the Christmas and Cocos (Keeling) Islands, South Kuroshio, Hawaii, and Eastern Philippines) are more hybrid‐rich than others. Our analysis revealed that mean surface current velocity, phylogenetic relatedness, and geographic isolation were the best predictors of hybrid richness in a given location. Main conclusions: Phylogenetic distance between coral reef fish species may serve as a pre‐condition for hybridisation to occur, lying between introgression and reproductive incompatibility. We also propose a novel mechanism, with oceanic currents driving long‐distance larval dispersal events, transporting stray species to geographically remote sinks to maintain hybrid‐rich zones. [ABSTRACT FROM AUTHOR]

High-frequency (HF) radar measures sea surface currents from the radio waves transmitted and received by antenna on land. Since the data quality of HF radar measurements sensitively depend on the radio wave environment around antenna, Antenna Pattern Measurements (APM) plays an important role in evaluating the accuracy of measured surface currents. In this study, APM was performed by selecting the times when the background noise level around antenna was high and low, and radial data were generated by applying the ideal pattern and measured pattern. The measured antenna pattern for each case was verified with the current velocity data collected by drifters. The radial velocity to which the ideal pattern was applied was not affected by the background noise level around antenna. However, the radial velocity obtained with APM in the period of high background noise was significantly lower in quality than the radial velocity in a low noise environment. It is recomended that APM be carried out in consideration of the radio wave environment around antenna, and that the applied result be compared and verified with the current velocity measurements by drifters. If it is difficult to re-measure APM, we suggest using radial velocity in generating total vector with the ideal pattern through comparative verification, rather than poorly measured patterns, for better data quality. [ABSTRACT FROM AUTHOR]

Research on coastal ocean circulation patterns over long time periods is significant for various marine endeavors: environmental protection, coastal engineering construction, and marine renewable energy extraction. Based on sea surface current data remotely observed using a shore-based high frequency radar (HFR) system for one year (2016), spatiotemporal characteristics of surface flow fields of sea surface flow fields along the west coast of Ireland are studied using harmonic analysis, rotary spectral analysis and representative flow fields over different seasons and the whole year. Coastal surface currents in the study area are strongly affected by tidal dynamics of the M2 constituent, showing significant characteristics of regular semidiurnal tide, such as M2 and S2. The energy spectrum distribution indicates that the tidal constituents M2 and S2 are the dominant periodic energy constituents in a counterclockwise spectrum, which mainly presents rotating flow; the representative diurnal tidal constituents is the constituent K1, and its energy spectrum distribution is mainly clockwise. A comparison between probable maximum current velocity (PMCV) and measured maximum current velocity (MMCV) is presented. It shows that although tidal current characteristics in the study area are significant, the main driving force of the currents at the time of the maximum currents is wind energy. These results provide new insights into a region of huge societal potential at early stages of sustainable economic exploitation where few data currently exist.

Near Inertial Oscillations (NIOs) are ocean oscillations forced by intermittent winds. They are most energetic at mid-latitudes, particularly in regions with atmospheric storm tracks. Wind-driven, large-scale NIOs are quickly scattered by ocean mesoscale eddies (with sizes ranging from 100 to 400 km), causing a significant portion of the NIO energy to propagate into the subsurface ocean interior. This kinetic energy pathway illustrates that the wind energy input to NIO is critical for maintaining deep ocean stratification and thus closing the total energy budget, as emphasised by numerous modelling studies. However, this wind energy input to NIO remains poorly observed on a global scale. A remote sensing approach that observes winds and ocean currents co-located in time and space with high resolution is necessary to capture the intermittent air-sea coupling. The current satellite observations do not meet these requirements. This study assesses the potential of a new satellite mission concept, Ocean DYnamics and Surface Exchange with the Atmosphere (OSYSEA), to recover wind-forced NIOs from co-located winds and currents. To do this, we use an Observation System Simulation Experiment (OSSE) based on hourly observations of ocean surface currents and surface winds from five surface moorings covering latitudes from 15° to 50°. ODYSEA wind and current observations are expected to have a spatial resolution of 10 km with about a 12 h sampling frequency in mid-latitudes. Results show that NIOs can be recovered with high accuracy using the ODYSEA spatial and temporal resolution, but only if observations are made over a wide area of 1800 km. A narrower swath (1000 km) may lead to significant aliasing.

This paper presents the systematic design of an elliptical ring monopole wideband antenna using characteristic mode analysis. The modal analysis is used to analyze the bandwidth and the radiation performance of the radiating patch. The resonance frequencies of three characteristic modes are close to each other with similar modal current distributions and characteristic fields. These three characteristic modes are simultaneously excited by an effective feeding technique. The proposed model achieves wideband characteristics. The proposed model is printed on an inexpensive FR4 substrate with a size of 20 mm × 18 mm × 1.6 mm and has a wide impedance bandwidth of 124.4% in the range of 3.6–15.46 GHz. The prototype has been fabricated and the measured results show good agreement with simulated results. The antenna will cover WLAN, WiMAX, Wi-Fi, and X-band applications.

The kinetic energy transfer between the atmosphere and oceans, called wind work, affects ocean dynamics, including near-inertial oscillations and internal gravity waves, mesoscale eddies, and large-scale zonal jets. For the most part, the recent numerical estimates of global wind work amplitude are almost five times larger than those reported 10 years ago. This large increase is explained by the impact of the broad range of spatial and temporal scales covered by winds and currents, the smallest of which has only recently been uncovered by increasingly high-resolution modeling efforts. However, existing satellite observations do not fully sample this broad range of scales. The present study assesses the capabilities of ODYSEA, a conceptual satellite mission to estimate the amplitude of wind work in the global ocean. To this end, we use an ODYSEA measurement simulator fed by the outputs of a km scale coupled ocean–atmosphere model to estimate wind work globally. The results indicate that compared with numerical truth estimates, the ODYSEA instrument performs well globally, except for latitudes north of 40∘N during summer due to unresolved storm evolution. This performance is explained by the wide-swath properties of ODYSEA (a 1700 km wide swath with 5 km posting for winds and surface currents), its twice-a-day (daily) coverage at mid-latitudes (low latitudes), and the insensitivity of the wind work to uncorrelated errors in the estimated wind and current.

This communication proposes a compact 4-shaped monopole annular ring UWB antenna design. The proposed structure contains multiple radiating strips inside the annular ring, in the form of a 4-shaped and a 50Ω microstrip feed line. A tapered structure with a feed point is chosen to achieve wideband characteristics. The proposed model is printed on a low-priced FR4 substrate with a size of 0.180λ 0 × 0.225λ 0 (20 × 25mm 2). The proposed model achieves a fractional bandwidth of 133.74% in the 2.7 to 13.6 GHz range with S11

The South China Sea (SCS) is the largest marginal sea in the western Pacific and comprises an ideal natural laboratory for studying deep-sea processes. Previous investigations in the region have primarily focused on surface currents and to a lesser extent on intermediate currents, while observations of bottom currents are scarce, hindering our understanding of deep ocean sediment transport and deposition mechanisms. Here, we carried out grain size measurement and end-member modelling analysis (EMMA) on samples collected by a sediment trap at a water depth of 1500 m in the Xisha Trough in the northwestern South China Sea. Grain size data shows that terrigenous material has particle sizes ranging from 0.3 μm to 745.9 μm and a peak at 4–11 μm. The EMMA result suggests that finer sediment accounting for an average of 48.2% of the sediment represents material transported by deep water currents (End-member 1; EM1), and this is primary sediment source in the region. Coarser sediment contributing to 32.9% of the sediment is the second most significant source (EM3), representing the deposits of surface currents influenced by monsoons and deposited close to the source. A third sediment type (EM2) constituting approximately 18.9% of the sediment exhibits a wide range of grain sizes and rapid changes, indicating transport in the bottom nepheloid layer triggered by anticyclonic eddies. Our study identifies three primary sediment transport mechanisms to the northwestern SCS, bridging present-day observations with paleoceanography and providing a crucial basis for future research on paleoceanographic reconstructions. • Samples were collected in sediment trap at a water depth of 1500 m in the northwestern South China Sea from 2009 to 2012. • Finer particles (c. 48.2% of the sediment) likely comprise sediment transported by deep-water currents. • Coarser particles (c. 32.9% of the sediment) indicate near-source deposition, likely transported by surface currents influenced by monsoons. • The end member with a large grain size range probably originated from resuspended sediment transported by anticyclonic eddies. [ABSTRACT FROM AUTHOR]

This study examines the effect of surface currents on the bulk algorithm calculation ofwind stress estimated using the scatterometer data during 2007–2020 in the Indian Ocean. In the study region as a whole, the wind stress decreased by 5.4% by including currents in the wind stress equation. The most significant reduction in the wind stress is found along the most energetic regions with strong currents such as Somali Current, Equatorial Jets, and Agulhas retroflection. The highest reduction of 11.5% is observed along the equator where the Equatorial Jets prevail. A sensitivity analysis has been carried out for the study region and for different seasons to assess the relative impact of winds and currents in the estimation of wind stress by changing the winds while keeping the currents constants and vice versa. The inclusion of currents decreased the wind stress (consistent with scatterometer winds) and this decrease is prominent when the currents are stronger. This study showed that the equatorial Indian Ocean is the most sensitive region where the current can impact wind stress estimation. The results showed that uncertainties in the wind stress estimations are quite large at regional levels and hence better representation of wind stress incorporating ocean currents should be considered in the ocean/climatic models for accurate air-sea interaction studies that are not based on remotely sensed winds. [ABSTRACT FROM AUTHOR]

This communication proposes a compact 4-shaped monopole annular ring UWB antenna design. The proposed structure contains multiple radiating strips inside the annular ring, in the form of a 4-shaped and a 50Ω microstrip feed line. A tapered structure with a feed point is chosen to achieve wideband characteristics. The proposed model is printed on a low-priced FR4 substrate with a size of 0.180λ0 × 0.225λ0 (20 × 25mm²). The proposed model achieves a fractional bandwidth of 133.74% in the 2.7 to 13.6 GHz range with S11<-10dB and covers the 3.1-10.6 GHz unlicensed band approved by FCC in 2002 and X-band applications. The antenna exhibits stable and Omni-directional radiation patterns in the operating frequency range. The analysis of the proposed monopole antenna using characteristic modes is performed to obtain a physical understanding of the radiation process occurring on the radiating antenna. The modal significance curves and the modal current distributions are used to analyze the radiating antenna using the first six characteristic modes. The measurement and simulation results show a good agreement. [ABSTRACT FROM AUTHOR]

Surface currents in the equatorial Pacific play a key role in determining thermohaline structures in the upper ocean, thus acting to modulate the tropical Pacific mean state and interannual variability. In turn, the thermohaline structure and its variability can also affect surface currents by changing ocean density and pressure gradients. Currently, the thermohaline effects on the surface currents have not been well understood, especially for the salinity role. In this study, the thermohaline effects on surface currents in the equatorial Pacific are systematically evaluated based on an analytical scheme, which is used to calculate surface currents in producing the Ocean Surface Current Analysis Real‐time (OSCAR) product. Several calculations are performed with the original OSCAR‐based scheme and an improved scheme including the salinity effect. It is demonstrated that the thermohaline effects are generally dominated by the temperature role, but the salinity effect plays a non‐negligible role near the dateline and in the far eastern Pacific. The thermohaline effects are quantified and highlighted on seasonal and interannual time scales. For annual mean surface currents, temperature and salinity effects work together to weaken zonal currents north of the equator, with their reduced amplitude by about half. Seasonally, the thermohaline effects are dominant in spring due to the weakened wind effect. Interannually, the salinity effect explains up to 25% of the total variance in the far eastern equatorial Pacific. This study highlights the substantial contributions of thermohaline anomalies to surface current variabilities, in which salinity can play an important contributing role. Plain Language Summary: Surface currents in the equatorial Pacific play a fundamental role in the El Niño‐Southern Oscillation (ENSO) phenomenon because they transport heat and salt. Temperature and salinity affect surface currents by changing ocean density and pressure gradients; but currently, their impacts on surface currents in the equatorial Pacific have not been well understood, especially for the salinity role. In this article, we examine the temperature and salinity effects on surface currents in the equatorial Pacific by including salinity contribution in a scheme used to produce a state‐of‐the‐art surface current product. Relatively, the temperature effect is dominantly important across the tropical Pacific; but near the dateline and in the far eastern Pacific, the salinity effect plays a non‐negligible role. For annual mean zonal currents, the temperature and salinity effects act to weaken the mean currents north of the equator by half. Seasonally, their effects are dominant in spring when the trade winds weaken. Interannually, the salinity effect makes a positive contribution (up to 25%) to the total variance of zonal currents in the far eastern Pacific. This study highlights the salinity effect on surface currents on multi‐timescales, which is important for modeling and predictions of ENSO. Key Points: The scheme producing the Ocean Surface Current Analysis Real‐time product is used to evaluate temperature and salinity effectsThermohaline effects act to weaken mean zonal currents north of the equator by half, with a dominant role in springThe effect of salinity is more important than that of temperature near the dateline, and non‐negligible in the far eastern Pacific [ABSTRACT FROM AUTHOR]

Observations from six Lagrangian Surface Wave Instrument Float with Tracking drifters in January‐February 2020 in the northwestern tropical Atlantic during the Atlantic Tradewind Ocean‐atmosphere Mesoscale Interaction Campaign are used to evaluate the influence of wave‐current interactions on wave slope and momentum flux. At observed wind speeds of 4–12 ms−1, wave mean square slopes are positively correlated with wind speed. Wave‐relative surface currents varied significantly, from opposing the wave direction at 0.24 ms−1 to following the waves at 0.47 ms−1. Wave slopes are 5%–10% higher when surface currents oppose the waves compared to when currents strongly follow the waves, consistent with a conservation of wave energy flux across gradients in currents. Assuming an equilibrium frequency range in the wave spectrum, wave slope is proportional to wind friction velocity and momentum flux. The observed variation in wave slope equates to a 10%–20% variation in momentum flux over the range of observed wind speeds (4–12 ms−1), with larger variations at higher winds. At wind speeds over 8 ms−1, momentum flux varies by at least 6% more than the variation expected from current‐relative winds alone, and suggests that wave‐current interactions can generate significant spatial and temporal variability in momentum fluxes in this region of prevailing trade winds. Results and data from this study motivate the continued development of fully coupled atmosphere‐ocean‐wave models. Plain Language Summary: Six surface current‐following drifters were deployed in the northwestern tropical Atlantic during the Atlantic Tradewind Ocean‐atmosphere Mesoscale Interaction Campaign to study how surface current gradients influence wave properties. In theory, when spatial variability in currents exists, surface currents are in the opposite direction as the waves will cause a shift in wave frequency and shoaling of wave energy, leading to wave steepening. Increased wave slopes, due to opposing surface currents, may lead to increased whitecapping and wave breaking. Conversely, surface currents in the same direction as the waves are expected to flatten waves. Wind directions were relatively constant, owing to prevailing trade winds. Wave slopes varied by up to 10% at a given wind speed due to the variability of surface currents. This suggests that surface current variability may influence air‐sea exchanges of gas, heat, and momentum through their interaction with waves. The effect of surface current variations on waves is often not incorporated into model parameterizations, so these findings may be useful in the development of more fully coupled atmosphere‐ocean‐wave models. Key Points: Six Lagrangian surface drifters observed wave spectra in an area of moderate mesoscale activity in the northwestern tropical AtlanticSurface current and wind‐wave directions were opposed over 30% of the time; during this time wave mean square slope was elevatedWave‐current interactions caused variations in air‐sea momentum flux of at least 6% at wind speeds over 8 ms−1 [ABSTRACT FROM AUTHOR]

This article provides a detailed presentation of the dual-aperture synthetic aperture radar (SAR) data processing scheme for the retrieval of ocean surface radial velocities.This scheme includes processing of raw SAR data, coregistration of along-track interferometric samples, magnitude and absolute phase calibration, and coherent averaging (multilooking). Several approaches for the absolute phase calibration are provided and compared. Some of the attempted approaches can potentially be used over open ocean (i.e., in scenes that do not contain any land). Main goal of attempting different approaches for the absolute phase calibration was to determine their relative performance, and determine the potential feasibility of some approaches over open ocean. The data processing scheme is applied to a RADARSAT-2 dual-channel MODEX-1 acquisition over a section of the Florida Current. For the dataset used in this study, different absolute phase calibration methods yielded similar radial velocity estimates, with relative mean and RMS differences within approximately 0.1 m/s. Estimates from SAR ATI were also compared to estimates from NASA’s OSCAR dataset. Comparison of visually identified currents showed close spatial overlap between estimates from the two sources. The RMS difference was found to be approximately 0.30 m/s. This difference can be attributed to the physical and temporal differences between the estimates.

Abstract We propose a conductively coupled terahertz metallic metamaterial exhibiting analog of electromagnetically induced transparency (EIT), in which the bright and dark mode antennae interact via surface currents rather than near-field coupling. Aluminum foil, which is very cheap and often used in food package, is used to fabricate our metamaterials. Thus, our metamaterials are also flexible metamaterials. In our design, aluminum bar resonators and aluminum split ring resonators (SRRs) are connected (rather than separated) in the form of a fork-shaped structure. We conduct a numerical simulation and an experiment to analyze the mechanism of the proposed metamaterial. The surface current due to LSP resonance (bright mode) flows along different paths, and a potential difference is generated at the split gaps of the SRRs. Thus, an LC resonance (dark mode) is induced, and the bright mode is suppressed, resulting in EIT. The EIT-like phenomenon exhibited by the metamaterial is induced by surface conducting currents, which may provide new ideas for the design of EIT metamaterials. Moreover, the process of fabricating microstructures on flexible substrates can provide a reference for producing flexible microstructures in the future.

Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (∼10 m) and temporal (∼1 min) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10–20 min). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.

This paper presents the systematic design of an elliptical ring monopole wideband antenna using characteristic mode analysis. The modal analysis is used to analyze the bandwidth and the radiation performance of the radiating patch. The resonance frequencies of three characteristic modes are close to each other with similar modal current distributions and characteristic fields. These three characteristic modes are simultaneously excited by an effective feeding technique. The proposed model achieves wideband characteristics. The proposed model is printed on an inexpensive FR4 substrate with a size of 20 mm X 18 mm X 1.6 mm and has a wide impedance bandwidth of 124.4% in the range of 3.6-15.46 GHz. The prototype has been fabricated and the measured results show good agreement with simulated results. The antenna will cover WLAN, WiMAX, Wi-Fi, and X-band applications. [ABSTRACT FROM AUTHOR]

This communication presents the design of an asymmetrical hexagonal-shaped wideband antenna based on Characteristic Mode Analysis (CMA). An L-shaped slot is used to change the distribution of mode surface currents. Current modes and characteristic fields of the desired modes are studied to improve bandwidth. A simple 50 Ω microstrip line is used to excite the selected modes and obtain wideband impedance matching. The antenna is printed on a low-cost FR4 substrate with a size of 30 mm × 29 mm × 1.6 mm. The design approach is experimentally validated through fabricated prototype antenna. It exhibits impedance bandwidth of 110.22% from 2.7 to 9.33 GHz, and thus covers most wireless systems, such as WPAN, WiMax, WLAN, RFID and C-band. [ABSTRACT FROM AUTHOR]

Simulating upper layer of Bay of Bengal through three-dimensional ocean circulation models is a challenging task. In this study, the surface current from the Nucleus European Modelling of Ocean (NEMO) based global ocean assimilation system is assessed against the high frequency (HF) radar data along the Andhra Pradesh coast on a daily scale during southwest monsoon 2016. The temporal variation of NEMO simulated surface current with HF radar data shows that the NEMO model captures the zonal current better than the meridional current. Both NEMO and HF radar show that the mean surface current average over latitude (15.8°–16.3°N) is westward for zonal surface current and southward for meridional current with maximum at 40–60 km away from the coast. Further, the monthly mean HF radar derived surface current indicates the strong south-westward flow of surface current dominated during July 2016 with speed more than 50 cm/s which is also well simulated by NEMO analysis. It also captures the cold core eddy during 15–25 July 2016 with very small north-eastward shift with respect to HF radar. The scatter plot of collocated surface zonal and meridional current average over the box (81.5–82.5°E; 15.5–16.5°N) clearly shows that NEMO analysis has the correlation of more than 0.5 for both zonal and meridional current. Research highlights: The simulation of upper layer of Bay of Bengal (BoB) through three dimensional ocean circulation models is a challenging task. In this study, the surface current from the high resolution NEMO based global ocean assimilation system is compared against the observed High Frequency (HF) radar data along the Andhra Pradesh Coast during the southwest monsoon 2016. NEMO analysis captures the mean and variability of surface current very well with HF radar. However, it underestimates the mean surface current which may be due to coarser model resolution and complex non-linear processes in the coastal region. The strong cold core eddy during 15–25 July 2016 is observed along the coast which is well simulated in NEMO model with small north-eastward shift with respect to HF radar. The scatter plot of collocated surface current from NEMO analysis and HF radar data average over the Andhra Pradesh Coast (APCO; 81.5–82.5°E; 15.5–16.5°N) clearly shows that NEMO analysis has the correlation of more than 0.5 for surface current. [ABSTRACT FROM AUTHOR]

Search and rescue (SAR) modeling applications, mostly based on Lagrangian tracking particle algorithms, rely on the accuracy of met-ocean forecast models. Skill assessment methods are therefore required to evaluate the performance of ocean models in predicting particle trajectories. The Skill Score (SS), based on the Normalized Cumulative Lagrangian Separation (NCLS) distance between simulated and satellite-tracked drifter trajectories, is a commonly used metric. However, its applicability in coastal areas, where most of the SAR incidents occur, is difficult and sometimes unfeasible, because of the high variability that characterizes the coastal dynamics and the lack of drifter observations. In this study, we assess the performance of four models available in the Ibiza Channel (Western Mediterranean Sea) and evaluate the applicability of the SS in such coastal risk-prone regions seeking for a functional implementation in the context of SAR operations. We analyze the SS sensitivity to different forecast horizons and examine the best way to quantify the average model performance, to avoid biased conclusions. Our results show that the SS increases with forecast time in most cases. At short forecast times (i.e., 6 h), the SS exhibits a much higher variability due to the short trajectory lengths observed compared to the separation distance obtained at timescales not properly resolved by the models. However, longer forecast times lead to the overestimation of the SS due to the high variability of the surface currents. Findings also show that the averaged SS, as originally defined, can be misleading because of the imposition of a lower limit value of zero. To properly evaluate the averaged skill of the models, a revision of its definition, the so-called SS∗, is recommended. Furthermore, whereas drifters only provide assessment along their drifting paths, we show that trajectories derived from high-frequency radar (HFR) effectively provide information about the spatial distribution of the model performance inside the HFR coverage. HFR-derived trajectories could therefore be used for complementing drifter observations. The SS is, on average, more favorable to coarser-resolution models because of the double-penalty error, whereas higher-resolution models show both very low and very high performance during the experiments.

Located between the sub-Antarctic Front and the Antarctic Polar Front, the Prince Edward Islands (PEIs) provide an essential breeding ground for top predators and are an ideal location to investigate disturbances linked to climate change. This study provides the first seasonal characterisation of surface hydrography at the PEIs, using satellite and reanalysis products from 1993 to 2016. Sea surface temperature (SST) showed consistently higher values to the north, while wind, currents and eddy kinetic energy all showed higher values to the south of the islands. The highest SST (8 °C) occurred in summer and the lowest (2 °C) in winter, with a one-to two-month delay between the maximum incoming solar radiation (in December–January) and the highest SST (in February). The highest wind speed occurred in July (10.8 m s−1) and the minimum in February (7 m s−1). Geostrophic currents were four-times larger than Ekman currents, showing lower speeds between April and June (0.25–0.30 m s−1) and a peak in August (0.45 m s−1). There were no significant differences for SST and Ekman currents between the regions upstream and downstream of the PEIs. In contrast, surface total and geostrophic current velocities were weaker downstream because the islands act as a barrier to the flow. A zone of lower wind speed, coinciding with enhanced positive wind stress curl (WSC), favouring downwelling, occurred directly upstream throughout the year. Although WSC over the PEIs was negative (upwelling-favourable), no corresponding cooling was evident in SST. This seasonal characterisation provides a baseline against which interannual/decadal variability and future changes in these parameters can be assessed. [ABSTRACT FROM AUTHOR]

Aim: For 80 years, popular opinion has held that most of Madagascar's terrestrial vertebrates arrived from Africa by transoceanic dispersal (i.e. rafting or swimming). We reviewed this proposition, focussing on three ad hoc hypotheses proposed to render this unlikely scenario more feasible: (a) Could hibernation have helped mammals to reach Madagascar? (b) Could the aquatic abilities of hippopotamuses have enabled them to swim the Mozambique Channel? (c) How valid is the Ali‐Huber model predicting that eastward Palaeogene surface currents allowed rafts to reach Madagascar in 3–4 weeks? Finally, we explored the alternative hypothesis of geodispersal via short‐lived land bridges between Africa and Madagascar. Location: East Africa, Madagascar, Mozambique Channel. Taxa: Fish, amphibians, reptiles, birds, mammals. Methods: We established colonization timeframes using molecular divergence dates estimated for Malagasy vertebrate lineages. We reviewed the likelihood of the "torpid waif" and "swimming hippopotamus" hypotheses, and re‐investigated Ali and Huber's model of Eocene jet‐like currents by tracking particle trajectories in currents simulated using the Institut Pierre‐Simon Laplace Earth System Model. Finally, we summarized recent geological findings from the Mozambique Channel, and used them to compile palaeosedimentological maps using PLACA4D. Results: Madagascar's vertebrate fauna has complex origins. Hibernation is probably an adaptation to Madagascar's hypervariable climate, rather than a facilitator of mammal dispersal. Hippopotamus physiology precludes the ability to cross an oceanic channel deeper than 4 m and hundreds of km wide. The Ali‐Huber model of Palaeogene currents considerably underestimated the time required to cross the Mozambique Channel under simulated palaeogeographic conditions. New geological data indicate the existence of three short‐lived land bridges between Africa and Madagascar at 66–60 Ma, 36–30 Ma and 12–05 Ma. Main conclusion: The three Cenozoic land bridges afford a more grounded hypothesis for the dispersal of Madagascar's extant biota than transoceanic rafting or swimming, although vicariance, island hopping and limited rafting also played a role. [ABSTRACT FROM AUTHOR]

DIVAnd (Data-Interpolating Variational Analysis, in n-dimensions) is a tool to interpolate observations on a regular grid using the variational inverse method. We have extended DIVAnd to include additional dynamic constraints relevant to surface currents, including imposing a zero normal velocity at the coastline, imposing a low horizontal divergence of the surface currents, temporal coherence and simplified dynamics based on the Coriolis force, and the possibility of including a surface pressure gradient. The impact of these constraints is evaluated by cross-validation using the HF (high-frequency) radar surface current observations in the Ibiza Channel from the Balearic Islands Coastal Ocean Observing and Forecasting System (SOCIB). A small fraction of the radial current observations are set aside to validate the velocity reconstruction. The remaining radial currents from the two radar sites are combined to derive total surface currents using DIVAnd and then compared to the cross-validation dataset and to drifter observations. The benefit of the dynamic constraints is shown relative to a variational interpolation without these dynamical constraints. The best results were obtained using the Coriolis force and the surface pressure gradient as a constraint which are able to improve the reconstruction from the Open-boundary Modal Analysis, a quite commonly used method to interpolate HF radar observations, once multiple time instances are considered together. [ABSTRACT FROM AUTHOR]

This study examines the effect of surface currents on the bulk algorithm calculation ofwind stress estimated using the scatterometer data during 2007–2020 in the Indian Ocean. In the study region as a whole, the wind stress decreased by 5.4% by including currents in the wind stress equation. The most significant reduction in the wind stress is found along the most energetic regions with strong currents such as Somali Current, Equatorial Jets, and Agulhas retroflection. The highest reduction of 11.5% is observed along the equator where the Equatorial Jets prevail. A sensitivity analysis has been carried out for the study region and for different seasons to assess the relative impact of winds and currents in the estimation of wind stress by changing the winds while keeping the currents constants and vice versa. The inclusion of currents decreased the wind stress (consistent with scatterometer winds) and this decrease is prominent when the currents are stronger. This study showed that the equatorial Indian Ocean is the most sensitive region where the current can impact wind stress estimation. The results showed that uncertainties in the wind stress estimations are quite large at regional levels and hence better representation of wind stress incorporating ocean currents should be considered in the ocean/climatic models for accurate air-sea interaction studies that are not based on remotely sensed winds.

Globally, coral reefs are degrading due to a variety of stressors including climate change and pollution. Active restoration is an important effort for sustaining coral reefs where, typically, coral fragments are outplanted onto degraded reefs. Coral outplants, however, can experience mortality in response to a range of stressors. We pair results of outplant monitoring observations with satellite‐based measurements of multiple oceanographic variables to estimate the relative importance of each driver to coral outplant survival. We find that when considering mean environmental conditions experienced by outplants during the monitoring period, particulate organic carbon (POC) levels are most important in determining outplant survival, with certain levels of POC beneficial for outplants. Sea surface temperature anomalies (SSTA) are also important determinants of outplant survival, where survival is greatest in regions with minimal or slightly negative anomalies. Survival also increases with increasing distance to land, likely due to a reduction in negative ridge‐to‐reef effects on coral outplants. When considering the range (min–max) of environmental conditions experienced during the monitoring period, large fluctuations in photosynthetically active radiation (PAR) and POC are most important in determining outplant survival. Increasing outplant depth can help to counter the negative impacts of large fluctuations in environmental variables. We find that a variety of remotely sensed oceanographic variables have significant impacts on survival and should be considered in coral restoration planning to help evaluate potential restoration sites and ultimately maximize coral outplant survival. [ABSTRACT FROM AUTHOR]

A two‐dimensional analytical model is presented for predicting the air‐gap magnetic flux density due to the permanent magnet (PM) and armature currents in brushless surface inset PM (BSIPM) machines. The virtual current theory is used to model the effects of the stator slots and rotor grooves. In this approach, the BSIPM machine is initially modelled as a machine without any stator slot and rotor groove, and the rotor groove and stator slot effects are modelled by a set of surface currents. By extracting the governing Laplace and Poisson equations and applying the boundary conditions, the magnetic flux density due to the PM and armature current is obtained in different parts of the machine. The accuracy of the proposed analytical model is confirmed by comparing its results with those of the finite‐element method and experimental results. The proposed model is also compared with the subdomain analytical model in terms of computational speed and accuracy. [ABSTRACT FROM AUTHOR]

The influence of ocean currents on marine population connectivity is critical to territory planning, and such phenomena should be considered in the design and implementation of marine protected areas (MPAs), marine spatial planning strategies, and restoration plans, among other developments. Knowledge of the influence of currents is also vital in understanding the relationship between oceanographic drivers and ecosystem configurations. However, despite their importance, ocean currents and their role in coral connectivity remain poorly constrained in the Seaflower Marine Reserve, an area that hosts the most productive open-ocean coral reef system in the Caribbean and that was declared a biosphere reserve in 2000. We herein characterize the larva transport patterns associated with surface currents that control connectivity in the reserve. To achieve this aim, we simulated the advection of buoyant coral larvae of Acropora palmata during nine spawning events. Larval dispersal patterns were obtained through offline coupling of a high-spatiotemporal resolution hydrodynamic field and a biophysical Lagrangian model for particle dispersion. Ocean current fields were generated using a Regional Ocean Modeling System (ROMS) that was appropriately configured for the region. Larval dispersion was simulated using an Individual-Based Model (Ichthyop). Our results show that there are heterogeneous connectivity patterns during the spawning events at seasonal and inter-annual scales. This seems to be associated with high spatiotemporal dynamic variability in the region, such as the Caribbean Current bifurcation close to the Nicaraguan Rise, the intrusion-formation of mesoscale and sub-mesoscale eddies, and the semi-permanent presence of the Panama-Colombia Gyre. We also found that Serranilla, Providencia, Quitasueño, and Serrana act as the most important sinks. In contrast, the northernmost reefs, Serranilla, B. Alicia, and B. Nuevo, seem to be the most important sources of larvae, highlighting that these areas need to be incorporated into the current MPA zonification and that this could lead to the improvement of MPA effectiveness. Our findings also suggest the need to implement MPA networks between Jamaica and Colombia to allow biological populations to become resilient to environmental changes and less prone to local extinctions.

High Frequency Radar (HFR) technology refers to land based remote sensing instruments capable of measuring surface currents and ocean waves at ranges up to 200 km or more. HFR technology is widely acknowledged as a cost-efficient tool to monitor coastal regions and has potential use to monitor coastal regions all over the world. Globally, the number of HFR stations is steadily increasing. Regional networks provide real-time data in support of operational activities such as search and rescue operations, fast response in case of maritime accidents and spill of pollutants, and resource management. Each operator needs a general understanding of the working principles in order to ensure that instruments are managed properly. A set of harmonized quality assurance and quality control procedures is recommended, along with an effective approach to HFR data discovery and dissemination, to provide high quality measurements to the end users. Different documents providing best practices for operation and maintenance have emerged in the past years. They are oriented either to Direction Finding (DF) or Beam Forming (BF) systems, or to specific manufacturer’s radar systems. The main objective of this paper is to offer a comprehensive “Best Practices” document in an effort of ensuring consistency between different deployments and harmonized operations of HFR systems. This, regardless of system manufacturer, antenna design and setup. A homogeneous approach is given when possible, general concepts and definitions are introduced in order to provide a framework for both data processing and management steps. Examples are also given from the European HFR users with focus on Near Real Time data flow suitable for operational services.

Coupling between the surface and near‐bottom currents in the Gulf of Mexico (GoM) has been reported in many case studies. However, geographical variations of this coupling need more examination. In this study, surface geostrophic currents derived from satellite‐observed sea surface height and subsurface currents from a collection of deep ocean moorings are used to examine the surface and bottom coupling in different parts of the GoM. The short‐period (30–90 days) fluctuations generated by the Loop Current (LC) and the LC eddies (LCEs) have a more vertically coherent structure and stronger deep ocean expressions than the long‐period fluctuations (>90 days). In addition, the strength of the coupling is modulated by the long‐period variations of the LC and LCE sheddings. Moreover, the surface and bottom coupling varies geographically. In the LC region, the surface fluctuations along the eastern side of the LC are important in causing the bottom current fluctuations through baroclinic instability under the LC and through traveling topographic Rossby waves (TRWs) north of the LC. In the central deep GoM, the bottom currents are affected by the upper fluctuations of the northern LC through both local baroclinic instability and remote TRW propagation. In the northwestern GoM, the bottom current fluctuations are largely related to the remote surface variability from the west side of the LC by TRWs propagating northwestward. This study will help us better understand mechanisms of the bottom current fluctuations that are important for the dispersal of deep ocean materials and properties. Plain Language Summary: Bottom currents in the Gulf of Mexico (GoM) are important in many ways, such as transporting fish larvae and spilled oil. However, in contrast to surface currents that are easily derived from the abundant satellite observations, our understanding of the bottom currents is very limited. In this study, the relationships between surface and near‐bottom current fluctuations in different parts of the GoM are investigated by combining a collection of historical near‐bottom current measurements and satellite data. The results show that the bottom current variability is linked to local processes such as current instability or remote processes through planetary wave propagation. The surface and bottom coupling is mainly related to the short‐period variability generated along the Loop Current and the Loop Current eddies and has a significant influence on the bottom currents. Although the long‐period fluctuations have weak bottom expressions, they can modulate the strength of the short‐period coupling. Studying the coupling between the surface and bottom current fluctuations in the GoM can advance our understanding of mechanisms of the bottom currents that are important for the deep ocean ecosystem and industrial operations. Key Points: Surface and bottom currents in the Gulf of Mexico can be coupled locally by baroclinic instability and remotely by topographic Rossby wavesFluctuations generated along the Loop Current have stronger deep ocean expressions at the period of 30–90 days than at longer periodsSurface and bottom coupling in the Gulf of Mexico varies geographically and is modulated by the Loop Current and the Loop Current eddies [ABSTRACT FROM AUTHOR]

Surface circulations off the Andaman Islands are least studied, much of the fundamental understanding of the circulations are from model studies. A pair of High-Frequency Radar (HFR) systems, as a part of the Indian Coastal Ocean Radar Network operated by the National Institute of Ocean Technology, MoES measures surface currents as far as 200 km from the coast since 2010. The present study focuses on the coastal circulation and its variability along the east coast of the Andaman Islands. The general circulation pattern in this region shows a poleward flow during December and January with November and February as transition months, and in the remaining months the flow is more or less equatorward. The current present in the HFR coverage is strongly influenced by the tides. Semidiurnal tidal constituents dominate surface current with a maximum variability of 24%. The amplitude of M2 component varied between ~5 and 10.5 cm s-1. The major tidal constituents matched well with tide gauge measurements in the study region. These findings point towards the strong influence of remote forcing on the currents along the east coast of the Andaman Islands. [ABSTRACT FROM AUTHOR]