The eastern English Channel is a highly energetic macrotidal region where sediment transport is driven by the interaction between tidal currents and wind forcing. A high-resolution three-dimensional hydrodynamic–sediment modelling system was calibrated and evaluated against multiple observational datasets, including satellite-derived suspended particulate matter, hydrodynamic fields, and seabed sediment maps. The model reproduced the main spatial patterns of sediment distribution and yielded realistic suspended sediment concentrations in both the surface and near-bed layers. The calibrated configuration was then used to analyse sediment transport during two contrasting winter periods: typical winter conditions (December 2021) and an extreme wind–tide co-occurrence event associated with Storm Eunice (February 2022). The results highlighted the key role of Cape Gris-Nez (CGN) in shaping local hydrodynamics in the Dover Strait, where strong tidal currents and flow constriction generated complex circulation patterns. A clockwise recirculation gyre developed in Wissant Bay during flood tide under both conditions, promoting sediment retention. Under typical conditions, surface concentrations remained below 0.1 g L ^-1 , and the suspended sediment was dominated by fine particles. During the extreme event, flood currents intensified markedly, whereas ebb currents remained comparatively weak, thereby increasing tidal asymmetry. The intensified flood currents increased sediment mobilisation, with near-bed concentrations locally exceeding 1 g L ^-1 and marked enrichment in the sand fraction throughout the water column; the sand fraction accounted for up to 70

The Red River (RR) plume region of the Gulf of Tonkin (GoT) plays an important role in driving coastal dynamic variability and regulating sediment and nutrient transport and budgets, and is therefore vital for coastal ecosystems and maritime activities. High-frequency radar (HFR) surface current measurements were used to characterize surface circulation and assess passive-tracer dispersion from August to December 2024, improving understanding of particle transport, dispersal, and fate in this region. The coastal circulation in the region, found to be strongly influenced by winds, tidal forcing, riverine input, and coastal bathymetry, exhibited a large spatio-temporal variability during the analysis period with the occurrence of small-scale structures, i.e., submesoscale eddies. The dispersion under varying forcing conditions and an extreme event-the typhoon Yagi-was analyzed by particle tracking and Lagrangian diagnostics. The results revealed that the dispersion within the RR plume region predominantly approached a Richardson super-diffusive regime after 24 h of tracking. Under the influence of typhoon Yagi, the dispersion was approximated by a ballistic regime after 12 h of tracking, with the spreading rate 10 times faster than that during normal conditions. In addition, the presence of Lagrangian Coherent Structures (LCSs), i.e., eddies next to the river outflow jets, coastal plume fronts, and zones of surface current convergence and divergence in the vicinity of river outlets, significantly influenced the dispersion behaviorof tracers in the RR plume region. Overall, this study pro-vides new insights into how coastal circulation and material dispersal in the RR plume respond across a wide range of weather conditions, including extreme events.

Coastal water dynamics along the Vietnamese middle southern coast (VMSC) region, part of the South China Sea, are highly complex with large spatiotemporal variability whose drivers are not yet well understood. For the first time, high-resolution surface current data from high-frequency radar (HFR) measurements were obtained in this region during the early (transition) phase of the Asian summer monsoon. The data were used for comparison with simulation results from a circulation model, SYMPHONIE, and ultimately to optimize the wind forcing in the model. Both modeling and HFR were able to show the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimize the wind forcing, namely the ensemble perturbation smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW). Both methods achieved a significant reduction (∼ 36 %–40 %) in the error of the surface current velocity fields compared to the measured data. Optimized winds obtained from the two methods were compared with satellite wind data for validation. The results show that both optimization methods performed better in the far field, where topography no longer affects the coastal surface circulation. The optimization results revealed that the surface circulation is driven not only by winds but also by other factors such as intrinsic ocean variability, which is not entirely controlled by boundary conditions. This indicates the potential usefulness of large velocity datasets and other data fusion methods to effectively improve modeling results.

Abstract. Surface current velocity measurements by high-frequency radars (HFR) were analyzed to characterize the surface circulation and to investigate the dispersion of passive tracers in the Red River (RR) plume region within the Gulf of Tonkin (GoT) from August to December 2024. The coastal circulation in the region, found to be strongly influenced by winds, tidal forcing, riverine input, and coastal bathymetry, exhibited a large spatio-temporal variability during the analysis period with the occurrence of small-scale structures, i.e., permanent submesoscale eddies. The dispersion under varying forcing conditions and an extreme event – the typhoon Yagi – was analyzed by particle tracking and Lagrangian diagnostics. The results revealed that the dispersion within the RR plume region predominantly followed a Richardson super-diffusive regime after 24 hours of tracking. Under the influence of typhoon Yagi, the dispersion quickly followed a ballistic regime after 12 hours of tracking, with the spreading rate ten times faster than that during normal conditions. In addition, the presence of Lagrangian Coherent Structures (LCSs), i.e., eddies next to the river outflow jets, coastal plume fronts, and zones of surface current convergence and divergence in the vicinity of river outlets, significantly influenced the dispersion behavior of tracers in the RR plume region.

In an attempt to reconciliate air-sea momentum flux estimates derived from open sea observations, from large eddy simulation output fields, and from wind-wave tank measurements, a series of dedicated experiments were conducted in the wind-wave tank of the Large Air-Sea Facility of Marseille, France. The turbulent friction velocity, upon which the momentum flux depends, was estimated from wind measurements by applying four classical methods including the eddy-covariance method and the inertial-dissipation method. The collected data were used to investigate some characteristics of the wave-influenced boundary layer that were predicted by previous simulations, and to quantify a wave-dependent term of the turbulent kinetic energy equation, the so-called imbalance term ϕimb. Our results show that the turbulent stress decreases toward lower heights where the effect of waves is large, as in the simulations, and that ϕimb is in the range 0.3 to 0.7, which is comparable to the value found with open sea data (0.4). These preliminary results have to be confirmed with wave-following probes, because the estimated eddy-covariance flux slightly varied with height, thus it could not be strictly considered to be equal to a constant total flux.

The study aims to assess turbulent dispersion processes in the coastal regions of northeastern Vietnam, in order to meet the major challenge of monitoring the fate of particulate materials in this part of the East Vietnam Sea, home to the most famous tourist sites and economic sites. The study area is strongly influenced by the freshwater discharge of the Red River, which creates a large river plume, greatly affecting coastal circulation and turbulent dispersion. The monsoon wind profoundly alters the dynamics of the river plume, pushing light surface water seaward over long distances, in summer, and toward the coast, in winter. Sea surveys were organized for the first time in this region in 2022 and 2023 to better characterize the processes controlling coastal flow variability and turbulent dispersion in the plume region and surrounding waters. Surface drifters, released in the plume region, were tracked during short periods of time, lfrom one to a few days. Current velocity profiling and CTD profiling have been also done. Estimates of the relative dispersion based on surface drifter measurements have revealed that the dispersion regime is local, mainly ballistic and Richardson, induced by turbulent eddies whose size does not exceed a few km. Local wind variability, combined with variations in bathymetry, considerably affects the transport pathways of real drifters and modifies the dispersion regime. A coastal circulation model was used to better assess dispersion processes over the entire study area and for a wide range of variability in the main forcing terms. Virtual surface drifters were tracked in the model velocity field during the surveying periods. The results revealed that, on scale of several days, the transport of passive tracers is considerably affected by irregularities in current velocity fields associated with zones of current convergence and divergence. The results also demonstrated that merging observations with model outputs significantly improves the representation of small scale features of current variability, turbulent mixing, and horizontal stirring of tracers in the plume region.

The eastern English Channel (EEC) experiences a dynamic interplay of extreme events, including powerful winds, tides, and complex bottom relief features, collectively shaping the region's hydrosedimentary transport dynamics. Extreme winds, frequently observed in the EEC, play a pivotal role in influencing surface currents and wave patterns. Coupled with strong tidal forcing, these events lead to intricate interactions with the seafloor topography, creating a complex hydrodynamic environment. The resulting effects on sediment transport are significant, with the potential for altered erosion and deposition patterns along the coastal areas of the EEC. Understanding this multiscale interaction is crucial for predicting and managing the impact of extreme events on hydrosedimentary transport, contributing to effective management of coastal ocean environment. In order to study all these processes, we used a 3D hydrodynamic model (MARS3D, IFREMER) coupled with a sediment transport model (MUSTANG, IFREMER) and calibrated it for the English Channel. This model employs nested grids of different extents and resolutions: (i) the northwest European continental shelf area with a 5 km resolution, (ii) the English Channel area with a 1 km resolution, and (iii) local zones along the eastern coast of the English Channel with a 100m resolution. The larger model transfers boundary conditions to the higher-resolution model. Such a cascade of resolutions allows for the consideration of both large-scale hydrodynamic processes and the replication of smaller-scale processes (eddies, turbulent current oscillations). Four fractions of suspended matter, including two size classes of mud and two size classes of fine sand, were specifically chosen for modeling based on in-situ data. The model then has been validated using the data collected in 2020-2021. The model can be employed to replicate the dynamics and sedimentary processes over a multi-year period. This enabled the estimation of the quantity and flux of suspended matter, as well as potential changes in the sediment transport regime induced by extreme weather conditions. The modeling outcomes highlighted tides as the primary driving force behind hydrosedimentary transport, surpassing the influence of wind. This effect is evident in the formation of eddies near capes, emerging for a few hours after the onset of the tide phase. These eddies create a counterflow of matter along the French coasts, opposing the general current direction that moves toward the North Sea. In the regions characterized by the highest velocities in offshore areas, larger particles (sands) prevail within the eddies, while the slower sections closer to the coast are predominantly composed of finer mud particles. Additionally, the wind introduces instabilities in the current structure, leading to an increased resuspension of fine sediments.

The transport and spatial distribution of materials within river plumes and neighboring waters are strongly influenced by the plume morphology and fronts bounding the plume area. Thus, identifying locations of plume fronts and assessing frontal mixing activity is particularly important for understanding the spatial distribution of biogeochemical tracers and the connectivity between source (river mouth) and offshore regions. Statistical properties of horizontal dispersion in the Red River (RR) plume region were obtained from drifter experiments, conducted in the Gulf of Tonkin during summer 2022, and also from realistic numerical modeling. Adopting a Lagrangian perspective, a method is proposed to characterize the RR plume morphology based on the extraction of Lagrangian Coherent Structures (LCS) from the surface flow field using Finite-Size Lyapunov Exponents (FSLE). Maxima of the attracting FSLE field (FSLE ridge lines) enabled us to identify zones of current convergence and shear in the flow, which are essential for characterizing the dynamics associated with plumes and mapping the fronts delimiting them. It became clear that the body of the RR plume is formed by the individual river plumes of the RR deltaic system. FSLE ridge lines allowed identification of the offshore expansion of individual plumes and revealed a variety of forms, semi-circular or hook-like filaments, characterizing the plume shape. The ballistic regime of dispersion was found inside the plume region, with the dominant direction perpendicular to the coastal flow direction revealing anisotropic shear dispersion. It was observed at scales of 0.3 to 3 km (submesoscale range), with a maximum value of 10-13 day-1, as derived from the analysis of the scale dependent relative dispersion of real drifters. In model simulations, the relative dispersion appeared scale independent in the range below 2 km, indicating a significant effect of grid resolution on the model's representation of lateral mixing.

The southeastern Bay of Biscay has been described as a "dead end" for floating marine litter, often accumulating along small-scale linear streaks. Coastal Current Convergence Structures (CCS), often associated with vertical motions at river plume edges, estuarine fronts, or other physical processes, can be at the origin of the accumulation. Understanding the formation of CCS and their role in the transport of marine litter is essential to better quantify and to help mitigate marine litter pollution. The Lagrangian framework, used to estimate the absolute dispersion, and the finite-size Lyapunov exponents (FSLE), have proved very effective for identifying CCS in the current velocity field. However, the quality of CCS identification depends strongly on the Eulerian fields. Two surface current velocity data sets were used in the analysis: the remotely sensed velocities from the EuskOOS High-Frequency Radar (HFR) network and velocities from three-dimensional model outputs. They were complemented by drifting buoy velocity measurements. An optimization method, involving the fusion of drifting buoys and HFR velocities is proposed to better reconstruct the fine-scale structure of the current velocity field. Merging these two sources of velocity data reduced the mean Lagrangian error and the Root Mean Square Error (RMSE) by 50 % and 30 % respectively, significantly improving velocity reconstruction. FSLE ridgelines obtained from the Lagrangian analysis of optimized velocities were compared with remotely sensed concentrations of Chlorophyll-a. It was shown that ridgelines control the spatial distribution of phytoplankton. They fundamentally represent the CCS which can potentially affect marine litter aggregation. Analysis of the absolute dispersion revealed large stirring in the alongshore direction which was also confirmed by spatial distribution of FSLE ridgelines. The alignment between FSLE ridgelines and patterns of high Chlorophyll-a concentration was observed, often determining the limits of river plume expansion in the study area.

Transport and dispersion processes in the ocean are crucial, as they determine the lifetime and fate of biological and chemical quantities drifting with ocean currents. Due to the complexity of the coastal ocean environment, numerical circulation models have difficulties to accurately simulate highly turbulent flows and dispersion processes, especially in highly energetic tidal basins such as the eastern English Channel. A method of improving the results of coastal circulation modeling and tracer dispersion in the Dover Strait is proposed. Surface current velocities derived from Lagrangian drifter measurements in November 2020 and May 2021 were optimally interpolated in time and space to constrain a high-resolution coastal circulation MARS model, with careful attention given to selecting ensemble members composing the model covariance matrix. The space-time velocity covariances derived from model simulations were utilized by the optimal interpolation algorithm to determine the most likely evolution of the velocity field under constraints provided by Lagrangian observations and their error statistics. The accuracy of the velocity field reconstruction was evaluated at each time step. The results of the fusion of model outputs with surface drifter velocity measurements show a significant improvement (by similar to 50 %) of the model capability to simulate the drift of passive tracers in the Dover Strait. Optimized velocity fields were used to quantify the absolute dispersion in the study area. The implications of these results are important, as they can be used to improve existing decision-making support tool or design new tools for monitoring the transport and dispersion in a coastal ocean environment.

The middle southern coasts of Vietnam (11-15ºN) are featured by a large upwelling in summer which is governed by various physical processes i.e. regional circulations, monsoonal wind, and tidal rectification. Through the HF radar measurements acquired during April-May, 2019, a transition period of the Asian monsoon, three prominent patterns of local circulation have been revealed: a northern current intrusion as a result of South China Sea Western Boundary Currents (SCSWBC); a strong southeast jet occurring at ~30km from the coast; and a seaward current appearing for several days. In order to acquire a better understanding of the regional hydrodynamics, a 3D circulation model SYMPHONIE was run covering the measurement periods of HF radar and AWAC. Sea surface current (SSC) time series obtained from HF radar and AWAC were found to be strongly correlated (R=0.58, RMSE=0.10, MAE=0.07 for u- and R=0.72, RMSE=0.13, MAE=0.11 for v-components) (Fig 1). Three subdomains (nearshore, middle-range and far-range) have been identified within the radar coverage region for detailed analysis and comparison with the model. The comparison revealed that during the first 20 days, when the SE wind prevailed, the model does not seem to capture well the variability of SSC (Fig 2). However, when the wind changed to S-SW direction, the model results show a good agreement with HF radar measurements. Besides, an eddy forming near the coast on April 16 has been well represented by both model and HF radar (Fig 3). The uncertainty of modeled velocity fields can result from the uncertainty in forcing fields. Thus, the next step of our research will be to optimize the forcing field (i.e., wind) using HF radar data with an expectation to achieve better model results of SSC variability. Figure 1 Comparison of SSC time series from HF radar (black solid line) and AWAC (red asterisk points)Figure 2 Space averaged U- and V-velocity components of SSC from SYMPHONIE model (dash line) and HF radar measurements (solid line) in three subdomains: nearshore (a, d); middle range (b, e) ; and far range (c, f). Wind vector during the measurement period (g).Figure 3 An eddy in SYMPHONIE model simulation (left) and HF radar measurements (right) on April 16, 2019. Color scale represents curl of surface velocity superimposed by SSC vectors. Isobaths are shown by black lines.

A method of assessing the mean eddy viscosity profile (EVP) in the sea surface boundary layer (SBL) under variable wind conditions is proposed. Performance of the method is tested using observations by an ADCP-equipped platform in the coastal environment of the northwestern Mediterranean Sea under variable (3–12 m s −1 ) wind conditions. EVP retrievals are made by a variational method strongly constrained by the Ekman dynamics, with the wind and velocity observations assumed to be uncertain within the prescribed error bars. Results demonstrate a reasonable agreement of the EVPs with KPP shape functions for stronger (8–12 m s −1 ) wind conditions and appear to be consistent with the classical Pacanowski–Philander parameterization of the viscosity profile based on the Richardson number. For weaker (3–5 m s −1 ) winds, the EVP retrievals turn out to be less accurate, which is primarily attributed to the decay of the wind-driven turbulence energy in the SBL. Feasibility and prospects of the retrieval technique are discussed in the context of uncertainties in the structure of the background flow and limitations of the microstructure and ADCP profiling.

Near-ground measurements by Sonic anemometer were performed during 2 years at a nearshore experimental site in Dunkirk, France for automatic detection of sea-breeze (SB) and nocturnal low-level jet (NLLJ) events. The SB detection is based on a recurrent neural network algorithm (RNN) with the accuracy of event identification equal to 95%. We found 67 and 78 SB days in 2018 and 2019 respectively. NLLJ detection algorithm uses wavelet transformation and shows better performance than the known existing methods. A total of 192 and 168 NLLJ days were found in 2018 and 2019 respectively. Our estimations show that during the NLLJ event, the peak power production can increase up to 40 times, compared to normal days. To evaluate the skill of detection algorithms, sonic and lidar wind measurements were performed simultaneously. The comparison shows a good agreement and highlights that Sonic anemometer measurements are very efficient for meteorological event detection.

This study focused on the detection of mesoscale meteorological phenomena, such as the nocturnal low-level jet (NLLJ) and sea breeze (SB), using automatic deterministic detection wavelet technique algorithms (HWTT and SWT) and the machine learning recurrent neural network (RNN) algorithm. The developed algorithms were applied for detection of NLLJ and SB events from ultrasonic anemometer measurements, performed between January 2018 and December 2019 at a nearshore experimental site in the north of France. Both algorithms identified the SB and NLLJ days successfully. The accuracy of SB event detection by the RNN algorithm attained 95%, and we identified 67 and 78 SB days in 2018 and 2019, respectively. Additionally, a total of 192 and 168 NLLJ days were found in 2018 and 2019, respectively. To demonstrate the capability of the algorithms to detect SB and NLLJ events from near-ground ultrasonic anemometer measurements, analysis of the simultaneous wind lidar measurements available for 86 days were performed. The results show a good agreement between the RNN-based detection method and the lidar observations, detecting 88% of SB. Deterministic algorithms (HWTT and SWT) detected a similar number of NLLJ events and provided high correlation (0.98) with the wind lidar measurements. The meteorological phenomena studied can significantly affect the energy production of offshore wind farms. It was found that the maximum hourly average peak power production could be to 5 times higher than that of the reference day due to higher wind speed observed during NLLJ events. During SB events, hourly average peak power production could be up to 2.5 times higher. In this respect, the developed algorithms applied for analysis, from near-ground anemometer measurements, may be helpful for monitoring and forecasting the meteorological phenomena capable of disturbing the energy production of offshore wind turbines.

Particle pair statistics from synthetic drifter trajectories reconstructed from realistic, high-resolution numerical simulations (SYMPHONIE model) and HF radar velocity measurements are used to investigate the dispersion properties in the Gulf of Tonkin (GoT). This study takes an approach based on two-particles statistics providing the relative dispersion, relative diffusivity and Finite Size Lyapunov Exponent (FSLE) estimates. In the GoT, the relative dispersion follows the predictions from the theory of two-dimensional turbulence with two inertial subranges identified in the kinetic energy spectrum with the spectral slopes −5/3 and −3. The time evolution of dispersion shows an exponential growth during 5–8 days, followed by a power law regime during the next 6–20 days. Fixed-length indicators from the relative diffusivity and the FSLE reveal a local dispersion at large and intermediate scales (above Rossby radius of deformation) and non-local dispersion in sub-mesoscale range (below Rossby radius of deformation). The effect of river runoff on the local hydrodynamics and dispersion processes is assessed using the numerical model simulations without river discharge. The results show that in the model, the river plume, when present, highly impacts the Lagrangian statistics. High gradients of buoyancy reinforce the sub-mesoscale circulation in a large region along the Vietnamese coast and modify the scales and intensity of turbulent dispersion. However, a clear change of dispersion regime in the sub-mesoscale range is not identified, suggesting that the mesoscale circulation in the GoT largely governs particle spreading even at small scale.

Characteristics of the coastal surface currents in Vietnam are often challenging to be well-described due to the lack of reliable data. Thus, a remote sensing technology – high-frequency (HF) WavE RAdar (WERA) has been implemented to monitor the surface currents along the mid-southern coast of Vietnam, which is largely influenced by the strong South China Sea Western Boundary current system (SCSWBC). The HF radar data obtained from a measurement campaign in May, 2019 were checked for outliers and noise by RMSE threshold. Thereafter, the velocity fields were reconstructed using 2DVar-EOF gap filling and interpolation method [1], [2] for further analysis. The reconstructed surface current maps showed that the main current direction in this area during summer was northeast and the flow was driven by the southwest monsoon which usually starts from May [3]. The eastern sector of the domain was noticed with remarkably strong northeast current. At the onset of summer monsoon, the jet was weaken and deviated, thus, strongly modified the dynamics of the region [4]. A current divergence was observed in the northeastern and eastern parts of the study area. Some other interesting features of surface circulation were revealed during the second week of measurements, on 9-10 of May. Those features are characteristics of the east Vietnam cyclonic eddy and the southeastward offshore current [3]–[5]. On May 9, the cyclonic eddy expanded further to the south and prevailed over the entire area within a week. Spectral analysis was performed to identify the dominant frequencies of the surface current variability. The results demonstrated the capability of HF radars in monitoring sub- and mesoscale motions of near-shore surface currents.

The nocturnal low-level jet (LLJ) and orographic (gravity) waves play an important role in the generation of turbulence and pollutant dispersion and can affect the energy production by wind turbines. Additionally, gravity waves have an influence on the local mixing and turbulence within the surface layer and the vertical flux of mass into the lower atmosphere. On 25 September 2017, during a field campaign, a persistent easterly LLJ and gravity waves were observed simultaneously in a coastal area in the north of France. We explore the variability of the wind speed, turbulent eddies, and turbulence kinetic energy in the time–frequency and space domain using an ultrasonic anemometer and a scanning wind lidar. The results reveal a significant enhancement of the turbulence-kinetic-energy dissipation (by 50%) due to gravity waves in the LLJ shear layer (below the jet core) during the period of wave propagation. Large magnitudes of zonal and vertical components of the shear stress (approximately 0.4 and 1.5 m2 s−2, respectively) are found during that period. Large eddies (scales of 110 to 280 m) matching the high-wind-speed regime are found to propagate the momentum downwards, which enhances the mass transport from the LLJ shear layer to the roughness layer. Furthermore, these large-scale eddies are associated with the crests while comparatively small-scale eddies are associated with the troughs of the gravity wave.

The surface circulation in the Gulf of Tonkin (GoT) was analyzed using 2.5 year-long dataset from the High-Frequency radar (from April 2014 to October 2016). High temporal resolution of the measurements and large coverage from HFR dataset enable us to characterize the variability of surface circulation in the GoT in a wide range of scales: from tidal to annual scale. A number of techniques of data including rotary spectral analysis (RSA), principal component analysis (PCA), harmonic tidal analysis, coherent analysis, etc. were used to identify the dominant modes of variability. The tidal motions, accounting for approximately 62% of the total variability, revealed the dominance of diurnal components (K1 and O1) with 4 times larger magnitude than that of semi-diurnal constituent (M2). At seasonal scale, the monsoon wind plays an important role in driving the surface circulation in the GoT. This was supported by a tight correlation (0.7) between the wind stress and current velocities and by a large contribution (more than 50%) of the Ekman-driven component to the total variability of currents in the offshore area. Along the shore, large seasonal variability of circulation was highlighted. During the year, the seaward extension of the coastal current is primarily controlled by the cross-shore wind stress while the flow intensity is modulated by the Red River discharge.

The sea surface velocity measurements obtained during the period of 2014–2016 using two high-frequency radars (HFR), which were installed on the northern coast of Vietnam, were reprocessed by using a variational approach (EOF/2dVar) to provide an extended dataset. The high temporal resolution of measurements and large spatial coverage of the radar data enabled the assessment of the surface circulation in the data-poor region of the southern Gulf of Tonkin (GoT) and the characterization of its temporal variations in a wide range of scales: from tidal frequency to annual cycle. It was found that tidal motions account for a large percentage (approximately 60%) of surface current variability in the GoT. The temporal variability of the kinetic energy of the surface currents is tightly correlated with wind variations in the monsoon-dominated atmospheric circulation system. Stratification, induced by the discharge of the Red River, also affects the tidal currents within the coastal region (~ 40 km wide) by modifying the tidal ellipses polarizations. Close to the shore, the prominent coastal flow experiences large temporal variations in response to wind and buoyancy forcing. The seasonal variability of the zonal wind component was found to play a significant role in coastal flow speed variations through the cross-shore mass exchange. The Red River freshwater discharge controls the seaward extension of the coastal flow and causes asymmetry in the seasonal variability of flow velocity. The data obtained and analysis results could be useful for the improvement of the regional circulation models, for further studies of the physical and biological processes in the GoT, and for marine resource management.