Caractérisation de la dispersion de traceurs passifs dans un écoulement côtier soumis à un régime macrotidal (Étude d'impact de la dynamique sur la qualité de l'eau le long du littoral du Nord-Pas de Calais et de la Picardie)

Caractériser la circulation et l'évolution de la qualité des eaux côtières en Manche orientale est un enjeu majeur pour de multiples applications dans une région riche en bio-diversité et en ressources halieutiques. L'hydrodynamisme, dominé par un régime megatidal, joue un rôle essentiel dans les processus de dispersion et de mélange le long du littoral du Nord-Pas-de-Calais. La physionomie en "entonnoir" de la Manche orientale donne lieu à une dyssimétrie de l'onde de marée qui induit un flot plus intense et plus bref que le jusant, engendrant une dérive résiduelle vers la mer du Nord accentuée par les vents dominants de sud-ouest. Ce contexte atypique confère une structure complexe à l'écoulement côtier, dans une région dite "à influence d'eau douce". L'utilisation du modèle hydrodynamique MARS3D a permis de mettre en évidence certains phénomènes complexes à l'échelle régionale et comprendre la gense et la dynamique du fleuve côtier. Grâce à la modélisation haute résolution, à l'échelle dite locale, la circulation dans l'enceinte du port de Boulogne-sur-Mer a pu être caractérisée. Il a été démontré qu'un tourbillon anti-cyclonique, caractéristique du régime de flot, permet une évacuation rapide des traceurs passifs transitant par le port. Lors du jusant, une dérive intense vers le sud piège les traceurs dans le port. Ce piégeage peut être accentué ou atténué selon le sens du vent ou le débit fluvial. Une étude de modifications morphologiques de la rade a été réalisée afin de montrer l'utilité de la modélisation pour proposer des solutions innovantes pouvant améliorer la qualité de l'eau. Un travail similaire a été mené en baie de Somme pour comprendre les problèmes relatifs à la qualité de l'eau. Certaines activités sont soupçonnées d'être à l'origine de contamination microbienne et en particulier les moutons de pré-salés, accusés de contribuer majoritairement à l'apport en E. Coli. En assimilant les traceurs passifs au matériel bactérien, nous avons pu appréhender la problématique de la qualité de l'eau. Notre étude a permis de mettre en exergue les conditions environnementales propices à la rétention ou à l'évacuation de matières contaminantes émises par de nombreuses sources alimentant la baie. La convergence des courants, et l'accumulation du matériel contaminant, ont été révélées au voisinage du Crotoyce qui peut expliquer en partie, la qualité médiocre de ces eaux. Ces schémas de circulation interne à la baie de Sommesuivant les conditions environnementales contribue à améliorer les études sur la qualité de l'eau et les futurs aménagements côtiers durables.Characterizing the circulation and the coastal water quality evolution in the EEC is a major stake in many applications, particularly in a such bio-diversified region. The hydrodynamism plays an essential role in dispersal and mixing processes along the Nord-Pas-de-Calais shores. The EEC funnel physiology gives place to a tidal asymmetry which induces flood conditions stronger but shorter then ebb conditions, involving a residual northern drift, strengthened by the dominant south-west winds. This atypical context confers a complex structure of the coastal flow in a ROFI (Region Of Freshwater Influence). Using the hydrodynamical model MARS3D permitted to underline complexes phenomenon at regional scale and understand the genesis and dynamic of the so called, "coastal river". Thanks to high resolution modelling, circulation pattern inside Boulogne-sur-Mer harbor has been characterized. We identified an anti-cyclonic eddy, characteristic of the flood conditions and responsible of a rapid evacuation of tracers passing through the harbor. In opposition, during ebb, a southward drift traps passive tracers inside the port. This trapping effect can be limited or emphasized depending on wind directions and freshwater inputs. Besides, harbor morphological changes have been assessed in order to propose innovative solution to improve water quality inside the harbor. A similar work was developed in the "baie de Somme" (France) to understand water quality existing issues. Many activities are suspected to supply microbial contamination within the bay, especially the marshes sheep accused to be the major contributor in E. Coli. Assuming passive tracers as bacterial material we have studied dispersion and transport processes within the bay. This study permit to underline environmental conditions favorable to evacuate or maintain pollution emitted from several water sources supplying the bay. A current converging zone has been shownsurrounding the Crotoy area (were the water quality is bad), and simulations reveal an accumulation place for most of the different water bodies relative to the fresh water inputs. This work on circulation pattern in the Bay of Somme regarding environmental conditions, contribute to improve water quality studies and further sustainable coastal managements.DUNKERQUE-SCD-Bib.electronique (591839901) / SudocSudocFranceF

Fusion of Lagrangian drifter data and numerical model outputs for improved assessment of turbulent dispersion

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 ∼ 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.</p

Surface circulation characterization along the middle southern coastal region of Vietnam from high-frequency radar and numerical modeling

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.</p

HF Radar activity in European coastal seas: next steps toward a Pan-European HF Radar network

High Frequency Radar (HFR) is a land-based remote sensing instrument offering a unique insight to coastal ocean variability, by providing synoptic, high frequency and high resolution data at the ocean atmosphere interface. HFRs have become invaluable tools in the field of operational oceanography for measuring surface currents, waves and winds, with direct applications in different sectors and an unprecedented potential for the integrated management of the coastal zone. In Europe, the number of HFR networks has been showing a significant growth over the past 10 years, with over 50 HFRs currently deployed and a number in the planning stage. There is also a growing literature concerning the use of this technology in research and operational oceanography. A big effort is made in Europe toward a coordinated development of coastal HFR technology and its products within the framework of different European and international initiatives. One recent initiative has been to make an up-to-date inventory of the existing HFR operational systems in Europe, describing the characteristics of the systems, their operational products and applications. This paper offers a comprehensive review on the present status of European HFR network, and discusses the next steps toward the integration of HFR platforms as operational components of the European Ocean Observing System, designed to align and integrate Europe's ocean observing capacity for a truly integrated end-to-end observing system for the European coasts

The Global High Frequency Radar Network

Academic, government, and private organizations from around the globe have established High Frequency radar (hereinafter, HFR) networks at regional or national levels. Partnerships have been established to coordinate and collaborate on a single global HFR network (http://global-hfradar.org/). These partnerships were established in 2012 as part of the Group on Earth Observations (GEO) to promote HFR technology and increase data sharing among operators and users. The main product of HFR networks are continuous maps of ocean surface currents within 200 km of the coast at high spatial (1–6 km) and temporal resolution (hourly or higher). Cutting-edge remote sensing technologies are becoming a standard component for ocean observing systems, contributing to the paradigm shift toward ocean monitoring. In 2017 the Global HFR Network was recognized by the Joint Technical WMO-IOC Commission for Oceanography and Marine Meteorology (JCOMM) as an observing network of the Global Ocean Observing System (GOOS). In this paper we will discuss the development of the network as well as establishing goals for the future. The U.S. High Frequency Radar Network (HFRNet) has been in operation for over 13 years, with radar data being ingested from 31 organizations including measurements from Canada and Mexico. HFRNet currently holds a collection from over 150 radar installations totaling millions of records of surface ocean velocity measurements. During the past 10 years in Europe, HFR networks have been showing steady growth with over 60 stations currently deployed and many in the planning stage. In Asia and Oceania countries, more than 110 radar stations are in operation. HFR technology can be found in a wide range of applications: for marine safety, oil spill response, tsunami warning, pollution assessment, coastal zone management, tracking environmental change, numerical model simulation of 3-dimensional circulation, and research to generate new understanding of coastal ocean dynamics, depending mainly on each country’s coastal sea characteristics. These radar networks are examples of national inter-agency and inter-institutional partnerships for improving oceanographic research and operations. As global partnerships grow, these collaborations and improved data sharing enhance our ability to respond to regional, national, and global environmental and management issues

The impact of high-frequency current variability on dispersion off the eastern Antarctic Peninsula

We present observations of high-frequency current variability on the continental shelf and the slope of the Antarctic Peninsula using Lagrangian surface drifters deployed as part of the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project. Here we focus on high-frequency processes such as tides and inertial oscillations that are typically smoothed out of large-scale spatially averaged, and/or temporally averaged, observed current fields. We investigate the role that this class of motion plays in the transport of physical or biogeochemical properties. Lateral displacements on the shelf and slope are found to be larger than displacements in deeper waters where tidal currents are negligible. We apply this result in a parameterization of the lateral dispersion during an off-line drifter modeling study. The outcome is an improvement on the modeling of Lagrangian drifting particles compared with a standard random walk scheme