Utilisation de mesures de courants cotiers superficiels par radar HF pour valider et optimiser les modeles numeriques de circulation littorale en mer a maree

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Contrôle des conditions aux frontières ouvertes d'un modèle de circulation côtière avec une méthode variationnelle d'assimilation de données (application au Golfe du Lion)

La dynamique de la circulation marine en région côtière peut être appréhendée par des moyens d'observation et de modélisation. Si le domaine côtier bénéficie de réseaux de mesure adaptés en résolution et en couverture, la mise en oeuvre d'un outil de modélisation s'avère délicate, particulièrement dans la spécification de conditions initiales et de conditions aux frontières ouvertes appropriées. Cette question de l'estimation des conditions aux limites peut être considérée comme relevant d'une problématique plus générale de détermination des paramètres de fonctionnement d'un modèle à partir de l'information apportée par les observations. Ainsi posé, le problème est abordé par l'outil mathématique du contrôle optimal qui conduit à une opération d'assimilation de données par méthode d'adjoint. L'adaptation et l'application de cette formulation inverse à un problème de modélisation océanographique contrôlé par ses conditions frontières font l'objet de ce travail de thèse. Une approche 4D-variationnelle d'assimilation pour le contrôle des conditions frontières est ainsi développée et implémentée sur la base du modèle aux équations primitives OPA (version toit rigide') et de son adjoint existant. L'algorithme d'inversion est validé sur trois expériences d'identification : (i) les perturbations du forçage barotrope sur la frontière ouverte d'un modèle non stratifié sont détectées par l'assimilation de courants de surface pris à l'intérieur du domaine ; (ii) le développement d'un courant de densité dans un canal semi infini et homogène est retrouvé à partir de profils hydrologiques distribués en aval ; (iii) l'observation du " spin up " d'une circulation de plateau induit par un courant côtier permet d'identifier les conditions amonts prévues par le mécanisme de séparation d'un front de Kelvin barocline en incidence sur un talus. Cette méthode d'assimilation est alors proposée pour des applications s'inscrivant dans la problématique scientifique des échanges côte - large sur la zone du Golfe du Lion. Riche en observations et largement ouverte sur le bassin méditerranéen occidental, cette région est soumise à l'influence du courant Liguro-Provençal qui s'écoule en bord de plateau. L'algorithme d'inversion permet de contrôler cette influence régionale sur la circulation de plateau ; il est donc évalué à travers des expériences réalistes menées dans le mode diagnostique. En particulier, la faculté de réseaux d'observation à détecter des épisodes d'intrusion du courant côtier sur le golfe est traitée suivant leur couverture spatiale et temporelle.The marine circulation dynamics in coastal zones can be assessed by observation and modelisation. If coastal domains are quantitatively observed (routine remote sensing, periodic in situ surveys, long term measurements), consistent model set up remain relatively elusive, especially for the prescription of appropriate initial and open boundary conditions. This issue raised by the specification of model boundary conditions is proposed to be posed in the optimal control framework, which leads to a data assimilation problem solved by an adjoint method : these model parameters are determined according to information from observations. A four-dimensional variational (4DVAR) assimilation method for open boundary control is then developed and implemented on the basis of the primitive equation circulation model OPA and its adjoint. The algorithm is validated with three academic twin experiments: (i) the detection of perturbations on the barotropic forcing along the open boundary of a non-stratified model are obtained by assimilating surface currents taken inside the domain ; (ii) the development of a density current inside a semi-infinite homogeneous channel is retrieved from a synthetic data set figuring density profiles distributed downstream; (iii) the observation of the spin up of a coastal circulation induced by a baroclinic Kelvin front incident to a shallow shelf topography permits to identify the upstream boundary conditions linked to the separation at the shelf break. The Gulf of Lions, which is largely opened to the north-western Mediterranean basin, appears to be an appropriate study case to investigate mechanisms and magnitudes of coastal lateral export to the open ocean. The assimilation method is applied in situations of regional influence on the gulf circulation. It is assessed in the hind cast mode for monitoring applications, especially on the data related aspect. The regional forcing provided by the Liguro-Provençal current flowing along the shelf break is controlled to study the ability of the available observational networks to detect transient cross shelf intrusions according to their spatial and temporal coverage.TOULON-BU Centrale (830622101) / SudocSudocFranceF

Etude expérimentale des instabilités méso à sub-méso échelle du courant Nord Méditerranéen

En océanographie les flux de matière sont très abondamment étudiés depuis longtemps, car ils contrôlent les flux biogéochimiques qui forcent les écosystèmes marins. Les récents problèmes de pollutions en mer rendent cruciale la connaissance du trajet et de la dilution des divers contaminants. L'un des processus majeurs de flux de matière sont les échanges côte large à travers un talus continental sont les tourbillons. Deux algorythmes de détection et identification des paramètres des tourbillons ont été développés en prenant en compte les spécificités de l'étude. L'un, validé par les cartes radar servant de références l'autre, restant à parfaire, sur une analyse temps-fréquence de série temporelles de vitesse. Les campagnes ECOLOPHY ont permis d'identifier les tourbillons de sub-méso échelle. La plupart sont confirmés par l'analyse du champ de température à l'horizontale et à la verticale. Les gyres détectés sont de type anticyclonique. ils sont positionnés entre le Courant nord méditerranéen et la côte, avec une taille allant de 12 à 28 lm de diamètre.Matters fluxes have been studied in details for a long time because they control biogeochemical fluxes which influence marine ecosystems. The knowledge of the path and the dilution of contaminants is made critical in case of sea pollution problems. Oneof the major processes which controls matter flux is the coastal exchanges through the continental shelf vy vortices. two zlgorithms of detection and identification of vortex parameters were developed, taking into account the specificities of the study. The first one, validated by radar maps acting as a reference, is based on a classical kinmatic method, the second one, which could be improved is based on a time-frequency analysis of velocities time series. ECOLOPHY campaigns enaled us to identify submesoscale vortices. Most of them were confrimed by the analysis of the horizontal end vertical temperature field. they are located between the Nothern Current and the coast, their diameter size anges from 12 to 28 km.AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

Impact of cross-reef fluxes on the Ouano lagoon circulation.

International audienceIn meso-tidal lagoons the coral reef barrier can be temporarily covered by water at high tide and emerged at low tide, inducing particular cross-reef dynamics. In order to understand the importance of crossreef fluxes on lagoon dynamics a field survey had been conducted on Ouano Lagoon (New Caledonia). The lagoon water dynamics were analyzed and compared to a numerical model output. The main current was found to be northward during the flood, with inward flows through the ocean passes and through the south passage and outward flows through the north passage. During the ebb, currents are usually reversed. Concerning tidally averaged fluxes, they are mainly inward at the ocean passes and southward in passages. Finally, numerical tests were performed to evaluate the impact of cross-reef fluxes on this circulation. The results of this study are, then, discussed

Submesoscale vortex structures at the entrance of the Gulf of Lions in the Northwestern Mediterranean Sea

The meanders of a baroclinic coastal current in the Northwestern Mediterranean Sea have already been reported in the literature. These meanders can be surrounded by vortices. Such vortices have been observed in the western part of the Gulf of Lions but the location and the mechanism of their formation are poorly documented. In this paper, we use the current measurements of a one-year experiment, which was conducted in the eastern part of the Gulf of Lions to detect and characterize the vortex activity. A vortex detection algorithm based on few velocity data was developed. Current measurements were available at the sea surface (HF radars) and in the water column from 50 to 140 m depth (four current meter moorings). SST images and hydrologic data were also used. Results focus on observations that are coherent 50 m and at the surface. Vortices are anticyclonic, of submesoscale size and present maximal velocities of 30-50 cm/s. The drift speed of the vortices is comparable to but less than the velocity of the Northern Current. These observations enable to estimate the minimum vortex occurrence in this area. The presence of vortex structures is strongly correlated with a specific sequence of wind patterns. (C) 2010 Elsevier Ltd. All rights reserved

Assimilation de données par méthode variationnelle utilisant l’adjoint d’un code de circulation côtière tridimensionnel

The optimal control technique using an adjoint method is applied to fit the parameters of a 3-D primitive equation coastal model due to a data assimilation procedure. Two of the main unknowns of coastal modelling operations - open boundary conditions and turbulent mixing processes in stratified conditions - are successively considered. The aim of the boundary control procedure is to identify boundary forcing to ensure the best fit between data and model results by minimising a function that measures model and data discrepancies. A methodology for proceeding from the continuous to the discrete formulation of a control problem is given. The robustness of the method is tested for the typical barotropic case of a semi-enclosed basin Kelvin wave. The data assimilation method of optimal control is then applied to the turbulence closure problem in the framing of coastal oceanic circulation. Numerical simulation of a schematic destruction of an halocline and an upwelling event resulting from a wind stress imposed at the sea surface in a strongly stratified two-layer case are realized. The adjoint method is used to find out an optimal set of viscosity and diffusivity coefficients which permits the retrieval of the closest computed simulation to hydrodynamic synthetic observations.Une méthode de contrôle optimal est utilisée pour identifier les conditions aux limites ouvertes et les coefficients de viscosité turbulente d’un modèle tridimensionnel de circulation côtière aux équations primitives au moyen d’une méthode d’assimilation de données par méthode variationnelle. L’objectif de cette méthode est d’assurer l’optimisation des paramètres inconnus du modèle en minimisant une fonction de coût mesurant l’écart entre la solution calculée et des observations. La minimisation de cette fonction de coût est réalisée en utilisant l’adjoint du linéarisé tangent du modèle côtier. Une méthode est donnée, dans le cas d’un modèle aux différences finies utilisant une grille décentrée, pour passer de la formulation continue du problème de contrôle optimal et du modèle adjoint à une formulation discrétisée. L’efficacité de la méthode est vérifiée à partir du cas de l’ajustement des conditions aux limites ouvertes pour une circulation associée à une seiche de Kelvin dans un bassin semi-fermé. Elle est ensuite testée vis à vis de l’optimisation du champ de coefficients de viscosité turbulente dans le cas de déstructuration soudaine d’une stratification préalablement existante, comme dans le cas d’une rupture d’une halocline marquée provoquée par un coup de vent. Ces coefficients sont également ajustés dans la phase d’établissement d’un upwelling côtier académique le long d’une côte rectiligne infinie

Detecting Habitats and Ecosystem Functions Considering the Mesozooplankton Size and Diversity Structures and Environmental Conditions in the Gulf of Lion, NW Mediterranean Sea

International audienceEnvironmental conditions and zooplankton size structure and taxonomic diversity were investigated in the Gulf of Lion in May 2010 and January 2011. The integrated physical and biological measurements provided a 3D view with high spatial resolution of the physical and biological variables and their correlations over the whole gulf. The effects of physical processes such as freshwater input, coastal upwelling, and water column mixing by winds on phytoplankton and zooplankton distributions were analyzed using these data. Several analytic tests were performed in order to define several ecoregions representing different habitats of plankton communities.Three habitats were distinguished based on statistical analysis performed on biological and physical variables: (1) the coastal area characterized by shallow waters, high chl a concentrations, and a steep slope of the normalized biomass size spectrum (NBSS); (2) the area affected by the Rhône with high stratification and flat NBSS slope; and (3) the continental shelf with a deep mixed layer, relatively low particle concentrations, and moderate NBSS slope. The zooplankton diversity was characterized by spatial differences in community composition among the Rhône plume area, the coastal shelf, and shelf break waters. Defining habitat is a relevant approach to designing new zooplankton sampling strategies, validating distribution models and including the zooplankton compartment in trophodynamic studies

Detecting habitats considering the mesozooplankton size structure and environmental conditions in the Gulf of Lion, NW Mediterranean Sea.

International audienceThe Gulf of Lion in northwestern Mediterranean Sea is characterized by a large continental shelf reaching up 40 miles width. Several hydroclimatic processes such as river discharges, strong winds and permanent current induce a complex hydrodynamics resulting in several habitats for plankton communities. The whole French continental shelf of the Gulf of Lion was sampled with a LOPC at two periods (May 2010 and January 2011). Zooplankton community’s characteristics and environmental parameters were investigated to describe biophysical interactions. Statistical tests allowed us to define 3 coherent ecoregions representing different types of habitat: (1) the coastal area, (2) the Rhône plume influence and (3) the continental shelf area. The zooplankton size distribution was investigated within the three areas using the normalized biomass size spectrum (NBSS). The size structure of zooplankton community differs following the areas and their environmental conditions. The coastal area was characterized by the steepest NBSS and highest chla concentrations, the upwelling and the formation of deep water participated to keep the water mixed. The Rhône plume showed the lowest NBSS slopes associated with strong water stratification. These waters were rich in nutrient promoted high chlorophyll a concentrations in the surface water layer and carried on the continental shelf a lot of organic matter. The continental shelf showed the lowest abundances and was hit by the Northern Current depending on the season.Defining habitats provides a firm basis to develop efficient sampling strategies for studying zooplankton dynamics, to validate zooplankton distribution models and to improve coastal management programs