The surface circulation of the northern South China Sea (NSCS) for the period of 1986-2006 is studied using the data of more than 505 satellite-tracked drifters from NOAA/AOML database and the data from ten drifters which were released by our lab in the Penghu channel and in the Luzon Strait in 2006. In this study, the spatial structure and the temporal variability of the surface currents, at mesoscale to seasonal cycle, are described in terms of Eulerian and Lagrangian statistics from the drifter velocities that have been processed. Maps of mean currents, velocity variance ellipses and mean kinetic energies were produced in domain of 0.5¢X¡Ñ0.5¢X. The mean flow map confirms that during the winter monsoon, there is a global cyclonic circulation in the NSCS and the southwestward current which passes Dongsha Island and is called the ¡§Dongsha Current¡¨. Its continuation is the southward coastal jet off Vietnam which is called the ¡§Vietnam Current¡¨. Maximum velocities in the Vietnam Current can exceed 100 cm/s. During the summer monsoon, the Dongsha Current became very weak and the drifters looped near Dongsha Island. The drifter data shows that part of Kuroshio water intrudes into South China Sea through Luzon Strait, Dongsha Current and Vietnam Current were coherent flows. Values of 5.12¡Ñ107 cm2/s, 2.56 days and 64.43 km were obtained for the diffusivity, Lagrangian time scale and spatial scale in the meridional direction in the NSCS, respectively. In the zonal direction, the statistics are half of the above values which show that the characteristic time and space scale are anisotropic. The mean velocity in the Dongsha Current is 0.33 m/s. The domain of Dongsha Current is subdivided into ten boxes of size 2¢X¡Ñ 2¢X in order to analyze the spatial structure of Lagrangian statistics. The result shows that the eddy kinetic energy is considerably higher than the mean kinetic energy in the region 113-121¢XE, indicating that the mesoscale motions are particularly frequent. The eddy kinetic energy gradually decreases toward the west and has a maximum value near the Luzon Strait. The drifters that flow southward along the east coast of Vietnam shows that the width of the Vietnam Current varies between 80 and 100 km and has a mean core speed of 60-150 cm/s. In winter, Maximum southward velocities could reach 130 cm/s at 38 km off the coast. In spring, a weaker maximum speed (around 100 cm/s) exists at about 18 km off the Vietnam coast. There were not enough drifter data in summer and fall.

The physical oceanography in the northwest Indian Ocean is largely controlled by the seasonal monsoon. The seasonal variability in circulation is complex. Many studies have investigated processes in the Persian (Arabian) Gulf and Arabian Sea, but little is understood about the Sea of Oman. This thesis incorporated observations from Argo floats, surface drifters and satellite imagery to study the deep and surface circulation in the northwest Indian Ocean. An assessment of four independent moorings located in the Sea of Oman and Arabian Sea, as well as a model skill comparison of the Simple Ocean Data Assimilation (SODA) model, contributed to understanding the dynamics in this region. Spatial patterns of surface current velocity produced from surface drifter data from 1995-2009 agreed with previously known surface currents. The Somali Current, East Arabian Current, Equatorial Current, Northeast/Southwest Monsoon Current, Great Whirl and Ras al Hadd Jet were all identified. During the Southwest Monsoon, flow direction was to the east and southeast in the Arabian Sea. The Somali Current flowed northeast along the Somali Coast extending to the East Arabian Current along the Oman coast. During the Northeast Monsoon, evaporation increased over the Arabian Sea, which resulted in a salinity gradient. This imbalance caused low-salinity surface water from the northeast Indian Ocean to flow into the northwest Indian Ocean as the Northeast Monsoon Current. Current direction reversed with the change of wind direction from the Southwest Monsoon to the Northwest Monsoon. Many characteristics seen at the surface were also identified in the subsurface as deep as 1500m. The comparison of moored observations to the Argo observations co-located in space and time showed reasonable agreement with the largest salinity difference of 0.23 and largest temperature difference of 0.78?C. The Murray Ridge mooring had a temperature correlation of 0.97 when compared to Argo observations. Argo observations were compared with SODA model numerical output from 1992-2001, and, after Argo, were assimilated from 2002-2009. With assimilation of Argo data into the SODA model, the temperature and salinity from the model numerical output improved, with most differences between model numerical output and Argo observations falling within one standard deviation.

Ce travail de thèse présente une étude sur la circulation océanique de surface s'écoulantle long des talus continentaux (aux abords de l’isobathe 200 m) de la mer Méditerranée centrale. L'objectif étaitde mieux renseigner le schéma de circulation de surface dans la région et les régimes de variabilité associés ens’appuyant essentiellement sur les observations altimétriques conventionnelles (AVISO/X-TRACK) le longdes traces satellite, disponibles sur la période 1993-2015. Cette étude est également basée sur une utilisationcombinée des données spatiales de SST, de mesures in-situ hydrographiques et de courantométrieconjointement avec l'altimétrie. L'analyse a d'abord permis de caractériser la variabilité saisonnière et deproposer un nouveau schéma de circulation de surface, y compris pour les saisons intermédiaires, incluant unenouvelle branche de courant Atlantique sur le plateau continental Tunisien. Le second point fort de ce travailréside aussi dans la caractérisation du transport de surface et des modes de variations inter-annuelles de l'eauAtlantique à partir de l'altimétrie côtière sur le long terme. L‘observabilité de la méso-échelle océanique de lazone d‘étude est enfin abordée à partir des mesures altimétriques haute résolution (AltiKa). Cela a permis dedétecter des structures physiques à plus fine échelle et d'affiner le détection des variations des courants les pluscôtiers. Au delà des aspects scientifiques, cette thèse a également permis des avancées méthodologiquesimportantes concernant l’utilisation combinée de différents jeux de données spatiaux et in-situ, aisémentapplicables pour les pays du Sud. / This work presents a study on surface ocean circulation flowing along the continentalslopes (near the 200 m isobath) of the central Mediterranean Sea. The aim was to better inform the surfacecirculation patterns in the region and the associated variability regimes based mainly on along-trackconventional altimetry (AVISO / X-TRACK) available over the period 1993-2015. This study is also based onthe combined use of spatial SST data, in-situ hydrographic and current measurements in conjunction withaltimetry. The analysis has first allowed to characterize the seasonal variability and to propose a new surfacecirculation pattern, including the intermediate seasons, and a new branch of Atlantic current on the Tunisiancontinental shelf. The second highlight of this work is the characterization of surface transport and inter-annualvariations of Atlantic water from coastal altimetry over the long term. The observability of the oceanic mesoscaleof the study area is finally addressed from high resolution altimetry measurements (AltiKa). This hasallowed to detect physical structures on a smaller scale and to refine the detection of variations in the mostcoastal currents. Beyond the scientific aspects, this thesis has also allowed important methodological advancesregarding the combined use of different spatial and in-situ data sets, easily applicable for the countries of theSouth.

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