River plumes often develop in complex environments, where variable coastal and bottom topography, ambient currents, winds and tides may play important roles in shaping the plume evolution. When all these factors are present, the plume dynamics may become intricate and unclear. The objective of this study is to understand the processes controlling the dynamics of a large river plume that is affected by strong boundary currents, variable winds and complex topography. The Mississippi River (MR) plume is the study case of this dissertation work, and focus is given to the interactions between the plume and the offshore circulation of the Gulf of Mexico (GoM). A series of numerical experiments was designed to investigate the impact of different factors on the development of a large scale river plume in scenarios with variable degrees of complexity. First, a box-like model with an idealized estuary was designed to address the general development of a mid-latitude river plume and assess the variability of the plume with changes in the outflow conditions at the river mouth. The structure and development of the plume in the flat-bottom, receiving basin was highly dependent on the degree of freshwater mixing at the source. Larger freshwater mixing enhanced the estuarine gravitational circulation and modified the dynamical balance at the estuary mouth. Those changes effectively modified the shape of the bulge and length/transport scales of the coastal current. Sloping-bottom conditions further modified the development of the plume. Secondly, a Northern GoM model was designed and numerical experiments were conducted to investigate the specific dynamics of the MR plume, in the presence of both shelf and basin-wide circulation. In particular, buoyancy-driven (due to the MR and all other major Northern GoM rivers) and wind-driven currents were studied on the shelf, while the extension of the Loop Current and associated frontal eddies were considered as major factors in the shelf to offshore interactions; wind-driven, shelfbreak eddies were also considered. Process-oriented experiments demonstrate that westerly and southerly winds promoted the development of a surface Ekman layer that enhances the offshore advection of plume waters. The steep topography in the vicinity of the MR Delta was a favorable condition for that process. When the MR plume was subject to a full-blown scenario (realistically-forced experiment nested within a large-scale model), complex interactions between wind-driven and eddy-driven dynamics determined the fate of the plume waters. Offshore removal is a frequent plume pathway, and the offshore transport can be as large as the wind-driven shelf transport. The offshore pathways depend on the position of the eddies near the shelf edge, their life span and the formation of eddy pairs that generate coherent cross-shelf flows. Strong eddy-plume interactions were observed when the Loop Current (LC) system impinged against the shelfbreak, causing the formation of coherent, narrow low-salinity bands that extended toward the Gulf interior. The offshore transport of MR water is a year-round process, but the interactions between the MR plume and the LC system have large inter-annual variability. Plume to LC interactions are determined by episodic northward intrusions of the LC system in the NGoM. The interactions are dictated by the proximity of the LC system to the MR Delta and by wind effects. On average, plume to LC interactions correspond to ~ 12 % of the year-round, total freshwater transport near the MR Delta, but this percentage can go up to 30 % in individual years. At the time of the plume to LC interactions, an average value of LC freshwater entrainment was estimated to be ~ 4,150 m3 s-1. The findings presented here are a major contribution toward the understanding of the cross-marginal and basin-wide transport of MR waters by a large-scale current system, and the connectivity to remote regions, such as the South Florida region and the Florida Keys.

L'upwelling du Sud Vietnam (SVU) joue un rôle clef dans la dynamique océanique et la productivité biologique en Mer de Chine du Sud. Cette thèse vise à quantifier la variabilité interannuelle du SVU et identifier les facteurs et mécanismes en jeu. Pour cela, un jeu de simulations numériques pluri-annuelles à haute résolution a été utilisé. Le réalisme du modèle a été évalué et optimisé par comparaison aux observations in-situ et satellites. Les résultats montrent que la grande variabilité du SVU est fortement pilotée par le rotationnel du vent estival, et liée à l'oscillation ENSO via son impact sur le vent. Cependant, cette influence du vent est significativement modulée par la variabilité intrinsèque océanique liée aux interactions entre la vorticité associée aux tourbillons océaniques et le vent, et dans une moindre mesure par la circulation océanique de grande échelle et les fleuves. Ces conclusions sont robustes aux choix effectués pour corriger la dérive de surface du modèle. / The summer South Vietnam Upwelling (SVU) is a major component of the South China Sea circulation that also influences the ecosystems. The objectives of this thesis are first to quantitatively assess the interannual variability of the SVU in terms of intensity and spatial extent, second to quantify the respective contributions from different factors (atmospheric, river and oceanic forcings; ocean intrinsic variability OIV; El-Niño Southern Oscillation ENSO) to the SVU interannual variability, and third to identify and examine the underlying physical mechanisms. To fulfill these goals we use a set of sensitivity eddy-resolving simulations of the SCS circulation performed with the ROMS_AGRIF ocean regional model at 1/12° resolution for the period 1991-2004. The ability of the model to realistically represent the water masses and dynamics of the circulation in the SCS and SVU regions was first evaluated by comparison with available satellite and in-situ observations. We then defined a group of sea-surface-temperature upwelling indices to quantify in detail the interannual variability of the SVU in terms of intensity, spatial distribution and duration. Our results reveal that strong SVU years are offshore-dominant with upwelling centers located in the area within 11-12oN and 110-112oE, whereas weak SVU years are coastal-dominant with upwelling centers located near the coast and over a larger latitude range (10-14oN). The first factor that triggers the strength and extent of the SVU is the summer wind curl associated with the summer monsoon. However, its effect is modulated by several factors including first the OIV, whose contribution reaches 50% of the total SVU variability, but also the river discharge and the remote ocean circulation. The coastal upwelling variability is strongly related to the variability of the eastward jet that develops from the coast. The offshore upwelling variability is impacted by the spatio-temporal interactions of the ocean cyclonic eddies with the wind stress curl, which are responsible for the impact of the OIV. The ocean and river forcing also modulate the SVU variability due to their contribution to the eddy field variability. ENSO has a strong influence on the SVU, mainly due to its direct influence on the summer wind. Those results regarding the interannual variability of the SVU are robust to the choice of the surface bias correction method used in the model. We finally present in Appendix-A2 preliminary results about the impacts of tides.