Les circulations océaniques en mer des Salomon : modélisation haute-résolution et altimétrie spatiale.

The Pacific decadal variability, such as the low-frequency modulation of El Niño Southern Oscillation, has been hypothesized to be influenced by changes in the strength or water mass properties of the meridional circulation associated with the subtropical cells (STC). The STCs provide a connection between subtropical waters and the equatorial Pacific at thermocline level, by both an interior pathway and by the low latitude boundary currents. The LLWBC branch of the South Pacific STC has been pointed as the main source of the Equatorial Undercurrent and of the Equatorial Cold Tongue by both observational and modeling studies. Those currents transit in the poorly documented Solomon Sea, located in the western boundary of the tropical South Pacific. The very intricate bathymetry of the semi-enclosed Solomon Sea complexifies the STC pathways to the equator. In this study, we characterize the Solomon Sea fine-scale thermocline mean circulation and its annual cycle. As available observations of the region are sparse, our approach is based on modeling. High-resolution is required to realistically represent the complex topography. It is achieved through the implementation of a hierarchy of Ocean General Circulation Models (OGCMs) : a 1/12° resolution model of the Solomon Sea is interactively into a ¼° regional model of the southwest Pacific, itself embedded through open boundary conditions in a global 1/4° OGCM. The thermocline circulation involves an inflow from the southern open Solomon Sea which is distributed via WBCs between the three narrow straits that connect this region to the equatorial Pacific. The system of WBCs appears to be complex, with a double system of currents. It provides connections of subtropical water to the EUC at different longitudes, which may be associated to different climate impacts. The seasonal variability of the circulation results from the combination of equatorial dynamics, of remotely-forced Rossby waves north of 10°S, and of the spinup and -down of the subtropical gyre as a response of Rossby waves forced south of 10°S. The Solomon Sea is also a highly variable region. Indeed, the highest levels in sea level variability in the entire South Pacific are found in the Solomon Sea. Specifically reprocessed along-track data adapted to coastal areas were used in addition to standard gridded data to explore sea level and western boundary currents in this region. Track data appear especially helpful for documenting the fine structure of surface coastal currents. Sea level anomalies (SLA) in the Solomon Sea principally evolve at seasonal and interannual time scales. The annual variability of the boundary currents that emerged from altimetry is phased by Rossby waves arriving in the Solomon Strait, and it compared quite well with the variability seen at the thermocline level, as based on our numerical simulations. The interannual signature corresponds to the basinscale ENSO mode. The western boundary current interannual transport anomalies counterbalance changes in western equatorial Pacific warm water volume, confirming the phasing of South Pacific western boundary currents to ENSO. Water mass modifications are characterized in our high-resolution model in a Lagrangian quantitative framework. We show that strong diapycnal mixing, partly due to internal tide waves dissipation, is responsible for a reduction of the temperature and salinity vertical gradients. More specifically, diapycnal mixing erodes the high salinities associated with the waters of subtropical origin carried by the STC, and this erosion is associated to a downward heat transfer. Therefore, the Solomon Sea might damp the spiciness anomalies formed in the southeast Pacific and advected equatorward to the EUC by the LLWBC branch of the South Pacific STC, with reduced impact of the STC on the Tropical Pacific climate variability.La mer des Salomon, qui voit transiter les eaux subtropicales alimentant la thermocline du Pacifique équatorial via les courants de bord ouest (WBC), pourrait moduler le climat du Pacifique tropical. Les objectifs de cette thèse sont de caractériser les circulations océaniques en mer des Salomon, notamment dans la thermocline, leurs variabilités, ainsi que les modifications des masses d'eau transitant par la mer des Salomon pour rejoindre l'équateur. Pour cela, une approche parallèle de modélisation haute résolution et d'analyse des observations disponibles a été utilisée. Un double système de WBC est modélisé dans la thermocline. Une partie des eaux du New Guinea Coastal Undercurrent rejoint le New Ireland Coastal Undercurrent, qui se rétroflecte partiellement dans l'Equatorial Undercurrent, produisant ainsi une connexion directe à la thermocline équatoriale. Le cycle saisonnier de cette circulation est forcé par le régime des ondes équatoriales et par le régime des ondes de Rossby. Les WBC sont intensifiés pendant les évènements El Nino et compensent la déplétion du volume d'eau chaude du Pacifique équatorial ouest. Les modifications des masses d'eau transitant en mer des Salomon avant de rejoindre le Pacifique équatorial ont été caractérisées à partir d'une approche lagrangienne. Un fort mélange diapycnal produit un transfert de chaleur vers le fond et une érosion du maximum de salinité présent dans la thermocline