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

Abstract : 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.
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Ocean, Atmosphere. Université Joseph-Fourier - Grenoble I, 2010. French


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Submitted on : Monday, November 22, 2010 - 11:18:15 AM
Last modification on : Sunday, November 24, 2013 - 11:07:16 AM

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Angélique Mélet. Les circulations océaniques en mer des Salomon : modélisation haute-résolution et altimétrie spatiale.. Ocean, Atmosphere. Université Joseph-Fourier - Grenoble I, 2010. French. <tel-00538257>

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