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Le rôle du Pacifique Tropical Sud-Ouest dans la fertilisation du Pacifique Équatorial : couplage dynamique et multi-traceur.

Melanie Grenier 1 
LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
Abstract : In my thesis, I study the sources of the Equatorial Undercurrent (EUC). The EUC is composed of water masses that feed the central and eastern equatorial Pacific, an area of prime importance due to its biological productivity and its impact on climate. First, I used an ocean general circulation model (OGCM) to describe the EUC in terms of its source trajectories, transports and mean hydrological characteristics. Then, I characterized the geochemical enrichment of the water masses that feed the equatorial Pacific, the EUC waters being the main contributors to the High Nutrients - Low Chlorophyll (HNLC) area. For this purpose, I used geochemical tracers : the neodymium (Nd) isotopes and the Rare Earth Element (REE) concentrations. Finally, I coupled both approaches to quantify the geochemical evolution of the thermocline waters of the southwest Pacific. The first part was motivated by questions raised over the last ten years concerning the impact of variations of transport and hydrological properties of the Cold Tongue on the low frequency modulation of El Niño Southern Oscillation (ENSO). The EUC sources exhibit a wide range of temperatures and salinities that could explain the heterogeneity of the Cold Tongue and contribute to the decadal variability of ENSO. In this context, I studied how the various sources contribute to the composition of the EUC and how this composition evolves between the western and the central equatorial Pacific. I used an OGCM and a Lagrangian tool to determine the main sources of the EUC and their trajectories and to quantify the evolution of their transports between the western part (156°E) and the central part (140°W) of the EUC. We also investigated the evolution of the hydrological properties of these sources between 10°N/10°S and the equator. We found that the EUC is mainly fed by water masses conveyed by the low-latitude western boundary currents (LLWBCs), among which the Salomon Sea supplies are dominant. All the sources undergo hydrological modifications during their transits to the EUC. Notably, we had to invoke a fraction of diapycnal mixing to explain the lightening of water masses conveyed by the LLWBCs. The second part of my work was based on the major question of what is yielding the chemical enrichment of the HNLC area. The equatorial Pacific area is of prime importance for its primary production, which contributes to the regulation of atmospheric carbon concentrations. It is therefore important to understand and quantify what drives the input of chemical elements to the surface waters. During the last decade, it has been suggested that the continental margins of the Papua New Guinea (PNG) provide the largest fraction of chemical supplies to the HNLC area. Following the first part of my work, we have a good description of the circulation in the region. This has allowed us to verify the above hypothesis by measuring in situ geochemical tracers, the REE, and more specifically, the isotopic composition of Nd (Nd IC). These tracers tag the origin of the continental material present in the seawater and help to quantify the processes by which this material penetrates into the sea. Thus, we diagnosed a progressive enrichment of the waters along the island coasts of the south tropical Pacific but showed that the maximum geochemical input occurred along the PNG coast. The coasts of South America are also suspected to provide geochemical supply for water masses flowing below the EUC. Results on both tracers converge toward the suggestion that the main process driving the geochemical supply is the dissolution of sedimentary material during the coastal contact of the water masses. The last part of my thesis couples the virtual tracer - the modeled trajectories - with the real tracer - the geochemical elements - in order to quantify the geochemical fluxes affecting the water masses that feed the EUC, and thus the HNLC area. This coupling helped us to determine the enrichment zones of the upper and lower layers of the thermocline transiting via the southern LLWBCs, between their subduction areas and the upstream stretch of the EUC. The main enrichment of the upper thermocline waters seems to occur as they flow past the island coasts of the southern tropical Pacific. The lower thermocline waters also benefit from a strong supply along the PNG coasts, favorable to a possible resupplying of the HNLC area.
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Submitted on : Wednesday, October 23, 2013 - 4:28:58 PM
Last modification on : Monday, July 4, 2022 - 10:17:45 AM
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  • HAL Id : tel-00876092, version 1


Melanie Grenier. Le rôle du Pacifique Tropical Sud-Ouest dans la fertilisation du Pacifique Équatorial : couplage dynamique et multi-traceur.. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2012. Français. ⟨tel-00876092⟩



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