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Modes interannuels de la variabilité climatique de l'Atlantique tropical, dynamiques oscillatoires et signatures en salinité de surface de la mer

Abstract : In this thesis, we investigate several topics related to the interannual climatic modes in the tropical Atlantic. Statistical analyses allows us to extract the two main dominant modes of interannual variability: an equatorial mode and a meridional mode. The equatorial mode is responsible for Sea Surface Temperature (SST) anomalies mainly found in the Gulf of Guinea and is linked to variations of the sea-level slope in the equatorial band. It is due to dynamic feedbacks between zonal wind, sea level and SST. The meridional mode is characterised by inter-hemispheric SST fluctuations and is controlled by dynamic and thermodynamic feedbacks between the wind, evaporation and SST. After quantifying the coupling between key variables involved in the meridional mode, we develop a conceptual model to explain the main mechanisms responsible for meridional mode oscillations. The model shows that the meridional mode results from the superposition of a self-sustaining mechanism based on positive and negative feedbacks generating regular oscillations of high frequency (2-3 years) and another low frequency oscillation mechanism (4-9 years) related to the influence of ENSO. As the evolution of these two modes is strongly linked to the meridional shift of the Intertropical Convergence Zone (ITCZ) and associated rainfall maximum, we identify the signature of these modes on Sea Surface Salinity (SSS) using in situ observations and a regional numerical simulation. Oceanic and/or atmospheric processes responsible for the signature of each mode are also identified through a mixed-layer salt budget in the validated model. The salt balance reveals that the atmospheric forcing, related to the ITCZ migration, controls the equatorial region while the advection, due to the modulation of current dynamics, the vertical gradient and mixing at the base of the mixed layer, explains SSS variations in regions under the influence of plumes. Finally, we study the Equatorial Kelvin wave characteristics and influences on the density that are involved in the meridional and equatorial mode connection processes, using a very simplified model of gravity wave propagation along the equator. After a brief description of this model, which was initially constructed to study dynamics in the equatorial Pacific, we apply it to the specific case of the equatorial Atlantic by validating its analytical and numerical solutions under adiabatic conditions. The results shows that the numerical model is in agreement with the analytical solutions for all the simulated physical fields, although these are more complex in the Atlantic than in the Pacific, in particular the evolution of the maximum depth anomalies of the mixing layer and density. This complexity is linked to the presence of a fresh tongue due to freshwater discharges from the rivers (Niger and Congo) that exist in eastern part of the basin, which impose a specific stratification to the Atlantic. We also study a particular event where such a wave was generated by the process of connection between the meridional and equatorial modes.
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Founi Mesmin Awo. Modes interannuels de la variabilité climatique de l'Atlantique tropical, dynamiques oscillatoires et signatures en salinité de surface de la mer. Planète et Univers [physics]. Université Toulouse 3 Paul Sabatier (UT3 Paul Sabatier), 2018. Français. ⟨tel-01916705⟩

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