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Mesoscale air-sea interactions in the western Tropical Atlantic

Description

Recent studies have shown the importance of mesoscale air-sea interactions in determining the ocean dynamics. In particular, it has been shown that mesoscale Thermal Feedback (TFB) and Current FeedBack (CFB) partly control the Western Boundary Currents dynamics and that the CFB also induces a sink of energy from mesoscale eddies to the atmosphere, causing a large dampening of the mesoscale activity. Nonetheless, the Wave FeedBack (WFB) impact on the atmosphere and the ocean is generally ignored although it could have a significant effect on e.g.,the energy exchange between the ocean and the atmosphere, and, thus, the TFB, CFB, and the ocean dynamic.

The western Tropical Atlantic (which we define as the region between 80°W-40°W and 0-20°N) is particularly well suited to unify our understanding of the mesoscale air-sea interactions in tropical areas, and to specifically tackle the question of the role of varying upper ocean stratification in determining the wind response : i) early results suggest that the so-called TFB and CFB of the mesoscale eddies on the atmosphere are particularly active in this region and may contribute to shape the eddy properties and regional mean state, ii) it gathers a broad range of upper ocean conditions influenced by the world largest river runoffs (Amazon, Orinoco) and tropical cyclone activity, and iii) the region will hold the EUREC4A intensive atmospheric field campaign in January-February 2020 (http://eurec4a.eu)and possibly intensive oceanic field campaigns (project EUREC4A-OA) both dedicated to air-sea coupling. 

The first part of the thesis project will consist in quantifying the TFB, CFB, and WFB in this region using a multisensory approach based on scatterometer winds and wind stress (QuikSCAT, ASCAT, CFOSAT, and possibly synthetic SKIM data), altimetry derived surface currents (AVISO), high resolution sea surface temperature products, and wave data from satellite derived products (e.g., AVISO, CFOSAT). Cross-validation between the different scatterometers, comparison of synthetic data built using intermediate resolution coupled simulations (1/12°), and confrontation of the satellite estimates with in-situ data obtained during the field campaigns will allow to cross validate the coupling coefficients inferred from satellite. Contrasting periods with different salinity conditions (SMOS) and ocean color (merged Globcolour products) will also help to disentangle the role played by river runoff and salinity stratification in theses sensitivities. 

In the second part of the thesis, a set of realistic regional mesoscale resolving high-resolution ocean (1/24°), atmosphere (1/12°) and waves coupled simulations (NEMO, WRF, WW3) will be designed over the area of interest in order to get deeper in the processes at play. Simulations will only differ by the degree of coupling they are taking into account (including or not the effect of the surface currents, mesoscale SST gradients and waves), and the presence or absence of salinity stratification. The sensitivity of the mean and seasonal surface conditions of the western Tropical Atlantic in the different simulations will be analysed. An eddy tracking procedure will be used in order to investigate the role of these feedbacks on the life cycle and properties of the meso- and submesoscale eddies observed within the western boundary current occurring in this region. 

Profil

Master in Oceanography or Atmospheric Sciences. Knowledge of Linux and programming skills (Python, Matlab, NCL, Fortran) are required. 

Description de la structure
Laboratoire d'accueil : LEGOS, Toulouse
Directeur(rice) de thèse/recherche : JOUANNO Julien
E-mail du directeur(rice) de thèse/recherche : jouanno@legos.obs-mip.fr
Responsable Cnes de l'offre : LIFERMANN Anne

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