Soutenance de thèse
Surface and subsurface evolution of mesoscale eddies under atmospheric forcings : case study in the Mediterranean sea
Mesoscale eddies are ubiquitous turbulent structures in the oceans, in thermal wind balance with a signature in density. Anticyclones with negative vorticity are associated with a negative density anomaly translating in a sea surface height (SSH) elevation, and conversely for cyclones. Statistical studies really began with eddy automated detections based on gridded altimetry products. The first quantitative studies were done in a composite approach : many observations are collocated with eddy contours and gathered into a single mean eddy picture. This approach combined with remote-sensing and Argo profiling floats provided eddy average signature in sea surface temperature (SST), salinity, chlorophyll but also air-sea fluxes. Previous studies did not significantly investigate eddy temporal evolution, apart from trajectory statistics. Eddies interact with heat and momentum air-sea fluxes interact over both short and long timescale, but their evolution remains unknown. We then investigate the mesoscale evolution submitted to atmospheric interactions, in both surface and at depth. Mediterranean eddies provide an ideal case study with extensive in situ measurements and occurrence of long-lived anticyclones. In a first part, we define a Lagrangian method tracking eddies in altimetric data at 1/8°. Eddy observation are collocated with in situ vertical profiles to measure eddy subsurface physical properties, and an outside-eddy reference background is defined to retrieve the eddy-induced anomalies. In a second part, evolution of eddy SST anomalies reveals a strong seasonal signal. Anticyclonic cold and cyclonic warm surface signatures shift from very rare in winter to predominant in early summer. This seasonal oscillation also recovered tracking individual structures. Collocated vertical profiles reveals this summer shift to occur only in near-surface. Hence an eddy-modulated vertical mixing is hypothesized to drive this evolution, with increased mixing in anticyclones. Getting to the mixed layer depth (MLD) in a third part, anticyclones are observed to enhance winter mixed layer deepening (up to 350m anomaly) and significantly delay spring restratification (up to 2 months). Eddy MLD anomalies do not scale with relationship from previous composite studies, and are rather impacted by the subsurface density profile. In a fourth part, we assess the accuracy of eddy evolution in a high resolution numerical experiment with the CROCO model. Eddy seasonal variations in both SST and MLD are retrieved. Increased mixing in anticyclone is confirmed and found to be sensitive to grid resolution. Near-inertial waves triggered by high frequency winds propagate more into the anticyclonic negative relative vorticity and enhancing mixing, in a remarkable example of scales interaction. Last, remaining interactions are discussed, in particular the role of Ekman pumping, atmospheric retroactions and importance of salinity. This study highlights the rich evolution occurring in mesoscale eddies with atmospheric interactions, blurred in composite approach but observable using Lagrangian tracking, and not yet properly retrieved nor studied in global models.
La soutenance se tiendra le lundi 9 octobre 2023 à 14h, dans la salle E314 de l’ENS Paris, département de Géosciences, 24 rue Lhomond 75005.
Composition du jury
Présidente : Sabrina Speich (LMD, ENS)
Rapporteur : Lionel Renault (LEGOS, IRD)
Rapporteur : Yves Morel (LEGOS, IRD)
Examinatrice : Anne Petrenko (MIO, Aix-Marseille Université)
Examinateur : Johannes Karstensen (GEOMAR Kiel)
Directeur de thèse : Alexandre Stegner (LMD, Polytechnique/Amphitrite)
Directeur de thèse : Franck Dumas (SHOM)
Invité : Xavier Carton (LOPS, UBO)