Le mercredi 27 janvier 2021, Sylvain WATELET présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Océanographie) sous la direction de Jean-Marie BECKERS .

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Reconstruction of the Gulf Stream variability since 1940 using a variational inverse method and study of its interaction with the North Atlantic Oscillation ».

Abstract

In this study, the Gulf Stream’s (GS) response to the North Atlantic oscillation (NAO) is investigated by generating an observation-based reconstruction of the GS path between 70° and 50°W since 1940. Using in situ data from WOD, SeaDataNet, ICES, Hydrobase3 and ARGO floats, a harmonized database of more than 40 million entries is created. A variational inverse method implemented in the software DIVA (Data-Interpolating Variational Analysis) allows the production of time series of monthly analyses of temperature and salinity over the North Atlantic (NA). These time series are used to derive two GS indices: the GS North Wall (GSNW) index for position and the GS Delta (GSD) index as a proxy of its transport. We find a significant correlation (0.37) between the GSNW and the NAO at a lag of 1 year (NAO preceding GS) since 1940 and significant correlations (0.50 and 0.43) between the GSD and the NAO at lags of 0 and 2 years between 1960–2014. We suggest this 2-year lag is due to Rossby waves, generated by NAO variability, that propagate westwards from the center of the NA. This is the first reconstruction of GS indices over a 75-year period based on an objective method using the largest in situ dataset so far. This enhanced tracking and quantification of the GS confirms and extends the temporal scope of this property: NAO+ phases lead to a stronger and more northward GS, and conversely for NAO− phases.

Le Jury sera composé de :

M. X. FETTWEIS (Président), MM. A. BARTH, J.M. BECKERS (Promoteur), H. GOOSSE (UCLouvain), G. MUNHOVEN (Secrétaire), S. NICOLAY, H. SAGEN (Institute of Marine Research, Bergen).

Conformément aux consignes institutionnelles relatives à l’épidémie du coronavirus, cette défense de thèse se fera en visioconférence via Lifesize.

Le lien public pour accéder à la défense est : https://call.lifesizecloud.com/7056159