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Ayah Lazar (LMD)

Titre : Inertial Instability in Oceanic Flows

Date et heure : Le 19-12-2012 à 13h30

Type : thèse

Université qui délivre le diplôme : Ecole Polytechnique

Lieu : Amphi Becquerel
Membres du jury :

Joël Sommeria, Legi Grenoble, rapporteur
Riwal Plougonven, LMD, rapporteur
Jean-Marc Chomaz, LadHyX, examinateur
Paul Billant, LadHyX, examinateur
Pascale Bouruet-Aubertot, examinateur
Eyal Heifetz, Tel Aviv University, co-directeur
Alexandre Stegner, LMD, co-directeur

Résumé :

Inertial instability of vortices is an instability that occurs when the equilibrium between the centrifugal force, the radial pressure gradient force and the Coriolis force is unstable. It is a selective destructive mechanism that induces small three-dimensional perturbations on anticyclones with strong vorticities. Submesoscale mechanisms, such as this, play an important role in the over-all energy budget of the ocean. Their strong ageostrophic flows can extract energy from balanced mesoscale states, and transfer it to smaller scales, from which it is dissipated through three-dimensional processes. Also it might be an over-looked source of vertical mixing in the ocean, transporting nutrient rich waters from the deep and effecting primary production.

We study the stability of circular vortices to inertial perturbations, taking into account stratification and vertical eddy viscosity, both of which are essential when dealing with oceanic flows. This is a comprehensive study of this phenomenon by means of linear stability analysis and high Reynolds laboratory experiments performed on the Coriolis platform. We study many types of circular vortices, provide a first estimate of the stability diagram and reach an asymptotic analytic marginal stability criterion, which is a further generalization to the well known generalized Rayleigh criterion. With laboratory experiments we corroborate our theoretical results, and find a useful signature of the instability on the surface dynamics. This signature is sensitive enough to detect a even weak instability that does not result in complete vortex breakdown. We try to use the same tool to study the stability of real oceanic vortices. Finally we investigate the dynamics of intense eddies in the Gulf of Eilat (Aqaba) with a general circulation model, and gain some insight on the surface circulation.

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