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Accueil du site > Évènements > Séminaires > Séminaires IRPHE > Archives IRPHE > 2022

Vendredi 20 Mai 2022 / IRPHE

publié le

Séminaire IRPHE

The turbulent life of downslope gravity currents

Orateur : Eletta Negretti, LEGI

Abstract : Downslope currents induced by density differences represent one of the key submesoscale processes that lead to energy transfers to larger/smaller scales, impact the thermohaline structure and the vertical exchange of water masses in the ocean, and hence play a crucial role for the overall ocean circulation and climate. During the presentation, I will talk about the core of my research, which concerns the dynamics of buoyancy driven flows, with special focus on gravity currents. First, I will describe their development without background rotation and over more complex terrain conditions, such as sudden topography changes. The different initial conditions expressed in terms of the local Richardson number, measuring the stability of the interface, and the initial slope angle, reveal to give rise to different flow regimes. Bottom friction shows to increase locally becoming as important as the entrainment, due to a strong correlation between the boundary layer and the interface in certain flow regimes. The developed theory predicts that for unbalanced currents, very long distances are required to reach the generally assumed equilibrium state. The bottom curvature has shown to have a minor influence on the overall current development, however the boundary layer may develop the Görtler instability. The focus of the second part of the talk is on rotating downslope intruding gravity currents, and in particular on the TUBE experiment. Herein, the rotating downslope gravity current has been produced using a novel experimental set-up with axisymmetric injection on an inversed conical slope into a two-layer stratified ambient, at the Coriolis Platform. This allowed azimuthal averages and long experimental durations. On the slope area, the gravity current has been characterized and shows to develop a thin Ekman drainage layer and to produce baroclinic vortices in the upper layer, as previously observed. Here they reveal to be also triggered by barotropic Rossby waves. These vortices appear for a larger parameter range as compared to previous studies. The intruding flow has shown to produce different global circulation patterns in the ocean interior in dependence on the initial ambient stratification. More importantly, the formation of vorticity and submesoscale vortices of different size and sign has shown to be closely related to boundary layer processes, that can be transmitted into the ambient interior only in intrusive-type currents and not in cases where the current follows the slope until the abyssal plain, with important consequences on the mixing and re-stratification processes that take place in the ocean interior. A particular form of intrusion is finally discussed, in form of ’avalanches’, when the injection exceeds the Ekman transport, so that the gravity current and the downward transport reaches a ’self organized criticality’. A preliminar statistical analysis will be presented.

Date et lieu : Vendredi 20/05 11h, salle de séminaires IRPHE.

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