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Instabilités agéostrophiques des écoulements baroclines dans l'atmosphère et dans l'océan et leur évolution non-linéaire

Abstract : For the atmosphere and oceans, which are forced mostly at low frequency, there is a time-scale separation leading to a weak coupling between the slow balanced and fast unbalanced components of the dynamics. Ageostrophic instabilities may provide a route to escape this separation and couple balanced and unbalanced motions. To understand these mechanisms systematic linear stability analyses in a two-layer rotating shallow water model are provided. A focus is made on the Rossby-Kelvin (RK) instability in a rectilinear channel and in the annulus for comparison with the experiments. We demonstrate the existence of crossover regions where baroclinic and RK compete having similar growth rates. We then revisit the linear stability problem for coastal currents and study various instabilities due to resonances between vortical (Rossby-like), frontal and coastal trapped waves. The nonlinear evolution of these instabilities is then studied in the shallow-water approximation with the help of high-resolution DNS and in a continuously stratified fluid using a mesoscale model. It is shown that saturation for the RK instability and the Kelvin-frontal (KF) instability is achieved through the breaking of the Kelvin mode forming a Kelvin front and leading to inertia-gravity waves and reorganization of the mean flow through dissipative and wave-meanflow interaction effects. We also show for coastal curents how nonlinear saturation of the ageostrophic instabilities and reorganization of the potential vorticity field lead to coherent vortex structures appearing and eventually detaching from the coast.
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Contributor : Jonathan Gula <>
Submitted on : Sunday, January 3, 2010 - 4:05:39 PM
Last modification on : Monday, December 14, 2020 - 9:41:20 AM
Long-term archiving on: : Thursday, June 17, 2010 - 10:14:44 PM


  • HAL Id : tel-00443696, version 1


Jonathan Gula. Instabilités agéostrophiques des écoulements baroclines dans l'atmosphère et dans l'océan et leur évolution non-linéaire. Océan, Atmosphère. Université Pierre et Marie Curie - Paris VI, 2009. Français. ⟨tel-00443696⟩



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