par- 4 février 2008
Lundi 4 février 11h30
Centre Ifremer de La Seyne/mer
Dynamique des tourbillons le long du talus
Alexandre Stegner, chargé de recherche CNRS au Laboratoire de Météorologie Dynamique présentera un séminaire le lundi 4 février à 11h30 en salle de réunion Porquerolles du Centre Ifremer de La Seyne/mer :
The first in situ observations of the EGYPT (Eddies and GYres Paths Tracking) programme in the eastern basin of the Mediterranean, backed by thermal images, reveal a large meso-scale anticyclone, propagating westward along the Libyo-Egyptian shelf ( ie in a direction opposite to the mean circulation). Surface drifters trajectories combined with a CTD transect quantify the horizontal and the vertical structure of this surface-intensified anticyclone. The observed westward drift speed is significantly faster than expected from the beta-effect only. To investigate this specific eddy dynamics we used a two-layer beta-plane model with steep continental slope and zonal boundary. An integral momentum balance for the upper layer is used to derive the vortex center velocity assuming a circular dome interface at the leading order. This approach reduces the problem to the calculation of the deep-flow pattern, depending on the interface shape and topography. When the topographic slope beneath the eddy changes rapidly most of the deep-flow pattern is shifted offshore. The corresponding anticyclonic deep-flow feedback provides additional along-slope propagation. This mechanism was also studied in laboratory experiments. A significant drift acceleration of anticyclonic vortices was found when a steep topography is introduced, in good agreement with the perturbation theory.