École Nationale Supérieure de
Techniques Avancées

Abstract

Oceanic meso-scale vortices and flows contribute significantly to the transport of water and heat in ocean. To understand their dynamics and their interactions with topographical features, it is essential to characterize the salt exchange, the movement of biological species or pollutants. In particular, surface currents and eddies, located in the first hundred or two hundred metres, play a crucial role in exchange between the coastal zone and the open sea. This is a study of modelling two experimental idealized phenomena that could participate actively in the horizontal and vertical exchanges in the ocean surface layer. Unlike the standard bidimensional Kármán street, the oceanic vortex streets which may occur behind isolated islands are affected by the earth’s rotation and the vertical stratification of the thermocline. These effects induce a selective destabilisation of anticyclonic vorticity regions. Several experimental studies were carried out on the LEGI Coriolis Platform, to characterize the dynamical processes of these instabilities. The LEGI rotating tank (13 m in diameter) allows us to reach a small aspect ratio, while using a shallow water configuration. Qualitative dye visualisations and PIV measurements allow us to identify different dynamic regimes, in a given range of Rossby and Reynolds numbers. We show that these instabilities occur at the anticyclonic boundary layer around the island. The other part of this work concerns the dynamic of a surface-intensified vortex interacting with the coast or the shelf. The first data sets of the EGYPT cruises in the eastern Mediterranean sea reveal a large meso-scale vortex travelling along the Libyo-Egyptian shelf. The observed westward drift speed is significantly faster than expected from the beta-effect only. Idealised experiments are carried out on the ENSTA turntable, to model a simple interaction of the coast or the shelf wih the dynamics of a surface anticyclone, in a two layer configuration. The drift speed of this surface vortex is double when it is initially at the base of the shelf, showing a feedback between the deep layer and the surface flow.