An experimental investigation of a boundary current instability
Résumé
This study deals with the instability of a surface boundary current maintained along a vertical lateral wall by the Coriolis force. The flowing water floats over a homogeneous denser layer. Experiments have been performed in the large “Coriolis” turntable (Grenoble, France). High resolution measurements of the horizontal velocity field, obtained by Particle Image Velocimetry (PIV), at different depths, bring consistent results to explain the mechanism of instability. We observe the formation of a countercurrent in the homogeneous lower layer. Instability first develops in this lower layer, in the form of small vortices generated in the viscous layer near the wall. This is essentially a barotropic instability. In a second stage, a baroclinic instability develops, leading to the meandering of the surface current. This interpretation is supported by measurements of the growth rates and phase difference between both layers, compared with the simple twolayer model of baroclinic instability.
