Internal gravity waves in a dipolar wind: a wave–vortex interaction experiment in a stratified fluid

An experimental study on the interaction of the internal wave field generated by oscillating cylinders in a stratified fluid with a pancake dipole is presented. The experiments are carried out in a salt-stratified water tank with constant Brunt–Väisälä frequency ( N ). Experimental observations of the deformation of the wave beams owing to the interaction with the dipole are presented. When the wave and the dipole propagate horizontally in opposite directions ( counterpropagating case), the phase line of the gravity wave beam steepens towards the vertical as it enters the dipolar field and it may even reach a turning point where the wave is reflected. When the dipole and the wave propagate in the same direction ( copropagating case), the wave beam is bent towards the horizontal and may be absorbed by the dipole. These observations are in good agreement with a two-dimensional ray-theoretic model, even if the flow is fully three-dimensional and, the vertical shear induced by the dipole being too strong, the hypothesis of slow variation assumed in the WKB approximation is not verified. When the waves encounter a critical layer, we show by rigorous measurement that momentum is transferred to the dipole. New three-dimensional effects of the dipolar velocity field on the propagating internal waves are also discussed. In particular, focusing and refraction of a wave beam occurring because of the horizontal structure of the background dipolar flow allow us to explain some of the observed features that cannot be accounted for through the two-dimensional ray theory.