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

Abstract

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.