OBSERVATION OF AN INTERNAL WAVE ATTRACTOR IN A CONFINED, STABLY STRATIFIED FLUID

When a container of water is vibrated, its response can be described i n terms of large-scale standing waves-the eigenmodes of the system. Th e belief that enclosed continuous media always possess eigenmodes is d eeply rooted. Internal gravity waves in uniformly stratified fluids, h owever, present a counter-example, Such waves propagate at a fixed ang le to the vertical that is determined solely by the forcing frequency, and a sloping side wall of the container will therefore act as a lens , resulting in ray convergence or divergence, An important consequence of this geometric focusing is the prediction(1) that, following multi ple reflections, these waves will evolve onto specific paths-or attrac tors-whose locations are determined only by the frequency. Here we rep ort the results of laboratory experiments that confirm that internal-w ave attractors, rather than eigenmodes, determine the response of a co nfined, stably stratified fluid over a broad range of vibration freque ncies. The existence of such attractors could be important for mixing processes in ocean basins and lakes, and may be useful for analysing o scillations of the Earth's liquid core and the stability of spinning, fluid-filled spacecraft.