Large-amplitude internal wave generation in the lee of step-shaped topography

Abstract

Laboratory experiments show that as uniformly stratified fluid flows over a smoothly-varying step, two wave-like phenomena result: boundary-trapped lee waves, characterized by an undular shear layer downstream of the base of the step, and internal waves, which propagate vertically away from the step. The frequency of both types of waves is an approximately constant fraction of the buoyancy frequency. The frequency of internal waves is moderately smaller than lee waves if the step size is small but, if the step size is large, the frequencies match more closely. Thus nonlinear dynamics act so that internal waves are generated not by topography alone but also by flow over the boundary-trapped lee waves that act as "fluidic" hills. This new mechanism for internal wave generation may be an important factor in mixing over rough topography in the ocean.