Nonlinear interaction of internal wave beams upon reflection at a sloping boundary

Internal gravity waves are supported by any fluid of varying density in height and propagate obliquely through the fluid with an angle determined by the wave frequency and the density gradient of the fluid stratification. Internal waves are suspected to play a key role in the dynamics of the ocean. Despite their great oceanographic importance, the dynamics of the internal waves are widely unknown. When an internal wave reflects from a sloping boundary, the nonlinear interaction of incoming and reflected beams gives rise to additional higher harmonic waves. Quantitative laboratory experiments were conducted on the nonlinear internal wave beam reflection. Strong internal wave beams were generated in a linearly stratified fluid and reflected by a sloping boundary. Second harmonic beams generated at the interaction region of incoming and reflected beam were clearly observed for different slope angles. The existence of a resonant slope orientation at which the second harmonic generation became strongest was precisely determined in the measurements. The resonant angle was always found to be smaller than the angle of the incoming beam and also depended on the beam profile. The observed sensitive dependence on the beam profile was unexpected and warrants further study.