Recovery of ordered periodic orbits with increasing wavelength for sound propagation in a range-dependent waveguide

Abstract: We consider sound wave propagation in a range-periodic acoustic waveguide in the deep ocean. It is demonstrated that vertical oscillations of a sound-speed perturbation, induced by ocean internal waves, influence near-axial rays in a resonant way, producing ray chaos and forming a wide chaotic sea in the underlying phase space. We study interplay between chaotic ray dynamics and wave motion with signal frequencies of 50-100 Hz. The Floquet modes of the waveguide are calculated and visualized by means of the Husimi plots. Despite of irregular phase space distribution of periodic orbits, the Husimi plots display the presence of ordered peaks within the chaotic sea. These peaks, not being supported by certain periodic orbits, draw the specific "chainlike" pattern, reminiscent of KAM resonance. The link between the peaks and KAM resonance is confirmed by ray calculations with lower amplitude of the sound-speed perturbation, when the periodic orbits are well-ordered. We associate occurrence of the peaks with the recovery of ordered periodic orbits, corresponding to KAM resonance, due to suppressing of wavefield sensitivity to small-scale features of the sound-speed profile.