Theory and observation of anisotropic and episodic internal wave effects on 100-400 Hz sound

Abstract

Propagation of sound through shallow-water internal waves of various types is discussed. The anisotropy of the waves imparts an anisotropy to their effects on sound. The internal waves are of two types: Long-wavelength internal tides and short-wavelength high-frequency waves. On the continental shelf both types of waves tend to move shoreward from deep water (i.e. have anisotropic motion and anisotropic correlation scales). The internal tides are less predictable than the surface tides that generate them. The short-wavelength nonlinear internal waves are also somewhat unpredictable, and also have anisotropic correlation scales, having crests of tens of kilometres in length but wavelengths of order 300 to 1000 m. Coupled-mode propagation dominates across-shelf sound propagation, which in the direction of short internal wave correlation scale. Refracted-mode propagation dominates along-shelf propagation. Data from two sea exercises illustrate the character of the waves and their effects on sound.