Examining the validity of approximations to fully three-dimensional shallow-water acoustic propagation through nonlinear internal gravity waves

The anisotropic nature of most oceanic sound speed structures, which have longer horizontal characteristic scale lengths than vertical ones, means that horizontal refraction of sound can often be neglected in computations. Cases of especially strong lateral sound-speed variation, such as in steep nonlinear internal gravity waves, where horizontal refraction effects are known to occur, can often be effectively studied by considering the behavior of adiabatically propagating vertical normal modes. However, this is not always appropriate. Here, we describe propagation of low-frequency (less than 1 kHz) sound on continental shelves in two situations where mode coupling and horizontal mode refraction occur either concurrently or in close proximity. These situations are propagation through crossing or interacting nonlinear wave packets, and propagation at low horizontal grazing angles into nonlinear internal wave packets having very short horizontal scale lengths.