Abstract:

Internal waves scatter low-frequency sound, complicating the interpretation of long-range acoustic transmission experiments. One can now ``invert'' acoustic data and infer parameters of a modeled internal wave spectrum (S. Flatte, this session). The quality of the result depends, among other things, on the realism of the model spectrum. For many phenomena (surface gravity waves, for example), changes in wave-field energy result in changes in spectral form, as well as level. It is appropriate to investigate this issue with regard to internal waves. Estimates of the vertical wave-number-frequency spectrum of internal waves are presented from six sites around the world (83(degrees) N to 2(degrees) S). Coded pulse Doppler sonar systems provide the required data. Wave-field energy varies by a factor of 100 in the global sample. The wave-number dependence of the spectrum varies with both wave frequency and overall spectral energy level. These spectral estimates are ``resolved'' in the sense that ocean velocity profiles are inverted to provide estimates of variance density on a band-by-band basis in the wave-number-frequency domain. The ``model mismatch'' issue is not escaped, however, since the waves are often nonsinusoidal. The spectrum alone provides an incomplete description of the field.