Statistics of mode amplitudes in shallow water environments: Effects of random linear internal waves and nonlinear internal wave packets.

Abstract

In shallow water environments there is presently an incomplete understanding of the relative acoustical importance of the random linear internal-wave field compared to nonlinear internal waves packets. Using the random coupled mode theory of [Creamer (1996)], the nonlinear internal wave mode coupling theory of [Colosi (2007)], and Monte Carlo numerical simulation we are able to demonstrate the important propagation physics when the two internal wave fields act alone, and simultaneously. Importantly, we find that for acoustic frequencies less than roughly 500 Hz and for propagation ranges of order 10s of kms mode propagation though the random internal wave field is very nearly adiabatic, and that cross mode coherences decay rapidly within the first 10 km. A surprising consequence of the decay of cross mode coherence is that the mean intensity will be to first order insensitive to nonlinear internal wave packets if they are located past the de-coherence range. It will also be shown that uncorrelated modes have important implications for the approach to saturation. The adiabatic nature of mode propagation though the random internal wave field may have important implications for shallow water acoustic remote sensing.