The mode coupling and effect on ambient noise vertical directionality caused by internal waves in shallow water

Abstract

Experiments show that internal waves can cause the fluctuation of the depth of thermocline in shallow water, which may bring about two results: one is the change of hit number of sound rays and sea bottom, the other is mode coupling. The former changes the sound intensity, and the latter changes the sound energy distribution. The previous studies show that internal waves cause mode coupling by changing the sound propagation paths, which can fill the noise notch. In the calculation, it is found that the notch is deepened when some solitary internal waves are present. The reason is that the sound wave excited by the distant sources which are located on the palne near the sea surface propagates through the soliton which is a single downward undulation. The mode coupling from low-order modes to higher-order modes happens first, then the reverse conversion occurs. However, the former coupling is stronger, the lower-order modes fall off and the filling ability is weakened, so the notch is deeper.