Abstract
Order-400 Hz sound propagated through continental-shelf internal solitary wave packets is shown by simulation to fluctuate strongly via mode coupling. The coupling is controlled by the modal input conditions, wave and packet dimensions, and the ambient sound-speed profile. In the case of a moving packet, relative modal phase changes dominate over the others, changing the coupling and controlling signal fluctuations. The phasing within the packet varies over minutes in a moving packet, causing fluctuations with comparable time scales. The directionality of energy flux between high-order acoustic modes and (less attenuated) low-order modes determines a gain factor for distantly detected signals. A significant finding is that a packet near a source will amplify low-order modes if the source favors high-order modes, giving amplification at distant ranges. Conversely, a packet far from a source can energize otherwise quiet higher modes. Intermittency in the coupling means that signal energy fluctuations and modal diversity fluctuations at a distant receiver are complementary, with energy fluctuations suggesting a source-region packet and mode fluctuations suggesting a receiver-region packet. In addition to the monochromatic results described above, the effects on pulses (broadband signals) will be compared with experimental observations. [Work supported by ONR Ocean Acoustics.]
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