Acoustic intensity fluctuations induced by South China Sea internal tides and solitons

Between late April and May 23, 2001, a suite of acoustic and oceanographic sensors was deployed by a team of U.S., Taiwan, and Singapore scientists in the northeastern South China Sea to study the effects of ocean variability on low-frequency sound propagation in a shelfbreak environment. The primary acoustic receiver was an L-shaped hydrophone array moored on the continental shelf that monitored a variety of signals transmitted along and across the shelfbreak by moored sources. This paper discusses and contrasts the fluctuations in the 400-Hz signals transmitted across the shelfbreak and measured by the vertical segment of the listening array on two different days, one with the passage of several huge solitons that depressed the shallow isotherms to near the sea bottom and one with a much less energetic internal wavefield. In addition to exhibiting large and rapid temporal changes, the acoustic data show a much more vertically diffused sound intensity field as the huge solitons occupied and passed through the transmission path. Using a space-time continuous empirical sound-speed model based on the moored temperature records, the observed acoustic intensity fluctuations are explained using coupled-mode physics.