A resonance explanation of mode coupling in SWARM data

The 1995 SWARM experiment off the coast of New Jersey acquired detailed range-dependent sound speed profile data along the acoustic path. In parabolic equation model simulations in this environment for a 1000 Hz source at 50 m depth, energy transfer to a patch above the thermocline was discovered; the transfer was special to that frequency. To identify the mechanism that caused this phenomenon, Fourier analysis of spatial (range) variation of sound speeds was performed. The results were analyzed using Zhou, Zhang, and Rogers´ well-known formula [J. Acoust. Soc. Am. 90, 2042-2054 (1991)]. The analysis has shown that internal wave structure over the 17 to 22 km range is responsible for energy transfer from lower to higher modes. Mode 17 is the lowest mode that has its amplitude peak above the thermocline, and it peaks at the location of the patch. Energy transfers to modes 6, 7, and 17 have been identified with the use of the formula.