Analysis of coupled oceanographic and acoustic soliton simulations in the Yellow Sea: a search for soliton-induced resonances

Abstract

The area south of the Shandong peninsula has been of great interest to both oceanographers and underwater acousticians due to the measurements of Zhou et al. [J. Acoust. Soc. Am. 90 (1991) 2042]. Over a period of years, in the summer, they observed anomalous losses in shallow water acoustic signals. They attributed the losses to acoustic mode conversions produced by solitary internal waves (solitons). Their hypothesis remains unsubstantiated due to a scarcity of oceanographic measurements in this area. In this simulation study the non-hydrostatic, 2.5-dimensional Lamb model was used to generate solitons in an adjacent, deeper area than that examined by Zhou et al. [J. Acoust. Soc. Am. 90 (1991) 2042]. Topographic variations and semi-diurnal tidal strength magnitudes were obtained from digital atlases and published data. For summer conditions, the Lamb model simulations showed that the existing semi-diurnal tidal flow over the topographic variations could lead to the formulation of internal bores and solitons. SAR observations of solitons near this region were used to select parameters and initialize the Lamb model. The resulting soliton simulations were comparable to the two-dimensional surface spectra seen in the SAR data. Soliton amplitudes of 2–7 m were indicated. Acoustic studies were made using the highly accurate finite element parabolic equation (FEPE) acoustic model applied to the initial soliton state data generated by the Lamb model. Mode decomposition of the acoustic fields in the deeper region showed that the mode conversions necessary for anomalous signal losses were present. These findings are consistent with the soliton hypothesis made by Zhou et al. [J. Acoust. Soc. Am. 90 (1991) 2042].