Effects of internal solitary waves on three-dimensional sound propagation and DOA estimation in the South China Sea

Internal solitary waves (ISWs) in the South China Sea, characterized by their large amplitude and strong nonlinearity, have attracted a lot of attention. In this paper, we use the Massachusetts Institute of Technology general circulation model (MITgcm) to construct the processes of generating and propagating ISWs in the South China Sea from the Luzon Strait to the continental shelf. The simulated east-west flow velocity is compared with remote sensing images, and the results demonstrate the accuracy of the ISW model in the South China Sea. Inserting the simulated three-dimensional (3D) sound speed from the ISW model into the Bellhop3D model, key findings emerge that ISWs cause depressions in the iso-sonic surface, which lead to drastic fluctuations in sound rays and the horizontal refraction effect in the progress of sound propagation. ISWs disturb the convergence zone of sound rays, but the position of the internal wave crest line is consistent with that of the collective deflection of rays. In addition, the results of direction of arrival (DOA) estimation under ISWs demonstrate that the internal wave increases the DOA estimation error. Together, ISWs induce fluctuations in sound propagation and degrade sonar performance. This study creatively combines ISWs with the underwater sound field and DOA estimation, which provides support for the 3D sound propagation mode and the arrangement of the sonar array in the internal wave environment.