Effects of topography and Kuroshio on sound propagation in the Okinawa Sea in winter

The topography in the Okinawa Sea is usually complex, and there is a strong Kuroshio front in winter. Characteristics of sound field, running from shallow to deep water across slope topography and across the front, are analyzed by numerical experiments. The environmental data provided by the Hybrid Ocean Model show that the Kuroshio front is above the slope of the Okinawa trough. Moreover, sound speed of the upper water varies in the horizontal direction, and the horizontal sound-speed change increases with the latitude increasing. The transmission loss is calculated by the parabolic-equation acoustic model. The distribution of sound energy in the environment with the existence of surface duct is analyzed by normal modes. Effects of the topography and a front on sound propagation are explained using ray diagrams. It shows that the sound propagation is mainly influenced by the topography when the frequency of source is lower than the cut-off frequency of surface duct. When the frequency of source is higher than the cut-off frequency of surface duct, the sound energy excited by the source located in the surface duct mainly propagates around the surface channel, and part of sound energy leaking from the surface duct transmits along the slope to the deep water. The sound energy excited by the source located below the surface duct mainly propagates along the slope to the deep water. The sound field in the depth range from the bottom of the surface duct to the conjugate depth appears as a sound shadow area due to the effect of slope bottom. It is also observed that the existence of a front can lead to significant differences of the transmission loss in surface duct at some depths of sound source.