Investigating acoustic propagation in the Sonic Duct related with subtropical mode water in Northwestern Pacific ocean

In this paper, the Subtropical Mode Water in the Northwestern Pacific Ocean was found to help form a Sonic Duct which is favorable for long range acoustic propagation. The duct was defined as the depth layer with the temperature smaller than 20 ℃ but larger than 16 ℃, within which the sound speed varies little due to the combined effects of the pressure and the temperature. Statistics of the duct obtained from the Argo floats shows that the duct thickness varies mainly within the range 200 ~ 400 m, and that in summer, the duct was located below the seasonal thermocline; in winter, the duct would overlap with the mix layer near surface. Acoustic propagation analysis in the Sonic Duct suggests that higher frequency and larger duct thickness would result in the acoustic energy better trapped in the duct. Given that the duct thickness varies mainly within the range 200 ~ 400 m, the duct is always favorable for long range acoustic propagation in all seasons. Finally, a method based on Self-Organizing Map was proposed to estimate the Sonic Duct, and the results suggest that the method was promising in providing near real-time estimates of the duct parameters with sea surface information.