Structures and seasonal variation of sound velocity profiles in the central Philippine Sea

With the mass-calibrated Argo buoy data of 2019 from the central Philippine Sea, the sound velocity values at different water depths are calculated using Wilson's second equation, and the vertical structure, horizontal distribution patterns and seasonal variations of sound velocity are analyzed, and the relationship between sound velocity and seafloor topography was preliminarily discussed. It is found that the sound velocity in the study area has a typical three-layer structure in vertical direction, from the top to the bottom they are successively the mixed layer, the main thermocline layer, and the deep isothermal layer. The sound velocity in the water less than 100 m is mainly effected by the season, and the influence remains the same from 100 to 800 m in water depth. The influence is gradually weakening from 800 m to deep, and for the water deeper than 1200 m the sound velocity remains stable. The horizontal distribution of sound velocity shows that when the depth of the sound channel axis is between 900 and 1100 m, it is roughly shallow in the south and deep in the north, with little seasonal variation; when the water depth is less than 200 m, the sound velocity is high in the south and low in the north, for the water in depth of 200-700 m, the velocity is high in the north and low in the south, when the water is in the range of 800-1100 m, it is high in the middle and low in the south, for the area with water depth deeper than 1200 m the velocity is high in the south and low in the north. The sound velocity near the deep sound channel axis of the Kyushu-Palau ridge is significantly lower than that of the surrounding area due to the influence of topography.