We investigated the vertical and horizontal variation of the sound speed using the Thermodynamic Equation Of Seawater-2010 (TEOS-10) and monthly CTD observations at Cochin estuary and adjacent coastal waters (inlet to 20 km offshore) during 2018. The results showed that the sound speed varied from 1500 m/s in the Cochin inlet to 1549 m/s offshore. The seasonally changing river influx to the shelf resulted in a spatially varying salinity plume (0–35 psu) towards offshore, contributing significantly to the sound speed at near-surface waters. Such variation of saline stratification altered the sound speed up to ± 20 m/s in the Cochin inlet and adjacent coastal waters. We observed a subsurface-maximum zone of sound speed (1540 m/s) in June (monsoon season) due to the combined effect of upwelling and river discharge. This subsurface maximum formed only at the starting stage of upwelling, which disappears when the intensity of stratification decreases due to the mixing of upwelled water with the salinity plume. Correlation analysis showed that temperature was the actively prompting parameter in determining the sound speed at off Cochin waters during pre-monsoon (with a maximum of 1549 m/s recorded in April), while the combined effect of temperature and salinity influenced during monsoon and post-monsoon.

我们使用 2010 年海水热力学方程 (TEOS-10) 和 2018 年科钦河口和邻近沿海水域（离岸 20 公里的入口）的每月 CTD 观测值调查了声速的垂直和水平变化。结果表明声速从 Cochin 入口的 1500 m/s 到海上的 1549 m/s 不等。流入陆架的季节性变化的河流导致离岸空间变化的盐度羽流 (0-35 psu)，对近地表水域的声速有显着影响。这种盐分分层的变化将 Cochin 入口和邻近沿海水域的声速改变到 ± 20 m/s。由于上升流和河流流量的共同影响，我们在 6 月（季风季节）观察到了一个地下最大声速区 (1540 m/s)。这个地下最大值仅在上升流的开始阶段形成，当由于上升流水与盐度羽流混合而分层强度降低时，该最大值消失。相关分析表明，温度是确定季风前科钦海域声速的主动提示参数（4 月记录的最大值为 1549 m/s），而温度和盐度的综合影响在季风期间和季风后影响。 -季风。