Sound speed variation in the coastal waters off Cochin and signature of subsurface maxima

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.