Srinivasu, Kotta, Sanjana, M. C., Latha, G., Udaya Bhaskar, T. V. S., Rahaman, Hasibur, Thirunavukkarasu, A., and Venkatesan, R
The influence of upper ocean dynamics on the acoustic field in the South Eastern Arabian Sea (SEAS) is studied using in situ oceanographic/acoustic measurements from a moored buoy, along with satellite‐derived and climatological data sets. Upper‐ocean variability at the site is quantified using Mixed Layer Depth (MLD), Isothermal Layer Depth (ILD), Barrier Layer Thickness (BLT), Maximum Spice Depth (MSD), and Sonic Layer Depth (SLD), along with surface variability factors such as Sea Surface Temperature, Sea Surface Salinity, Spice, and Sea Level Anomaly. The mixed layer acoustic duct (MLAD) varies from 2 to 100 m, with BLT varying from 5 to 99 m, and a mean SLD of 43 m. A thick transition layer connects the mixed layer with the thermocline during winter. The observations reveal that maximum SLD, MSD, and BLT occurred during January–March. Unlike other seasons when SLD follows MLD, winter SLD is influenced by BLT, suggesting strong salinity stratification due to low‐salinity water intrusion from the Bay of Bengal by East India Coastal Current. During these months, the SLD varies from 80 to 100 m, with the corresponding minimum cut‐off frequency varying from 300 to 200 Hz. Results are correlated with estimated Sound Pressure Level (SPL) from Ambient Noise Measurements during November 2018 to November 2019. SPL variation follows SLD for low and mid‐frequencies, with the highest SPL noted during January‐February. Acoustic propagation simulations at 250 and 1,000 Hz revealed features like acoustic duct leakage and channeling, indicating energy transfers between the surface acoustic duct and deeper layers. Plain Language Summary: The surface duct occurs just below the sea surface whenever the processes of stirring by the wind and convection caused by surface cooling and evaporation produce a layer of quasi‐isothermal water called a mixed layer. Studies show a strong association between SLD and MLD over most parts of the global ocean, including the Arabian Sea. Salinity variability plays a crucial role in the formation of the barrier layer and its effect on SLD. Recent studies (Colosi & Rudnick, 2020, https://doi.org/10.1121/10.0002174; Richards & Colosi, 2023, https://doi.org/10.1121/10.0021311) have shown the effect of salinity on SLD, including considerations of ocean spice over the North Pacific Ocean. This study attempts to understand this aspect using moored‐buoy observation, satellite‐derived and climatological data. An Acoustic Recorder is mounted on an OMNI‐buoy mooring at 550 m depth, in the SEAS to measure the sound field. The influence of Upper Ocean dynamics on MLAD and its effects on acoustic propagation is examined through modeling. The variability in derived SPL over a year is addressed using SLD and acoustic propagation simulations. The link between oceanographic aspects and acoustic fields in warm tropical waters (i.e., SEAS) is brought out, which would be helpful in conducting at sea acoustic experiments and trials. Key Points: Evaluated the impact of upper ocean dynamics on under water acoustics at south western Bay of BengalImpact of Sonic layer variability on acoustic propagation at different frequencies using deep ocean Ambient Noise Measurement System [ABSTRACT FROM AUTHOR]