Study on Acoustic Variability Affected by Upper Ocean Dynamics in South Eastern Arabian Sea

Abstract

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.