Impact of River Runoff on the Upper Ocean Sound Speed Variability over the Bay of Bengal

Abstract:

The knowledge of sound propagation in the ocean is widely used in several scientific and strategic applications for underwater acoustic communication and remote observation of ocean interiors. In general, temperature is the dominant factor in controlling the sound speed structure in most of the regions. However, the Bay of Bengal (BoB) is such a region that experiences an intense spatio-temporal salinity contrast due to large freshwater input from monsoon rain and runoff from several adjacent rivers, which makes its northern part as the freshest region in the northern Indian Ocean. This study investigates the impact of river runoff on the sound speed variability of the BoB upper layers. The effects are analyzed by contrasting the simulated fields from a set of two mesoscale simulations (with and without river runoff) using the Regional Ocean Modeling System (ROMS). The sound speed is estimated from the simulated temperature and salinity fields. The river runoff spreads over the bay as freshwater plume and affects the sound speed in terms of modulating the salinity and temperature. The runoff reduces the salinity within the upper 80 m depth and hence reduces the sound speed with the maximum reduction (more than 12 m/s) in the surface of northern end. Freshening in the surface layers forms a strong near-surface stratification that modulates the vertical structure of temperature and hence the sound speed. The stratification leads to the formation of temperature inversion in the subsurface and a cooling underneath, which enhances the sound speed in the inversion layer and reduces underneath. Eventually, the river runoff enhances the vertical gradient of sound speed above the sonic layer depth (SLD), which leads to stronger surface ducting that supports formation of shadow zones below the SLD. This study shows the importance of proper simulation of spreading and movement of river runoff induced freshwater plumes to understand and predict the sound speed variability of the BoB.