Impact of the Irrawaddy Diluted Water on Upper-Layer Salinity in the Bay of Bengal

The Bay of Bengal (BoB) is characterized by notably low salinity levels primarily driven by riverine inputs. However, the pathways of Irrawaddy Diluted Water (IDW), an important source of freshwater, are still not well understood. This study integrates satellite-derived salinity data with reanalysis products and Lagrangian particle tracking simulations to investigate the transport dynamics of IDW. An analysis of the mixed layer salt budget reveals that the horizontal advection of IDW leads to salinity anomalies along the western coast of Myanmar and central BoB. The results indicate a consistent northwestward movement of IDW from the northern Andaman Sea toward the western coastline of Myanmar during the period from September to November. Concurrently, Ganges-Brahmaputra Diluted Water (GBDW) is observed to propagate southwestward along the eastern coastline of India. Consequently, IDW, rather than GBDW, serves as the primary freshwater source contributing to the low salinity signature along Myanmar's western coast. In addition to its northwestward trajectory, IDW also demonstrates westward expansion into the central BoB during November and December. The northwestward transport of IDW is primarily influenced by the discharge from the Irrawaddy River, whereas its westward expansion is jointly regulated by river discharge and the westward flow through the Preparis Channel during November and December. This study presents the first comprehensive observational and modeling evidence of the dual expansion pathways of IDW in the BoB underscoring the influence of monsoon-driven circulation.