Freshwater export pathways from the Bay of Bengal

Observations from surface drifters and Argo floats elucidate details of two upper-ocean pathways through which freshwater from the Bay of Bengal (BoB) is advected into the tropical Indian Ocean and Arabian Sea (AS). The western route is located along the east coast of India and feeds into the westward Northeast Monsoon Current (NMC) south of Sri Lanka during the winter monsoon, whereas the eastern path is located along the western margin of Sumatra reaching at times as far south as the Indonesian Throughflow plume near 10 degrees S. While the former pathway is highly seasonal and affected by mesoscale variability, the latter is a year-round feature and may be dominant from an annual perspective. The observations do not support the notion of a continuous current system comprised of the NMC and the West India Coastal Current around the Laccadive High in boreal winter that can directly export surface freshwater from the BoB to the AS. Further, the observations indicate an occasional leakage of low-salinity water by a westward coastal current between the eastward Southwest Monsoon Current (SMC) and Sri Lanka in boreal summer that may be caused by equatorial wave processes. The SMC, which exhibits considerable Fkman contributions in contrast to its geostrophically-balanced NMC counterpart in boreal winter, facilitates the transport of saltier AS water into the BoB, with subsequent spreading into the upper thermocline of the BoB setting up an estuary-like circulation pattern.