INFLUENCE OF GANGES-BRAHMAPUTRA DISCHARGE ON THE VARIABILITY OF SALINITY OBSERVED ALONG THE EAST COAST OF INDIA.

The Bay of Bengal (BoB) is unique amongst tropical oceans, being a semi-enclosed basin that receives a very large influx of freshwater through strong summer monsoon rainfall and runoff from the Ganga-Brahmaputra (GB), Irrawaddy and Indian peninsular rivers. To date, the paucity of salinity data has prevented a thorough description of the spreading of this freshwater into the Bay. The potential impact of the salinity on cyclones and regional climate in the Bay of Bengal is however a strong incentive for a better description of the water cycle in this region. Since May 2005, the CSIR-NIO conducts a program in which fishermen collect coastal seawater samples every 5 days at eight stations scattered along the Indian coastline. This new dataset reveals a seasonal salinity drop of more than 10 g kg–1 in the northern BoB at the end of the summer monsoon. This freshening signal originates from GB outflow and propagates southward as a narrow (~100 km wide) strip along the eastern coast of India, before reaching the southern tip of India after 2.5 months. The southward propagation of the freshwater along the coast is consistent with transport by the southward flowing East Indian Coastal current, while other processes are at stake in the ensuing erosion of this coastal freshening. This seasonal evolution has been consistently observed year after year since 2005. Beyond this climatological picture, the year-to-year anomalies of GB runoff also have a distinct influence on the coastal salinity in the northernmost part of the basin.