Movement of freshwater mass begins at the end of the summer monsoon

Fishermen have helped oceanographers discover a river in the sea that has been meandering its way along the eastern coast of the Bay of Bengal (BoB) after summer monsoon. A decade-long coastal salinity observations, carried out at eight collection points with local fishers from Paradeep downwards up to Colachel, allowed a detailed description of this uncommon oceanic feature.

The movement of the freshwater mass begins at the end of the summer monsoon and survives for nearly two-and-a half months. It also travels over 1000 km from the northern BoB to the southern most tip of India, say scientists.

A research paper on the formation of the “river in the sea flowing along the eastern coast of India” was recently published in the Bulletin of American Meteorological Society.

The presence of the river was confirmed through continuous monitoring of salinity levels for nearly a decade, said V. V. Gopala Krishna, Chief Scientist of the National Institute of Oceanography, Goa, the Principal Investigator of the project supported by the Ministry of Earth Sciences.

Sorbonne University and LOCEAN Laboratory, Paris, and the Indo-French Cell for Water Sciences, Goa, partnered in the research work.

The southwest monsoon roughly lasts from June to September. During this period, water vapour collected at the ocean surface by the powerful southwesterly winds is flushed over Indian continent and the BoB.

A large fraction of the monsoon shower reaches the ocean in the form of runoff and contributes to the freshwater flux into the BoB in equal proportion with rainfall over the ocean.

The large rivers — Ganges, Brahmaputra and the Irrawaddy, and three small others — Mahanadi, Godavari, and Krishna — together contribute approximately 1100 km of continental freshwater into the BoB between July and September.

This very intense freshwater flux into a relatively small and semi-enclosed basin results in an intense dilution of the salt contained in seawater, explained the paper.

The over 100 km-wide freshwater mass that is formed from river discharges and runoffs is transported down south by the East Indian Coastal Current, the western boundary current of the BoB. The freshwater signal generally becomes smaller and occurs later while progressing toward the southern tip of India, the paper said.

The salinity distribution in the BoB may impact cyclones and regional climate in the BoB. However, the paucity of salinity data prevented a thorough description of the coastal salinity evolution.

This lacuna was addressed by including fishermen in sea water sample collection process. Fishers collected seawater samples once in five days in knee-deep water at eight different coastal stations along the coastline. The samples were analysed at the modern lab of the institute and compared with open-ocean samples to obtain a picture of the salinity evolution, researchers said.

According to the research paper, the occurrence of this river in the sea along the eastern coast of India was probably not a generic feature that could be observed in many locations in the world.

The peculiar geography of the northern Indian Ocean that resulted in both a massive inflow of freshwater into the semi-enclosed northern BoB and the strong coastally trapped currents along the eastern coast of India were responsible for the formation of the river, the paper suggested.