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May 2015

Fishermen point scientists to ‘river in sea’

K. S. Jayaraman

Fishermen plying on the eastern coast of India have helped scientists discover a fresh water ‘river' that forms in the Bay of Bengal just after monsoon season1.

Arrows showing origin of the 'river in sea' n the Bay of Bengal all the way to the end. Locations from where fishermen colelcted water samples are named along the coast.
© Gopalakrishna, V. V. et al.
The ‘river in the sea’ forms in northern Bay of Bengal at the end of the monsoon and ‘vanishes’ gradually after a while. About 100 kilometres wide, it flows southward hugging the eastern coast of India and reaching the southern tip after two and a half months. The seasonal river in the sea was discovered by salinity measurements of sea water samples collected by fishermen along the coast.

The Bay of Bengal receives intense rainfall during the monsoon. This, and the run-offs from the rivers -- Ganges, Brahmaputra, Mahanadi, Godavari and Krishna -- bring around 1100 cubic kilometres of freshwater into the bay between July and September.

"This very intense freshwater flux into a relatively small and semi enclosed basin results in dilution of the salt in seawater," says one of the lead researchers V. V. Gopalakrishna, a scientist at the National Institute of Oceanography (NIO), Goa. The diluting effect gets concentrated in the upper 40 metres of the bay waters, resulting in a stark contrast between surface freshwater and saltier water below, he says.

The presence of low salinity water (called stratification in oceanography parlance) over the Bay of Bengal prevents vertical mixing of sea water. This results in the accumulation of more heat in the near-surface layers, Gopalakrishna says. The sea surface temperature remains above 28.5°C, a necessary condition to maintain deep atmospheric convection and rainfall. Similarly, strong salinity stratification close to the coast would mean more intense tropical cyclones, he says.