The salinity of the oceans that surround India play an important part in the complex interactions between atmosphere and sea that determines climate over the subcontinent. The Arabian Sea is saltier than the Bay of Bengal, and ocean currents that exchange waters between the two help maintain salinity levels.

However, a team of scientists from the Indian Institute of Science in Bangalore and the National Institute of Oceanography at Dona Paula, Goa, have found that it may not simply be a matter of currents moving water from one ocean basin to the other.

There was a ‘salt pump’ off the eastern coast of Sri Lanka that sprang to life from time to time during the south-west monsoon, lifting deep-flowing salty waters that originated in the Arabian Sea to the surface, according to a paper by P.N. Vinayachandran and others published recently in the journal Geophysical Research Letters.

The Arabian Sea got little rain, except off the west coast of India, said Prof. Vinayachandran. Given the evaporation that took place, its salt levels would increase. The Bay of Bengal, on the other hand, had a surfeit of fresh water, brought by rivers, like the Ganges, that flowed into it as well as the copious rain it received during the monsoon. This excess fresh water would reduce its salinity.

The salinity of the Arabian Sea was maintained by inflow from the southern Indian Ocean as well as the westward-flowing Winter Monsoon Current that carried water from the Bay of Bengal.

The Summer Monsoon Current, flowing eastward, took salty water from the Arabian Sea to the Bay. “This saltier water, however, slides under the lighter surface water of the Bay. Maintaining the salt balance of the Bay therefore demands upward mixing of this saltier, subsurface water,” the scientists noted in their paper.

Based on 2009 ocean cruise data, they found that “this saltier water erupted upward intermittently from its sub-surface abode to mix with the fresher surface water.”

The mechanism driving this ‘salt pump’ was not clear at present, according to Prof. Vinayachandran. sporadicsurges of wind and ocean currents during the monsoon might be playing a role.

“Nobody has seen anything like this elsewhere,” remarked Sulochana Gadgil, a leading atmospheric scientist who used to be at IISc. The Bay’s low-salinity surface waters allowed it to remain warm and sustain cloud systems. These cloud systems then often moved over to the land and provided a large part of the rain that India received during the monsoon. Understanding how changes to this ocean's salinity occurred was therefore important.