Also present was another scientist from the iisc, P N Vinayachandran who, in 2002, had collaborated with two scientists of the National Institute of Oceanography in Journal of Geophysical Research article, which showed for the first time that freshwater inputs from river runoff are critical to the Bay of Bengal’s low-salinity layer.

The meeting deliberated upon the existing knowledge on the Bay of Bengal and its land-atmosphere linkages. The participants noted, “The estimated freshwater influxes into the Bay of Bengal from local precipitation and through river discharge are 4,700 and 3,000 billion cubic metre (bcm) per year respectively.

The Bay loses only 3600 bcm per year. Thus, its annual freshwater input far exceeds the loss due to evaporation. This makes the Bay relatively less saline compared to other oceans.” Says Rajamani, this also helps the ocean maintain its low salinity area, and in turn influence the monsoon rainfall in most parts of the country.

How?
The density of seawater increases with salinity. The Bay of Bengal’s low-salinity zone is therefore a layer of less dense water that floats above denser waters below. Scientifically referred to as stratified layering, the phenomenon prevents mixing of surface water and the cooler waters below. Solar energy gets trapped in the top 10-20 metre-thick non-saline layer, keeping the sea surface temperature (sst) higher than other oceans. This is critical for rainfall. According to meteorologists, low pressure formation that causes monsoon begins at 28° c but peaks at 29° C. But while other oceans, including the Arabian Sea, take almost a month to increase their sst by 1° c, the corresponding time required by the Bay of Bengal is just 4 to 5 days. Its warmer low-salinity zone enables the Bay of Bengal to effect this temperature change, faster. This is very significant, considering that even small changes in the sst influence rainfall. “This relation between the low-salinity layer in the Bay of Bengal and the monsoon rainfall was established scientifically during Bay of Bengal Monsoon Experiment in 1999,” says Vinayachandran.

More evidence
Scientists also say that global circulation model studies have shown that its low-salinity zone also helps the Bay of Bengal retain freshwater from the river discharge closer to the coastal region during the monsoon period. And, then around the beginning October every year, when the monsoon withdraws, this water flows along the coast of India and around Sri Lanka into the southeastern Arabian Sea; here it plays a vital role in warming of the Arabian Sea during the pre-monsoon months. “This clearly shows that the effects of the rivers that originate in the Himalayas are not just local, but spread out over a large area on an annual scale,” the Current Science article observes.

Other impacts
Says R Ramesh, a biological oceanographer with prl, the disappearance of the low-salinity region can lead to an increase in marine phytoplankton population. Currently this layer prevents vertical mixing of water between the rivers dense and rare zone, and this in turn inhibits supply of nutrients from below, restricting phytoplankton growth. The Bay of Bengal does have an abundance of phytoplankton, but the productivity of these creatures is only one-fourth of that found in the Arabian Sea. Increased phytoplankton population might lead to a growth in the Bay’s population. Scientists fear that this fecundity would work to the ocean’s detriment.