KOLKATA: Monsoon depressions over Bay of Bengal are showing a decreasing trend, resulting in a substantial drop in rainfall across south Bengal and the Odisha coast, weather scientists observed at a South Asian conference on climate change held recently. Even though the reasons behind the phenomenon are still being investigated, a rise in surface temperature could be a trigger, believe metereologists.
While Kolkata experienced less low-to-moderate rainfall in recent years, heavy and very heavy spells have gone up, said MR Ramesh Kumar, chief scientist at the National Institute of Oceanography, Goa. "The number of monsoon depressions has gone down. It could have a profound impact on the weather in the region. It has already started showing an impact, which is evident in the reduced rainfall in south Bengal and Odisha. There could be multiple factors working behind this change. One of them could be that Bay of Bengal has become warmer by around 1 degree," said Kumar.
R Krishnan of the Centre for Climate Change Research Indian Institute of Tropical Meteorology endorsed the finding. "Low and moderate rainfall events used to be the mainstay of the monsoon. These would be punctuated with heavy or very heavy spells. But now we are witnessing a surge in the number of heavy rainfall events while the number of moderate showers has gone down. This has led to prolonged dry spells in monsoon. The monsoon depressions, on the other hand, have diminished," explained Krishnan.
Depressions and the monsoon are inter-dependent phenomena. While it is the monsoon clouds that develop into low-pressure zones, causing heavy rainfall, the latter are essential to ensure that it rains enough during the season. But it was too early to conclude that the number of depressions was on the wane, according to Devendra Pradhan, deputy director general of metereology, Kolkata. The number of depressions hasn't been consistently low over a substantial period of time, he argued. "Generally, we have around three depressions between June and September. We need to study the low-pressure patterns over a period of at least 20 years to arrive at a conclusion," Pradhan said.
This monsoon, south Bengal has witnessed two monsoon depressions so far. The chances of a third one were, however, negligible, felt Pradhan. "We are nearing the end of monsoon in Gangetic Bengal. A third depression may not happen. But more importantly, we have had a normal rainfall this season," Pradhan added.
He, however, admitted that heavy rainfall events were on the rise. "This does not necessarily indicate a climate change, though," said Pradhan.
Kumar, on the other hand, felt all was not well with the rainfall pattern in the east. "The amount of rainfall has gone down. We have also seen a fall in the number of depressions. All we now need to do is find the reasons," he said.
Kolkata receives around 375 mm rainfall in July and August. It goes down to around 260 mm in September and further slides to 130 mm in October.
