BENGALURU: Cherrapunji in Meghalaya, known to be the world’s wettest region with rainfall as high as 900 mm, and a day’s rain accounting for more than Bengaluru’s annual rainfall, also sees dry days in August-September. But this is not due to climate change, as some reports suggest, says Prof J Srinivasan, Chairman, Divecha Centre for Climate Change, Indian Institute of Science, Bengaluru.
“The reports misunderstand daily rainfall variation as climate change,” he added. The region goes without rain when moist air from the Bay of Bengal does not reach it, but goes towards Odisha. Srinivasan was speaking at a webinar, ‘What Really Drives Indian Monsoon’, organised by Divecha Centre for Climate Change, IISc, on Tuesday.
Talking to TNIE on the sidelines of the programme, he explained that normally, moist air rises when it hits a mountain, and causes rain. However, if it flows parallel to the mountain, it goes around it. “Moist air has to go up for the temperature to fall, and moisture to condense. Hence, small changes in wind direction can cause large changes in rainfall,” he said.
Climate change is related to change in rainfall patterns averaged over two 30-year periods. The rainfall influence on Western Ghats is the same as in Cherrapunji, he added.
India’s monsoon was unique this year, Prof Srinivasan said. “June was normal, July was slightly below normal, August was a disaster and September was very high. However, we don’t have sufficient data at present to understand the low rainfall in August. A similar pattern occurred in August-September 1993. We need to look into it.”
Aerosols and rainfall
Aerosols affect rainfall in two ways: One, if they reflect the sun’s radiation, they reduce the amount of energy available to drive the monsoon; Two, they reduce the number of liquid droplets, said Prof Srinivasan. Citing examples of aerosols affecting rainfall in remote areas (not just locally) too, he said aerosols released in Europe and China cooled the atmosphere between 30 degrees North and 40 degrees North.
Other factors affecting rainfall in India
Total radiative energy available, water vapour and vertical structure of the atmosphere.
The land-sea contrast (in surface temperature) which is often talked about as a factor influencing rainfall, cannot be invoked in India. This was because Indian land surface is cooler than surrounding oceans during the monsoon.
Net radiation (difference between solar energy absorbed and emitted by the atmosphere into space) and water vapour (amount of water vapour in the atmosphere between surface and top of the atmosphere)