Cherrapunji, in the East Khasi Hills district, Meghalaya has long been considered the wettest place on earth. Now, Mawsynram, 15 kilometres away from Cherrapunji, wins the title, say Indian meteorologists.
They were examining rainfall data from 1901 to 2019 available from 16 different rain gauge stations across northeast India to understand the impact of climate change on rainfall in the North East when they noticed this shift. After 1973, they found an abrupt change in rainfall patterns in the region.
While gauges in Gauhati, Pasighat, Imphal, Itanagar etc. showed minor reduction, Aizawal, Dibrugarh, Jorhat, Silchar and North Lakhimpur showed significantly decreased rainfall. Meanwhile, rainfall in Shillong, Dhubri, and Cherrapunji actually increased. Yet annual rainfall data from Mawsynram, available only from 1983 to 2010, was even higher. While the average rainfall at Cherrapunji was slightly below 12000 millimetres, Mawsynram recorded more than 12500 millimetres.
Interestingly, data between 1901 and 1973 showed that rains in Cherrapunji and Shillong used to start in April-May and peak in June. Now the maximum precipitation is during July.
“Annual mean rainfall, especially from the northeast monsoon, shows a decreasing trend in the North-East region”, says Hamsa Varikoden, IITM Pune.
What could be the reason?
An analysis of the factors involved showed that the surface temperature of the Pacific and the equatorial Indian Ocean have major influence on rainfall in the region. El Niño–Southern Oscillation, surface temperature at the Bay of Bengal and the Atlantic also influenced annual rainfall.
“Equatorial winds and the surface temperature of the Arabian Sea also had limited, but negative influence on annual rainfall”, says P A Francis, Indian National Centre for Ocean Information Services, Hyderabad.
“More than any of these factors, changes in land use are perhaps major contributors to the decrease in annual rainfall”, adds Jayanarayanan Kuttipurath, IIT Kharagpur.
Analysis of satellite data from 1992 to 2018 showed that, though crop land has been increasing, so have barren lands.
“Vegetation cover is decreasing at a rate of more than 100 square kilometres per year. Snow cover, too, is decreasing”, says Madan K Jha, IIT Kharagpur.
“Population in the region was only 361 million in 1951. In 2011, it increased to 1221 million! So there has been a rapid increase in built-up area”, adds Jayanarayanan Kuttipurath, his colleague.
Rainfall or climate change due to ocean circulation and other natural causes are beyond our control. But human involvement, such as deforestation, can be curtailed. And properly enforced legislation can prevent further reduction of rainfall in the region.
Environmental Research Letters, 16 (2): 024018 (2021)
School of Ocean Science and Technology, KUFOS, Kochi
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