Guwahati, Oct 23: Northeast India is gradually moving out of its tropical savannah climate zone, with the area under this climate type shrinking over seven decades as temperatures rise, a study by India Meteorological Department (IMD) scientists has found.
The tropical savannah zone, characterised by warm conditions throughout the year and a prolonged dry season, covered about 18% of the Northeast during 1951–1980. Its share fell to 12% during 1991–2020, according to the study published in MAUSAM, the IMD’s peer-reviewed journal.
At the same time, the temperate climate zone expanded to cover more than four-fifths of the region, up from about three-fourths during the earlier period.
The researchers from the IMD and the Indian Institute of Tropical Meteorology (IITM), Pune, attributed the shift to rising maximum temperatures.
They reconstructed India’s climate map using meteorological observations across successive 30-year periods, the standard reference for assessing long-term climate patterns.
The findings indicate that the Northeast’s climate is changing beyond the fluctuations associated with individual monsoon seasons, with potential consequences for agriculture, water resources and livelihoods.
The study also examined rainfall trends over the eastern plains, finding that annual rainfall in Gangetic West Bengal and Bihar declined between 1981 and 2020.
The researchers linked the decline to a recurring pattern of changes in North Pacific Ocean temperatures known as the Victoria Mode.
During certain phases, the pattern can alter atmospheric circulation, divert moisture away from the Gangetic plains and strengthen high-pressure conditions over Bengal and adjoining areas, suppressing rainfall.
Although the rainfall analysis focused on Gangetic West Bengal and Bihar, the broader climate-zone findings cover Northeast India.
The study’s findings are particularly relevant as the region faces increasingly uneven rainfall. East and northeast India recorded a rainfall deficit of 25.6% below normal this monsoon, the largest among India’s broad geographical regions, as El Niño conditions developed.
Meghalaya recorded a 60% shortfall, while Assam received 28% less rainfall than normal. Bihar reported a deficit of 32%. West Bengal, however, ended the season close to normal.
Rajib Chattopadhyay of IITM Pune, a co-author of the study, said the current monsoon deficit was largely associated with El Niño, but the long-term rainfall decline over Bengal and Bihar pointed to another influence.
The researchers found that rainfall over the Gangetic plains had been declining for four decades in step with changes in North Pacific temperatures, he said, highlighting the importance of tracking the Victoria Mode alongside El Niño.
IMD Director General of Meteorology Mrutyunjay Mohapatra, also a co-author, said a single season’s rainfall could not establish a long-term climate trend. However, the seven decades of observations showed that the region’s climate was itself shifting.
The changing climate zones could affect paddy cultivation, including the timing of sowing and harvesting, while rainfall variability could place pressure on water availability and agricultural livelihoods.
The researchers have recommended that long-term agricultural planning account for ocean patterns such as the Victoria Mode, alongside seasonal climate influences.
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