Gangetic West Bengal and parts of Northeast India have moved out of the tropical climate zone over seven decades, according to a study by scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune, published in MAUSAM, IMD’s peer-reviewed journal.
The tropical savannah zone, characterised by warm temperatures throughout the year and a long dry season, has shrunk across the Northeast. In Gangetic West Bengal, the region has shifted into the temperate climate zone. Meanwhile, rainfall over Bengal and Bihar has declined over four decades, the researchers found.
The study found that the area of Northeast India classified as tropical savannah shrank from about 18% of the region in 1951–1980 to 12% in 1991–2020.
Climate zones are defined by temperature and rainfall. The researchers redrew India’s climate map for successive 30-year periods, the standard span used to assess climate, from 1951–1980 to 1991–2020, using IMD observations.
The temperate zone, characterised by hot summers and mild, dry winters, now covers more than four-fifths of Northeast India, up from about three-quarters previously. The zone expanded during each successive period, with the authors linking the change to rising maximum temperatures across the region.
The researchers also found that annual rainfall over Gangetic West Bengal and Bihar declined between 1981 and 2020. They attributed the decline to the Victoria Mode, a recurring pattern of warmer and cooler waters in the North Pacific.
During one phase, the pattern steers moisture away from the Gangetic plains and creates a high-pressure area over Bengal and neighbouring regions, suppressing rainfall.
The authors warned that these changes could affect paddy cultivation, including the timing of sowing and harvesting, and pose risks to food security, water resources and livelihoods. They recommended incorporating ocean patterns such as the Victoria Mode into long-term agricultural planning.
East and Northeast India recorded the country’s largest regional rainfall shortfall this year, at 25.6% below normal, as El Niño, the periodic warming of the Pacific Ocean that tends to weaken the Indian monsoon, took hold. Meghalaya recorded a 60% shortfall, Bihar 32% and Assam 28%, although West Bengal ended the season close to normal.
“East and northeast India had the largest rainfall shortfall of any region this monsoon, and much of that is due to El Niño, but our analysis shows that rainfall over Gangetic West Bengal and Bihar had already been declining for four decades, in step with a pattern of ocean temperatures far away in the North Pacific, which is why tracking that pattern alongside El Niño matters for the region’s rice farmers,” said Rajib Chattopadhyay of IITM Pune, a co-author of the study.
“Bihar and much of the northeast received far less rain than normal this year as El Niño took hold, and while a single season tells us little about long-term change, our seven decades of records show that the climate of this region has itself been moving out of the tropical zone, with rainfall over the plains of Bengal and Bihar declining, so IMD will keep both the seasonal and the long-term picture in view in its forecasts and its advisories to the region’s farmers,” said Mrutyunjay Mohapatra, Director General of Meteorology and a co-author of the study.
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