Kolkata: Gangetic Bengal and parts of northeast India have shifted out of the tropical savannah climate zone into a temperate zone over seven decades, according to a new study by scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune.The findings, published in the Oct 2026 issue of IMD’s peer-reviewed journal ‘Mausam’, suggest long-term changes in temperature and rainfall are altering the climatic character of entire regions, with implications for agriculture and water and food security.The shift does not mean Bengal is becoming cooler. Rather, it reflects a changing combination of temperature and seasonal rainfall — including rising maximum temperatures.The tropical savannah climate’s share of the northeast has shrunk from 18% between 1951 and 1980 to 12% between 1991 and 2020, while the area classified as temperate expanded from roughly three-fourths to more than four-fifths of the region.Researchers analysed IMD rainfall and temperature data from 1951 to 2020 in successive 30-year periods, revealing a progressive expansion of the temperate category across the northeast.The study also found a worrying rainfall decline. Annual precipitation over Gangetic West Bengal and Bihar fell between 1981 and 2020. Researchers linked this to the Victoria Mode — a recurring pattern of sea-surface temperature variations in the North Pacific.During its negative phase, atmospheric circulation changes reduce moisture transport towards the Indo-Gangetic plains, suppressing rainfall over Bengal and neighbouring regions.“East and northeast India had the largest rainfall shortfall of any region this monsoon. 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, co-author.East and northeast India recorded a rainfall deficit of 25.6% in 2026 — the largest among India’s four broad meteorological regions. Meghalaya received 60% less than normal, Bihar 32% less and Assam 28% less, while Bengal ended close to normal.IMD director general Mrutyunjay Mohapatra, also a co-author, said: “Our seven decades of records show the climate of this region has been moving out of the tropical zone, with rainfall over the plains of Bengal and Bihar declining. Both seasonal and long-term changes must be considered in forecasts and advisories to farmers.”elihoods.
