Guwahati, Oct 23: The tropical savannah zone, which is warm throughout the year with a long dry season, has shrunk across the northeast and given way to a temperate climate in Gangetic West Bengal, while rainfall over Bengal and Bihar has declined over four decades, India Meteorological Department scientists report.
The Gangetic West Bengal has moved out of the tropical savannah climate zone into the temperate zone, and the area of northeast India in the tropical savannah zone has shrunk from about 18% of the region in 1951–1980 to 12% in 1991–2020, according to a study by scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology, Pune, published in MAUSAM, IMD’s peer-reviewed journal.
Climate zones are defined by temperature and rainfall, and the researchers redrew India’s climate map for successive 30-year periods, the standard span over which climate is measured, from 1951–1980 to 1991–2020, using IMD’s own observations.
The tropical savannah zone is warm throughout the year with a long dry season, while the temperate zone, which now covers more than four-fifths of the northeast, up from three-quarters, has hot summers and mild, dry winters. The temperate zone grew in every successive period, and the authors link the change to rising maximum temperatures over the region.
The researchers also found that annual rainfall over Gangetic West Bengal and Bihar declined between 1981 and 2020, and traced the decline to a recurring pattern of warmer and cooler waters in the North Pacific known as the Victoria Mode, which in one of its phases steers moisture away from the Gangetic plains and sets up an area of high pressure over Bengal and neighbouring regions that suppresses rainfall.
The authors warn that the change could affect paddy cultivation and the timing of sowing and harvesting in the region, and poses risks to food security, water resources and livelihoods, and they recommend that ocean patterns such as the Victoria Mode be factored into long-term agricultural planning.
This year, east and northeast India recorded the largest rainfall shortfall of any region in the country, 25.6% below normal, as El Niño, the periodic warming of the Pacific Ocean that tends to weaken India’s monsoon, took hold, with Meghalaya 60% short, 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.
“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.
