Northeast India’s tropical savannah climate zone shrinks as temperate zone expands: IMD study

Northeast India’s tropical savannah climate zone has contracted significantly over the past seven decades, while the temperate zone now covers more than 80 per cent of the region, according to a study by scientists from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune.

The study, published in MAUSAM, IMD’s peer-reviewed journal, found that the share of northeast India under the tropical savannah climate zone declined from around 18 per cent during 1951–1980 to 12 per cent during 1991–2020. The findings point to a long-term shift in the region’s climate, with rising maximum temperatures identified as a key factor behind the expansion of the temperate zone.

Researchers reconstructed India’s climate map across successive 30-year periods using IMD observations. The temperate zone, which accounted for around three-quarters of northeast India in the earlier period, now covers more than four-fifths of the region.

The tropical savannah climate is characterised by warm conditions throughout the year and a prolonged dry season, while the temperate zone generally experiences hot summers and mild, dry winters. The study found that the temperate zone expanded during every successive period analysed.

The findings come amid growing concerns over rainfall variability across eastern and northeastern India, which recorded the country’s largest regional rainfall deficit this monsoon season. Rainfall was 25.6% below normal across the region, with Meghalaya recording a 60 per cent shortfall, Bihar 32 per cent and Assam 28 per cent. West Bengal, however, ended the season close to normal.

The researchers also identified a longer-term decline in annual rainfall over Gangetic West Bengal and Bihar between 1981 and 2020. They linked the trend to the Victoria Mode, a recurring pattern of warmer and cooler sea surface temperatures in the North Pacific Ocean.

According to the study, a particular phase of this ocean pattern can divert moisture away from the Gangetic plains and create high-pressure conditions over Bengal and neighbouring areas, suppressing rainfall.

The researchers cautioned that these changes could have implications for agriculture, particularly paddy cultivation, by affecting sowing and harvesting schedules. They also flagged potential risks to food security, water resources and livelihoods, highlighting the need to account for large-scale ocean temperature patterns in long-term agricultural planning.

Rajib Chattopadhyay of IITM Pune, a co-author of the study, said the recent rainfall deficit across eastern and northeastern India was largely associated with El Niño, which tends to weaken the Indian monsoon. However, he noted that rainfall over Gangetic West Bengal and Bihar had been declining for four decades, indicating that the trend predates the latest monsoon season.

Chattopadhyay stressed the importance of monitoring the Victoria Mode alongside El Niño to improve understanding of rainfall patterns and support agricultural planning, particularly for rice-growing areas.

IMD Director General of Meteorology Mrutyunjay Mohapatra, another co-author, said the seven decades of observations showed that the region’s climate had been shifting away from the tropical zone, alongside declining rainfall over the plains of Bengal and Bihar.

He added that IMD would continue to consider both seasonal weather patterns and long-term climate trends while preparing forecasts and advisories for farmers.