PUNE: India’s summer heatwaves – the humid, suffocating kind that impact more than dry heat – are not just getting more frequent, but also more intense through a now-identified specific atmospheric chain reaction. A Jan 2026 study by scientists from IMD and Indian Institute of Tropical Meteorology (IITM) Pune, published in the Journal of the Atmospheric Sciences, is the first to explain the precise mechanism behind these “moist heatwaves” and the findings have direct implications for early warning, public health, and climate preparedness.IITM scientist Rajib Chattopadhyay told TOI that their earlier study identified two types of Indian summer heatwaves and classified them as dry and moist.
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“The dry variety, which mainly scorches the northwest plains, is not showing an increasing trend. But the moist variety, in which high humidity compounds with high temperature to make the body’s cooling system fail, shows a statistically significant and accelerating trend. The current study provides a hint towards the mechanism which can intensify moist heatwaves over India,” he said.Scientists traced the trigger to Rossby atmospheric wave patterns originating near Europe’s west coast. These waves travel through a Europe-Middle East-Indian Ocean pathway and arrive over India as upper-air high-pressure systems, suppressing clouds and baking the surface. “But what turns a bad heatwave worse is a second actor – the anomalous warming in southernmost Bay of Bengal. When that warm patch generates its own circulation pattern and the two systems arrive over northwest India simultaneously, they superimpose and amplify each other,” Chattopadhyay said.The anticyclone strengthens, lingers longer, and simultaneously pumps moisture westward into India, producing a lethal combination of heat and humidity. The team validated this using a mathematical atmospheric model run under 129 different experiment configurations.”The model results showed a clear pattern. When warming over the Bay of Bengal occurs at the same time as atmospheric waves arriving from Europe, temperature and the ‘feels-like’ heat index increase noticeably over northwest India. We also found that if the atmospheric wave pattern shifts slightly, the zone of stronger heat stress moves toward eastern and southeastern coastal parts of India,” Chattopadhyay said.Observational data suggested the humid heat signal in these regions has been strengthening in recent decades, he said. “In experiments examining climate conditions, we found this interaction between atmospheric waves and Bay of Bengal warming works most effectively when jet stream speeds are within the range seen under present-day conditions. This matters because climate change is expected to influence large-scale atmospheric patterns, including jet streams and wave behaviour.”
