Pune: India’s summer heatwaves – humid, suffocating heatwaves that have a greater impact than dry heat – are not only becoming more frequent but also becoming more intense through specific atmospheric knock-on effects that have now been identified. A study published in the Journal of Atmospheric Science in January 2026 by scientists from IMD and the Indian Institute of Tropical Meteorology (IITM), Pune, explains for the first time the precise mechanisms behind these “moist heat waves”, with the findings having direct implications for early warning, public health and climate preparedness.IITM scientist Rajib Chattopadhyay told TOI that their earlier research identified two types of summer heat waves in India and categorized them as dry and humid. “Dry varieties in the predominantly hot Northwest Plains did not show an increasing trend. However, wet varieties, where high humidity mixes with heat to disable the body’s cooling system, showed a statistically significant and accelerating trend. The current study provides clues to the mechanisms that exacerbate humid heat waves in India,” he said.Scientists traced its trigger to the Rossby atmospheric wave pattern off the west coast of Europe. These waves cross the Europe-Middle East-Indian Ocean path and reach over India as high-altitude high-pressure systems, suppressing clouds and baking the surface. “But what made the severe heat wave worse was the second factor – anomalous warming in the southernmost Bay of Bengal. When this warm region generates its own circulation pattern and the two systems arrive in northwestern India at the same time, they overlap and amplify each other,” Chattopadhyay said.The anticyclone intensifies and lasts longer while transporting moisture westward towards India, creating a deadly combination of heat and humidity. The team verified this using a mathematical atmospheric model run under 129 different experimental configurations.“The model results show a clear pattern. When warming in the Bay of Bengal coincides with atmospheric waves from Europe, temperatures and ‘felt’ heat indices increase significantly in northwest India. We also found that if there is a slight change in the atmospheric fluctuation pattern, areas of stronger thermal stress move towards the eastern and southeastern coasts of India,” Chattopadhyay said.Observational data show that the heat and moisture signal in these areas has been strengthening in recent decades, he said. “In experiments studying climate conditions, we found that the interaction between atmospheric waves and Bay of Bengal warming is most effective when jet stream speeds are within the range found under current conditions. This is important because climate change is expected to affect large-scale atmospheric patterns, including jet stream and wave behavior.“
