Experiencing an unusually hot summer? It may be an ‘ELLA' year, Pune study says

Time of India

a day ago

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Pune: As northwest India battles another blistering heatwave this week, a new study from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune, has revealed how a change in the Pacific Ocean patterns can make summers in India feel like the inside of an oven.
This dramatic change, from El Niño to La Niña, triggers what the researchers have now called 'ELLA years', essentially transition years when the heat can turn up across India, making days and nights hotter, longer, and more unbearable — especially for those without air-conditioning or working outdoors, the study found.
"In such years, many more districts face unusually hot days and nights, especially from Feb to May.
We found that even the winter months get warmer in the ELLA years, particularly in northern and northwestern states like Punjab, Rajasthan and Uttar Pradesh, making the heat season start earlier and last longer," said Rajib Chattopadhyay, an IITM scientist and one of the lead authors of the study.
During the pre-monsoon months of March to May, the study found a marked increase in the number of hot days in northwest India — particularly in Rajasthan, western Uttar Pradesh, and parts of Haryana and Delhi.
"In ELLA years, these districts saw many more extremely hot days compared to what's normally expected for that time of year. This suggests that residents in these areas face more frequent and prolonged spells of extreme heat, increasing risk of dehydration, heatstroke and power-grid stress," Chattopadhyay said.
The joint IMD-IITM research revealed that heatwave severity across India tended to be far greater when an El Niño event in the Pacific Ocean was followed by a La Niña, or ELLA (El Niño to La Niña).
The research found that during such years, more districts in the country tended to experience extreme summer temperatures, particularly between March and May, compared to years when the transition was not to La Niña.
Chattopadhyay said the study analyzed data from 1961 to 2020. "It found that during ELLA years, both day and night temperatures tended to remain unusually high over most of India. Notably, night-time temperatures remained elevated across southern and eastern India, leading to lower day-night temperature differences, which aggravate heat stress and discomfort.
We observed that during these transition years, the intensity and spread of hot extremes significantly increased, especially across the southern peninsula, east coast, and parts of central India," he said.
He said that in fact, the number of hot days — when daily temperatures exceed the district's 90th percentile — was almost twice as high in ELLA years compared to the non-transition years. For example, during Feb to April, districts in states like Odisha, West Bengal, Andhra Pradesh, Telangana and parts of Maharashtra and Karnataka saw up to 10 extreme hot days per month, compared to about five in other years.
El Niño events are known to trigger heatwaves in India by weakening the monsoon and causing atmospheric subsidence (downward air movement that warms the air). But this study showed that not all El Niño years have the same impact. The "asymmetry," as scientists call it, comes from whether or not the El Niño is followed by La Niña.
"Interestingly, almost all ELLA years in the past (e.g., 1982–83, 1997–98, 2015–16) were associated with Eastern Pacific El Niño events — the more intense variety — which then transitioned rapidly to La Niña conditions.
These patterns amplified heat buildup over the Indian region. We also found that during ELLA years, the Bay of Bengal and the South Indian Ocean are significantly warmer. This oceanic warming, combined with atmospheric changes like persistent high-pressure systems and increased moisture, created a perfect storm for heatwaves.
Warmer seas pump more heat and humidity into the air, particularly over eastern and southern India, which can cause hot and sticky nights and increase health risks," Chattopadhyay added.
The research highlighted the need for early warning systems that factor in global ocean trends and their regional effects. It also calls for region-specific heat action plans, especially for areas shown to be more vulnerable during ELLA years, the researchers said
Their study was recently published in Atmosphere-Ocean, an international scientific journal.
Pune: As northwest India battles another blistering heatwave this week, a new study from the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM), Pune, has revealed how a change in the Pacific Ocean patterns can make summers in India feel like the inside of an oven.
This dramatic change, from El Niño to La Niña, triggers what the researchers have now called 'ELLA years', essentially transition years when the heat can turn up across India, making days and nights hotter, longer, and more unbearable — especially for those without air-conditioning or working outdoors, the study found.
"In such years, many more districts face unusually hot days and nights, especially from Feb to May.
We found that even the winter months get warmer in the ELLA years, particularly in northern and northwestern states like Punjab, Rajasthan and Uttar Pradesh, making the heat season start earlier and last longer," said Rajib Chattopadhyay, an IITM scientist and one of the lead authors of the study.
During the pre-monsoon months of March to May, the study found a marked increase in the number of hot days in northwest India — particularly in Rajasthan, western Uttar Pradesh, and parts of Haryana and Delhi.
"In ELLA years, these districts saw many more extremely hot days compared to what's normally expected for that time of year. This suggests that residents in these areas face more frequent and prolonged spells of extreme heat, increasing risk of dehydration, heatstroke and power-grid stress," Chattopadhyay said.
The joint IMD-IITM research revealed that heatwave severity across India tended to be far greater when an El Niño event in the Pacific Ocean was followed by a La Niña, or ELLA (El Niño to La Niña).
The research found that during such years, more districts in the country tended to experience extreme summer temperatures, particularly between March and May, compared to years when the transition was not to La Niña.
Chattopadhyay said the study analyzed data from 1961 to 2020. "It found that during ELLA years, both day and night temperatures tended to remain unusually high over most of India. Notably, night-time temperatures remained elevated across southern and eastern India, leading to lower day-night temperature differences, which aggravate heat stress and discomfort.
We observed that during these transition years, the intensity and spread of hot extremes significantly increased, especially across the southern peninsula, east coast, and parts of central India," he said.
He said that in fact, the number of hot days — when daily temperatures exceed the district's 90th percentile — was almost twice as high in ELLA years compared to the non-transition years. For example, during Feb to April, districts in states like Odisha, West Bengal, Andhra Pradesh, Telangana and parts of Maharashtra and Karnataka saw up to 10 extreme hot days per month, compared to about five in other years.
El Niño events are known to trigger heatwaves in India by weakening the monsoon and causing atmospheric subsidence (downward air movement that warms the air). But this study showed that not all El Niño years have the same impact. The "asymmetry," as scientists call it, comes from whether or not the El Niño is followed by La Niña.
"Interestingly, almost all ELLA years in the past (e.g., 1982–83, 1997–98, 2015–16) were associated with Eastern Pacific El Niño events — the more intense variety — which then transitioned rapidly to La Niña conditions.
These patterns amplified heat buildup over the Indian region. We also found that during ELLA years, the Bay of Bengal and the South Indian Ocean are significantly warmer. This oceanic warming, combined with atmospheric changes like persistent high-pressure systems and increased moisture, created a perfect storm for heatwaves.
Warmer seas pump more heat and humidity into the air, particularly over eastern and southern India, which can cause hot and sticky nights and increase health risks," Chattopadhyay added.
The research highlighted the need for early warning systems that factor in global ocean trends and their regional effects. It also calls for region-specific heat action plans, especially for areas shown to be more vulnerable during ELLA years, the researchers said
Their study was recently published in Atmosphere-Ocean, an international scientific journal.