- A recent study finds that heat waves over southwest India are influenced by major climatic modes, including El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and the Indian Ocean Dipole (IOD).
- Researchers believe that this analysis will help meteorologists predict heat wave occurrences more accurately and confidently in the years to come.
- Heat waves disrupt carbon sink functions of terrestrial ecosystems, seawater oxygen levels and also lead to substantial crops yield loss, air quality degradation, and droughts.
The periodic warming and cooling of ocean surface temperatures, a phenomenon known as the El Niño–Southern Oscillation (ENSO), contributes to intense heat waves in southwest India (SWI), a new study shows. ENSO involves sea surface temperature and atmospheric pressure changes over the tropical eastern Pacific ocean. Other major climatic modes have also contributed to the heat waves in the region.
“Heat waves have adverse impacts on local communities as they lead to substantial crops yield loss, air quality degradation, degradation of ecosystem activities and biodiversity and human mortality,” said Ganaraj Dalal, lead author of the study, and a doctoral researcher in the Department of Environmental Science and Engineering at IIT-Dhanbad.
“Due to global warming, the intensity, frequency and duration of heat waves are going to rise further in the 21st century,” Dalal told Mongabay India. “In southwestern India, heat waves in the summer months (March-May) are severely affecting the flora and fauna of ecologically sensitive areas of the Western Ghats.” Faculty members in the same department, Tinesh Pathania and Vittal Hari, and Akash Koppa from EPFL Valais Wallis, a research institute in Switzerland, are the other authors of the study published in Climate Dynamics.
The study comes in the context of more frequent and intense heat waves in South Asia and elsewhere. There have been 223 heat wave events over the Indian subcontinent in the past few decades (1978-1999), causing approximately 5,500 deaths, the authors noted. India experienced about 24 days of heat waves in different parts of the country in 2024, its longest spell, as the head of India Meteorological Department (IMD), Mrutyunjay Mohapatra, noted in a recent interview. The country reeled under a third straight year of heat waves. Ironically, milder forms of heat waves claimed more lives during 2008-2019, another study showed.
Factors contributing to heat waves
Heat waves over southwest India are influenced by major ocean climate patterns. Apart from ENSO, these include the Pacific Decadal Oscillation (PDO) denoting alternating ‘cool’ and ‘warm’ phases every 20 to 30 years, and the Indian Ocean Dipole (IOD) that involves east-west contrasts in the oceanic and atmospheric conditions. Studies have also shown strong links of heat waves in India to the sea surface temperatures in the Atlantic ocean.
While PDO remained the leading factor behind the heat wave variability followed by IOD, post-1979, the scenario has changed remarkably with ENSO gaining prominence, the study pointed out. “It signifies the strengthening of the relationship between ENSO and SWI heat waves in the recent past,” Dalal said.
The study defined heat waves by the percentage of days when the daily maximum temperature exceeds the 90th percentile. These days were considered for the summer months of March–May with 1961–1990 as the base period. The researchers used long-term observational datasets from the India Meteorological Department from 1951 to 2020 along with climate model simulations. They showed an increasing trend in hot days with unusually high temperatures during the summer months.
“Through regression methods, climate perturbation and causality analysis using reliable climate data sets, the authors bring out the relative roles of Indian, Pacific and Atlantic oceans in producing heat wave conditions over this region during peak summer months,” summarised Vijayakumar P, assistant professor at the Department of Environmental Sciences, University of Kerala, who was not associated with this study.
“The study suggests that global warming-driven heat wave events are likely to increase in the ecosystem-rich southwest Indian region in the coming years,” Vijayakumar told Mongabay India.
Read more: What are El Niño and La Niña? How do they affect the Indian monsoon?
Impacts of heat waves on ecosystems
Heat waves significantly reduce terrestrial carbon sequestration, the process that captures and stores carbon dioxide in vegetation and soil in the first few feet of the Earth’s surface – thereby sustaining life and reducing its levels in the atmosphere, the study noted.
This is due to a reduction in gross primary productivity (GPP) or the total amount of carbon dioxide fixed by terrestrial plants per unit time through photosynthesis. “GPP is one of the most important characteristics of an ecosystem and in India it varies greatly across different parts, with the southwest and northeast showing the highest values,” Dalal said. “During a heat wave, GPP is seen to reduce significantly over the southwestern region of the biodiversity hotspots of the Western Ghats are located.”
Each year, plants absorb about a fifth of the carbon present in the atmosphere, acting as a major carbon sink. Carbon enters the leaves as carbon dioxide and gets converted through photosynthesis into sugars and starches. Water stress severely affects this action.
Another recent study which used satellite observations, field measurements and reanalysis found that increased heat waves and water shortage together affect ecosystem activity around the globe. “Compound drought–heat wave (CDHW) events are one of the worst climatic stressors for global sustainable development,” the study noted. “Limits on water availability are likely to play a more important role in constraining the terrestrial carbon sink than temperature extremes, and over 90% of the global population and gross domestic product could be exposed to increasing CDHW risks in the future, with more severe impacts in poorer and more rural areas,” it added.
Oceans are also under threat from heat waves. They can impact seawater oxygen levels, a critical factor in sustaining life. Another recent study has shown that heat waves can also trigger low-oxygen extreme events. The study recorded a marked increase in frequency, intensity as well as annual days of such events affecting their physical structure, chemical composition and biological functions.
“Oceans around the world are warming,” said Abhilash S, director of the Advanced Centre for Atmospheric Radar Research. “Marine heat waves can not only lead to cyclones and extreme weather events, but they can also affect marine life and thereby the livelihoods of people dependent on it,” he told Mongabay India.
Physical mechanisms of ocean climatic patterns
Dalal and colleagues used large-scale variables like outgoing longwave radiation (OLR), cloud cover, geopotential height and wind to see how different climate modes such as ENSO, Pacific Decadal Oscillation (PDO) and Indian Ocean Dipole (IOD) affect the southwest Indian region. The study found that ENSO, PDO and IOD were significantly associated with heat wave variability over southwest India.
ENSO denotes sea surface temperature variations in the tropical Pacific ocean with relatively warm (El Niño) and cool (La Niña) phases occurring in an irregular cycle every two to seven years. Southern Oscillation refers to the atmospheric response to the underlying SSTs marked by the mean sea level pressure difference between Darwin, Australia and Tahiti in the Pacific ocean.
A study found that Outgoing Longwave Radiation (OLR), a measure of the energy emitted to space by earth’s surface, oceans and atmosphere, harboured a positive connection with ENSO in southwest India.
“High OLR means more sunlight is reaching the ground surface causing ground to warm further contributing to temperature rise,” Dalal explained. “This is further confirmed by decrease in cloud cover over SWI during El Niño years. That means during EL Niño years, SWI is becoming warmer. Similarly, an analysis of low-level winds showed a weakening of westerly winds (winds originating from the west to east) that bring moisture from the Arabian Sea to SWI, thereby increasing the possibility of heat waves.”
Meanwhile at lower levels of the atmosphere, divergence dominates with horizontal winds causing a net outflow of air — thereby more air leaving a vertical column of air — during March–May. On the contrary, at higher levels convergence dominates with horizontal winds causing a net inflow of air into the region, the study showed.
“This indicates that the atmospheric subsidence is happening, suggesting the weakened convection activity over the SWI region,” the paper noted. Subsidence denotes the sinking movement of air over a wide area within an area of high pressure. Convection means the vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts.
This situation is further exacerbated during the positive phase of ENSO (El Niño) when there are large-scale changes in the atmospheric winds across the tropical Pacific.
El Nino involves weakening of the Walker circulation, the east-west air circulation across the equatorial Pacific, driven by temperature and pressure gradients. Winds near the equator flow from areas of high pressure to low pressure, and forming near-surface easterlies called the trade winds. They influence the global exchange of momentum, heat, and water vapour within the tropics. It is most intense during La Niña, when longitudinal sea surface temperature gradients are at a maximum.
The weakening of the Walker circulation increases atmospheric subsidence and further weakens convective activity, thereby reducing the cloud cover over southwest India and hence forming a more conducive environment for heat waves, the study explained.
Read more: The Arabian Sea faces an increased frequency and duration of marine heat wave days, finds study
Influence of Pacific and Indian Ocean conditions
As with ENSO, IIT-Dhanbad researchers also observed that the Pacific Decadal Oscillation affects southwest Indian heat waves through the weakening of the Walker circulation over the Pacific and also by weakening the westerlies – thereby inhibiting the moisture transport towards the Indian mainland.
IOD denotes the difference in sea surface temperatures between the eastern and western parts of the Indian ocean. A positive IOD phase denotes cooler-than-normal sea surface temperatures in the eastern and warmer-than-normal conditions in the western tropical Indian ocean. This affects wind, temperature, and rainfall across the region.
Dalal and colleagues noticed a positive relationship of IOD with heat waves – irregular subsidence weakens convective activity and subdues the formation of clouds, thereby reducing precipitation during summer over India, the study noted.
“This analysis will help meteorologists predict heat wave occurrences more accurately and confidently in the years to come,” Vijayakumar commented.
Banner image: Fishing nets in Fort Kochi. Image by Bockomet via Wikimedia Commons (CC BY-SA 4.0).
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