Study finds global warming strengthens El Nino's impact on Indian monsoon

The link between El Nino and drought in India, once thought by scientists to be weakening, is in fact growing stronger under global warming, according to a new study led by Chinese researchers.

The study found that an El Nino event of the same intensity is projected to cause one-third greater reductions in Indian summer monsoon rainfall in the future than at present.

The findings come as the current El Nino is forecast to become the strongest in 150 years during 2026 and 2027, with climate models projecting peak warming of 2.8 to 3.6 C above normal in the central-eastern equatorial Pacific.

The research, conducted by scientists from the Institute of Tibetan Plateau Research at the Chinese Academy of Sciences, was published in Nature Communications on July 30.

"The Indian summer monsoon delivers roughly 80 percent of the subcontinent's annual rainfall, underpinning agricultural production and the livelihoods of more than a billion people across South Asia," said Wang Tao, the study's corresponding author and a researcher at the institute.

He said accurately predicting year-to-year fluctuations in summer rainfall is critical because El Nino — the periodic warming of sea surface temperatures in the tropical Pacific — has long been recognized as a key factor suppressing monsoon rainfall. However, whether that relationship has been strengthening or weakening has remained a subject of debate for decades.

Two landmark papers published in the journal Science in 1999 and 2006 argued that the connection was breaking down, citing reduced Eurasian snow cover as a compensatory buffer. Wang's team had challenged that conclusion in earlier research.

To further resolve the debate, the researchers examined year-by-year observations and found that two atypical El Nino events, in 1983 and 1997, had distorted the statistical relationship. In both years, Indian summer monsoon rainfall was above or near normal despite strong El Nino conditions.

Zhao Yutong, the study's first author and a postdoctoral researcher at the institute, said that in 1983, during the decaying phase of a strong El Nino, surface waters remained unusually warm while deeper ocean layers had already returned to normal. That weakened the ocean-atmosphere coupling, while delayed warming in the Indian Ocean enhanced moisture transport, resulting in above-normal rainfall.

In 1997, the El Nino was still developing and the atmosphere initially followed the typical drying pattern. However, disturbances from the midlatitudes and the Southern Hemisphere strengthened cross-equatorial winds in July and August, bringing additional moisture to India and keeping rainfall close to normal, Zhao said.

"No previous study had examined what happened in those two specific years," Wang said. "Everyone was running statistical correlations, but when you look at the mechanism behind each anomaly, you realize the relationship never collapsed."

According to Wang, the 1983 and 1997 cases show that although the risk of El Nino-induced drought in India is increasing, the impact of an individual event can still deviate from the long-term trend because of factors including its stage of evolution, the three-dimensional thermal structure of the ocean, interactions among ocean basins and midlatitude wind anomalies.

After accounting for the two outliers, the researchers found that the classic negative correlation remained robust throughout the historical record.

Comparing 1902-1962 with 1963-2023, the team found that each 1 C increase in El Nino warming reduced Indian summer monsoon rainfall from about 5.0 percent to 6.6 percent, representing a roughly one-third strengthening of the relationship. The researchers said the change is driven primarily by greenhouse gas emissions rather than natural climate variability.

Under a moderate emissions scenario, rainfall reductions are projected to reach about 8.7 percent for every 1 C of El Nino warming. Assuming no technological improvements in crop varieties or irrigation, agricultural losses caused by an El Nino of the same intensity could be 30 percent greater than those historically observed.

Wang said the findings underscore the urgency of reducing greenhouse gas emissions and limiting global warming.

"The earlier we limit warming, the more we can reduce the risk of extreme drought in the future," he said.

He said the strengthening of the El Nino-monsoon relationship occurs because global warming amplifies the atmospheric convection triggered by El Nino, pushing the tropical east-west circulation farther toward India and intensifying the sinking air that suppresses rainfall.

The impact extends beyond India, Wang said. El Nino-induced drying also affects precipitation over the Tibetan Plateau, where southern regions are likely to experience lower lake levels, vegetation browning and accelerated glacial retreat as monsoon moisture weakens.

Regarding the current El Nino, Wang said no unusual external factors have been detected so far, suggesting it could bring severe drought to India during 2026 and 2027.

He added that future research should shift from statistical risk assessment toward more precise prediction of anomalous El Nino events.