An unprecedented triple-dip La-Nina event, extended by climate change, triggered a peculiar trend in the 2022-23 winter season where air quality improved in north India while peninsular India recorded an increase in pollution levels, according to a new study released on Sunday.

The three consecutive years of La Niña conditions (2020-23) - a rare “triple-dip” phenomenon - had widespread impacts on the ocean and climate across the globe.

The study by a team of scientists led by Gufran Beig, Chair Professor, National Institute of Advanced Studies, emphasised that besides local emissions, rapidly changing climate is a significant factor affecting air quality.

Published in the Elsevier Journal, the study reveals that the air quality worsened in peninsular Indian cities in the 2022-23 winter season but improved in the northern part of India, contrary to the trends seen in recent decades.

Among the north Indian cities, Ghaziabad registered the most significant improvement with a reduction of 33 percent, followed by Rohtak (30 percent) and Noida (28 percent). Delhi, being the most critical and landlocked city, showed an improvement of around 10 percent.

On the contrary, Mumbai recorded the highest deterioration with a 30 percent increase in PM2.5 levels, followed by other peninsular Indian cities like Coimbatore (28 percent), Bengaluru (20 percent), Chennai (12 percent), etc.

Many North Indian cities almost reached the 5-year-long target set under the National Clean Air Programme in no time. What caused this had been a puzzle, the researchers said.

"The winter of 2022-23 coincided with the last phase of an unusual triple-dip La Niña event, the first in the 21st century. This phenomenon, influenced by climate change, impacted the large-scale wind pattern, playing a decisive role in preventing stagnation conditions in north Indian cities and thus improving air quality," said R H Kripalani, a climate scientist at the Indian Institute of Tropical Meteorology and co-author of the report.

In contrast, it led to calmer conditions in peninsular Indian cities, accelerating transboundary pollution and significantly deteriorating air quality, he added.

"The dominance of higher northerly winds at the transport level forced an influx, along with relatively slower winds near the surface, trapping pollutants in peninsular India and increasing PM2.5 concentration. Conversely, feeble western disturbances, unique wind patterns, and the absence of rain, clouds, and faster ventilation led to a significant improvement in air quality in the north," Kripalani said.

To validate their hypothesis, the scientists used the newly developed advanced NIAS-SAFAR air quality forecasting model, which combines a chemical-transport model with an indigenously developed modern artificial intelligence algorithm.

"Our findings are vindicated as the air quality in the winter of 2023-24, when La Nina ended, returned to normal levels," Beig said.

"The findings of the current paper suggest that we need to wake up to the fact that extreme and unusual occurrences in air pollution phenomena are directly or indirectly manifestations of climate change," he said.

"Such revelations, in all probability, are set to increase by leaps and bounds unless we focus on a long-term strategy to reduce the menace of anthropogenic emissions directly at the source. That would be a win-win situation for both air quality and climate change," he said.

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