Why winter of 2022-23 in Delhi was cleaner than usual, but Mumbai saw poor air

This change in wind patterns may have contributed to the intrusion of stubble-burning smoke from Punjab and Haryana into parts of peninsular India and Mumbai in the winter of 2022-23, a phenomenon that is not usually seen, according to Gufran Beig, chair professor at the National Institute of Advanced Studies and founder of the System of Air Quality and Weather Forecasting and Research (SAFAR), who is the lead author of the paper.

While the La Nina itself is a cyclical phase in the equatorial Pacific Ocean, involving cooler-than-normal sea surface temperatures, the La Nina event that began in 2020 and extended into early 2023 was a prolonged one that lasted for about three years. The El Nino Southern Oscillation (ENSO) events – the El Nino and La Nina episodes – impact weather worldwide, usually lasting for around nine to 12 months.

The paper, recently published in the journal ‘Science of the Total Environment’ by scientists from the National Institute of Advanced Studies, the Indian Institute of Tropical Meteorology, and the Ministry of Environment, Forest and Climate Change, points out that the impact of this “multi-year” La Nina may have been more potent on account of its duration, and its “unprecedented extended period is linked to changing climate.”

What the prolonged La Nina episode may have done to the weather is alter the wind pattern, which in turn was “mainly responsible for befuddling air quality.”

According to the paper, changes in the direction and speed of long-range transport level winds were observed in October and November 2022 compared to 2021. Winds blowing towards Delhi during this period are usually northwesterly, and they dump pollutants generated by stubble burning in Punjab and Haryana, in the National Capital Region and the Indo-Gangetic Plain. However, in November 2022, these winds were northerly and “relatively faster,” escaping most of the region in the NCR, including Delhi.

Instead, these winds carried pollutants from stubble-burning towards the west-south part of India, including Mumbai, where they were trapped along with locally produced pollutants by relatively calmer surface-level winds. The PM 2.5 level in Mumbai increased significantly during the stubble-burning period, the paper noted. Under the “normal circulation pattern,” this intrusion from stubble burning would have reached Delhi and the surrounding region of the Indo-Gangetic Plains.

Surface level winds from November 2022 to January 2023 were also “relatively stronger” in the north and slower in the south-west parts of the country compared to 2021-22, and this, along with the absence of ‘cold day’ conditions over northwest India till early January may have prevented parts of the region from experiencing stagnation conditions that can prevent dispersion of pollutants.

Air pollution in Delhi-NCR in the winter is linked to low temperature and low wind speed that keeps pollutants trapped close to the surface.

In Mumbai, these surface winds were calmer and may have reduced dispersal of pollutants, that is usually seen with high wind speeds since it is a coastal city, the paper observed.

In the La Nina phase, winds over the equatorial Pacific Ocean are easterlies. Easterlies from the Pacific and northeasterlies from the Bay of Bengal “appear to accelerate and enhance the northwesterly flow from north India and converge over the Indian peninsular (region), mainly over northern parts of the west coast of India. Thus, it supports the transport of pollutants from North India towards Peninsular India,” the paper explained.

This resulted in a cleaner winter over some parts of north India and more polluted air over parts of peninsular India. In Ghaziabad, for instance, the PM 2.5 level for the October-January period of 2022-23 was 33% lower than the average for the previous three years, while in Rohtak it was 30% lower. In Noida, it was 28% lower. Delhi saw a 10% fall in PM 2.5 levels in 2022-23 compared to the average of the previous three years, the paper noted.

In contrast, Mumbai registered a 30% increase in PM 2.5 levels followed by other peninsular Indian cities like Coimbatore, which saw a 28% increase, and Bengaluru which recorded a 20% increase.

Beig pointed out that with La Nina gone, air quality in the winter of 2023-24 returned to “normal levels” in both Delhi-NCR and Mumbai.