In an unusual turn of events during the winter of 2022-23, Delhi experienced a notable improvement in air quality, while Mumbai faced an unexpected surge in pollution levels. A recent research paper 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 attributes this environmental shift to the prolonged ‘triple dip’ La Nina event coupled with changing climate patterns.
The paper highlights that the extended La Nina, spanning three years from 2020 to early 2023, played a pivotal role in altering large-scale wind patterns. This shift influenced the intrusion of stubble-burning smoke from Punjab and Haryana into peninsular India, particularly affecting Mumbai, a phenomenon not commonly observed. 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), underscores this ‘sudden flip-flop’ of the pollution cycle. During October and November 2022, the paper notes changes in the direction and speed of long-range transport level winds. Unlike the typical north-westerly winds carrying pollutants from stubble burning to Delhi, the winds in November 2022 were northerly and faster, diverting pollutants westwards towards Mumbai. This deviation resulted in a significant increase in PM 2.5 levels in Mumbai, whereas Delhi experienced a reduction in pollution levels during the same period.
The La Nina phase, characterised by easterly winds over the equatorial Pacific Ocean, accelerated the north-westerly flow from North India towards Peninsular India. This phenomenon contributed to the cleaner winter experienced in parts of North India and elevated pollution levels in peninsular cities like Mumbai, Coimbatore, and Bengaluru. Ghaziabad, Rohtak, and Noida witnessed a considerable drop in PM 2.5 levels during the October-January period of 2022-23, reflecting a cleaner winter in comparison to the previous three years. In contrast, Mumbai observed a 30% increase in PM 2.5 levels, mirroring a pollution anomaly in peninsular cities.
With the conclusion of the La Nina event, air quality in the winter of 2023-24 returned to normal levels in both Delhi-NCR and Mumbai, emphasising the transient nature of these environmental fluctuations.
