In the winter of 2022, we encountered an unusual triple-dip La Nina—the cooling of surface temperatures in Pacific waters—of the century. This La Nina, in its withdrawal phase, altered the large-scale wind circulation, which resulted in two changes:
Slowing down the surface wind speed in most parts of western India, preventing proper ventilation of pollutants.
Reducing the frequency of cleaner winds originating from the ocean towards land to sweep away the pollutants.
Once the surface winds are slow, they allow the accumulation of particulate pollutants originating from emissions from various sources, including additional dust hot spots originating from redevelopment activities. That additional load, which is over and above normal emissions, tends to increase the pollution level in Mumbai.
In 2023, monsoon withdrawal got delayed in the Mumbai region until October. As we know, monsoon retreat is always accompanied by anticyclonic circulation, which slows down the surface wind speed beneath. As a result, pollution got trapped. When cooler winds blowing from Syadri Hills towards Mumbai met with warmer winds from the plane, the dust got arrested like a vortex, which was then pushed into the city, where it found stagnation. This was the major reason for the increased pollution levels in the second and third weeks of October.
The additional dust lifting in many parts of Mumbai also reduced visibility as it consisted of both finer and coarser particles. This increased hospital visits for people suffering from respiratory ailments.