Mumbai’s Prolonged October Heat and Pollution Explained



Mumbaikars have long believed that the sea breeze will protect them from pollution and that their air quality will never be as bad as that in Delhi. However, recent weeks have shattered this belief, as the oppressive October heat and pollution have made the air unbreathable. The air circulation patterns that determine temperature, humidity, and air quality in Mumbai now bring some bad news. Warming in the Middle East and the northern Arabian Sea is causing changes in wind patterns during the pre-monsoon, monsoon, and post-monsoon seasons. The increasing number of pre-monsoon heatwaves, heavy rain spells during the summer monsoon, and the recent October heat and pollution all indicate that these changes forced from afar are here to stay. As a result, careful urban adaptations will be necessary in the coming years.

One may wonder why the north Arabian Sea is warming and how it affects Mumbai. This story is essentially a result of global warming altering the seasonal transitions over the Arabian Sea and Mumbai. Due to global warming and the rise in atmospheric humidity, the dry desert regions of the Middle East and the Mediterranean are unable to release their surface heat to space. The water vapor in the atmosphere acts as a greenhouse gas, trapping the thermal energy radiated from the warm, arid surfaces. Consequently, these regions are experiencing rapid warming, second only to the Arctic Circle. The heating and associated low sea-level pressure pull the southwesterly winds over the Arabian Sea northward during the spring months. As a result, the northern Arabian Sea is warming rapidly, directly impacting the northern Western Ghats and northwest India throughout all seasons.

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So, why is Mumbai unable to cool down? The duration, intensity, and frequency of heatwaves in Mumbai and northwest India are affected by these wind shifts during the months of March, April, and May. Additionally, this warming of the Arabian Sea leads to more extreme monsoons with heavy rains in these regions. The transition from post-monsoon to northeast monsoon is also influenced by the warming of the Arabian Sea. Normally, as the monsoon retreats, winds over India shift to the northeasterly direction, bringing rain to the eastern coast and the eastern side of the Western Ghats. This year, however, the wind changes over the Arabian Sea resulted in surface cooling over Mumbai, offering relief from the usual humid conditions. But this relief was short-lived due to an anticyclonic (or anticlockwise) circulation anomaly that formed over the warm waters of the northern Arabian Sea in October. The southern edge of these anticlockwise winds brought in humidity from the ocean, while the northern edge pumped in air masses from central and northern India. This increase in surface humidity offset the slight cooling and caused uncomfortable heat events in October. The winds rising over the Ghats further heated the upper air, which can hold more moisture. This combination of humid maritime air and polluted air from the north-northeast created a deadly mix of heat, humidity, and pollution.

Another factor that contributes to the pollution problem is a phenomenon called temperature inversion. In October, the near-surface cooling with warming above creates a temperature inversion over Mumbai. This temperature inversion has been growing since around 1980 and plays a significant role in trapping pollution. Temperature inversion occurs when the surface temperature cools down, making the air heavier, while the air about a kilometer above the surface either cools less or warms up. This creates a stable vertical variation in air temperature, with heavy air below and lighter air above. This stability inhibits the mixing away of air pollution. Similar temperature inversions are responsible for the winter pollution crisis in Delhi.

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Given these challenges, how should we respond to the pollution problem in Mumbai? The city’s story is one of circulation changes forced from afar, occurring throughout the pre-monsoon, monsoon, and post-monsoon seasons. Combating this problem requires a combination of efforts to reduce pollution and adapt to the changes. Simple solutions like using masks can provide immediate relief and are affordable for everyone. In-home and vehicle air filters are affordable for some and offer another way to combat pollution. Implementing urban green infrastructure, creating special bus lanes, and focusing on improving public transportation can be effective long-term strategies. Additionally, affordable healthcare services are essential for addressing the health impacts caused by pollution. Monitoring air quality continuously and having reliable early-warning systems are crucial. The young and the elderly are most susceptible to the effects of heat and pollution, so early warnings will allow them to take preventive measures and minimize the consequences on human and animal lives.

In conclusion, the warming of the north Arabian Sea due to global warming and changes in wind patterns is impacting the air quality and temperature in Mumbai. The recent October heat and pollution have shattered the belief that the sea breeze will always protect the city. These changes, forced from afar, necessitate careful adaptations in urban planning. The best strategies to mitigate pollution include the use of masks, air filters, urban green infrastructure, improved public transportation, affordable healthcare, continuous air quality monitoring, and early-warning systems. By implementing these measures, the city can better cope with the challenges posed by heatwaves, humidity, and pollution.

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