Mumbaikars have lived under the assumption that the sea breeze is going to save them from pollution and that their air quality is never going to be discussed in the same strained breath as the notorious air over Delhi. But the last few weeks should have shattered this myth, with an oppressive October heat and pollution that rendered the air unbreathable.
The air circulation that determines the temperature, humidity, and air quality over Mumbai portends some bad news. Warming over the Middle East and the northern Arabian Sea is causing changes in the wind in the pre-monsoon, monsoon, and post-monsoon seasons. Recent increases in the number of pre-monsoon heat waves and heavy rain spells during the summer monsoon in addition to the October heat and pollution, point to changes forced from far away that are here to stay.
Such remote drivers of local air quality and heat will require careful urban adaptations in the coming years.
Why is the north Arabian Sea warming?
In particular, this story is one of global warming tinkering with the seasonal transitions over the Arabian Sea and Mumbai. Because of global warming and the rise in atmospheric humidity, the dry desert regions over the Middle East and the Mediterranean are unable to lose their surface heat to space. Water vapour is a powerful greenhouse gas and traps the thermal energy emitted from the warm, arid surfaces. These regions are as a result experiencing the fastest warming on the planet, after the Arctic Circle.
This heating and the associated low sea-level pressure pull the southwesterly winds over the Arabian Sea, northward of their normal position, during the spring months itself. Thus the Northern Arabian Sea is warming rapidly, affecting the northern Western Ghats and northwest India in all seasons.
Why is Mumbai unable to cool?
The duration, intensity, and frequency of heat waves over Mumbai and northwest India are affected by these wind shifts during March-April-May. The monsoon is then made more extreme with heavy rains over these regions. The post-monsoon transition to northeast monsoon is also affected by the warming of the Arabian Sea.
As soon as the monsoon retreats, winds over India begin to transition to the northeasterly direction, bringing rain over the eastern coast and to the east of the Western Ghats. This year, the wind changes over the Arabian Sea are resulting in a surface cooling over Mumbai, which would bring welcome relief from the typical humid conditions.
But this is not to be: an anticyclonic (or anticlockwise) circulation anomaly formed over the warm waters over the northern Arabian Sea during October. The southern edge of these anticlockwise winds swept humidity in from the ocean onto Mumbai, while the northern edge of the anticyclone pumped in the central and northern Indian air masses.
The increase in surface humidity is able to more than offset the slight cooling and produce crossings of the thermal comfort index, resulting in uncomfortable heat events in October. The winds rising over the Ghats are able to warm the upper air, which holds more moisture.
This combination of humid maritime air with polluted air coming from the north-northeast conspire to produce a deadly combination of heat, humidity, and pollution.
What is a temperature inversion?
The near-surface cooling with the warming above also causes a temperature inversion. An atmospheric temperature inversion in October has been growing over Mumbai since about 1980, and it is a key ingredient of a system that can trap pollution.
A temperature inversion occurs when the surface temperature cools down to render the air heavier while the air at about a kilometer above the surface either cools less or warms – altogether creating a stable vertical variation in air temperature, with heavy air below lighter air. This stability inhibits mixing away of the air pollution.
Such inversions are a key ingredient of the winter pollution nightmare in Delhi, for example, where natural winter circulation tends to produce such inversions.
How should we respond to the pollution?
Mumbai’s story is one of circulation changes forced from afar and extending from pre-monsoon into the monsoon and post-monsoon seasons. This is a bad mix of global warming and natural seasonal transitions. While local construction and traffic add to the pollution, that transported from central and northern India make it a great challenge to alleviate the city’s air quality woes.
Masks are the easiest and most affordable solutions for all, while air filters within homes and vehicles are affordable for some. Urban green infrastructure, special bus lanes bearing in mind the need for good public transportation, and affordable healthcare will be good long-term strategies to deal with such pollution.
The best of all adaptations, of course, will be to monitor air quality continuously, together with reliable early-warning systems. The young and the old are particularly susceptible to heat and pollution. Early warnings will help them take preventive action and thus greatly reduce the consequences of these hazards on human and animal lives.
Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.
- Mumbaikars have lived under the assumption that the sea breeze is going to save them from pollution and that their air quality is never going to be discussed in the same strained breath as the notorious air over Delhi.
- Warming over the Middle East and the northern Arabian Sea is causing changes in the wind in the pre-monsoon, monsoon, and post-monsoon seasons. Recent increases in the number of pre-monsoon heat waves and heavy rain spells during the summer monsoon in addition to the October heat and pollution, point to changes forced from far away that are here to stay.
- The duration, intensity, and frequency of heat waves over Mumbai and northwest India are affected by these wind shifts during March-April-May.
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