Explained | Why Mumbai’s cooler March may be a prelude to heat and heavy rain

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Mumbaikars have been pleasantly surprised this March because the whole month was cooler and drier than normal. Winds in March tend to flow from the Middle East to Mumbai, and portend the beginning of a sweltering heat and humidity build-up through May, before the rains arrive in June.

So what caused this March madness of cooler, drier air? And is this good news?

I was part of a study that recently reported the Middle East has been warming more rapidly than other tropical land regions. The northern Arabian Sea has also been warming, and the combination of this land and ocean warming have enhanced the duration, frequency, and intensity of heat waves over India in the pre-monsoon season. Meteorologists have also blamed the northern Arabian-Sea warming for the increase in heavy precipitation events over northwest India, going into Pakistan, in the monsoon season.

In addition, there has already been excess rainfall this spring over northwest India as well as a very rare pre-monsoon shower over Mumbai.

Rapid warming over the Middle East produces low sea-level pressure locally, which sets up a northward pressure gradient over the Arabian Sea – from the equator to the northern Arabian Sea. This pressure gradient pulls winds northward, disrupting winds that should actually be flowing directly east, from the Middle East towards Mumbai. So the winds came to Mumbai from the north-northwest, bringing cooler and drier air instead of the usual desert winds, which bring hot and humid air.

These wind changes created an unusual anticyclone over the Arabian Sea during March. An anticyclone causes a clockwise ocean circulation in which warm waters converge. As a result, sea surface temperatures have warmed by over 1º C just during March! This is tremendous.

A warmer Arabian Sea in the pre-monsoon season tends to favour a good monsoon. On the other hand, 2023 is expected to be an El Niño year, and an El Niño tends to suppress the monsoon, especially when it follows a La Niña winter (which has been the case for the last three years).

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But before we get to the monsoon, we still have the summer heatwave season to get through. As the seasonal land-heating continues into April and May, the southwesterly winds will begin to set out from the equator into the central Arabian Sea. The normal eastward winds from the Middle East towards Mumbai should also get stronger. These westerly winds will bring heat from the Middle East, over the warm Arabian Sea, into India. Winds will also sweep in from the northwest over the mountains of Afghanistan and Pakistan, setting up heat waves.

The anomalies of surface winds and sea surface temperatures. An anticyclonic wind circulation is clearly visible. This leads to warming of the North Indian Ocean, especially over the Arabian Sea. | Photo Credit: Dr. Baosheng Li/European Reanalysis 5, CC BY 4.0

We have already had an early heatwave this year, February. The evolution of winds and sea surface temperatures clearly show a rapid warming of the North Indian Ocean, especially the Arabian Sea, which augurs even more heat events.

Reports of unseasonal rain and hail, leading to devastating crop damages, in the northwest are just a preview of what may be in store in terms of heat waves and heavy rain over these parts. Since only about 50% of El Niño years have so far produced a drought over India, we may end up with a ‘normal’ monsoon. But it is quite likely that warming over the Middle East plus the Arabian Sea is going to once again produce heavy rainfall events over northwest India and Pakistan.

While Mumbaikars are basking in the unseasonably cool and dry weather, they can also expect heavy rains, and flooding, this monsoon even if the monsoon as a whole is in deficit.

Climate models seem surprisingly confident that there will be El Niño starting during the monsoon season. The caveat is that an El Niño forecast this early in the year tends to be inhibited by the ‘spring predictability barrier’: that is, tropical weather tends to be noisy in the spring months, making the prediction noisy as well. But we must still heed the forecast because the last three years, from 2020 to 2022, were La Niña years, which would have loaded the dice and which are more likely to roll an El Niño this time.

As usual, we must hope for the best but prepare for the rough roller-coaster ride of the heatwave and monsoon seasons.

Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.

• The Middle East has been warming more rapidly than other tropical land regions. The northern Arabian Sea has also been warming, and the combination of this land and ocean warming have enhanced the duration, frequency, and intensity of heat waves over India in the pre-monsoon season.
• A warmer Arabian Sea in the pre-monsoon season tends to favour a good monsoon.
• While Mumbaikars are basking in the unseasonably cool and dry weather, they can also expect heavy rains, and flooding, this monsoon even if the monsoon as a whole is in deficit.