Explained | How does a cyclone affect the monsoon’s onset?

Once seen as a very reliable system, the monsoon trough is now being kicked around in a game of climate-change football.

June 08, 2023 04:04 pm | Updated June 14, 2023 10:43 am IST

A snapshot of winds over the Arabian Sea and the Bay of Bengal at 3.15 pm on June 8, 2023.

A snapshot of winds over the Arabian Sea and the Bay of Bengal at 3.15 pm on June 8, 2023. | Photo Credit: Windy.com

The impact of global warming on the monsoons are manifest in the onset, withdrawal, its seasonal total rainfall, and its extremes. Global warming also affects the cyclones over the Indian Ocean and the typhoons over the northwestern Pacific Ocean.

We’re seeing cyclone formations in the pre-monsoon cyclone season, closer to the monsoon onset, arguably due to the influence of a warmer Arctic Ocean on winds over the Arabian Sea. The monsoon is of course also affected by the three tropical oceans – Indian, Atlantic, and Pacific; the ‘atmospheric bridge’ from the Arctic; and the oceanic tunnel as well as the atmospheric bridge from the Southern Ocean (a.k.a. the Antarctic Ocean).

A ‘bridge’ refers to two faraway regions interacting in the atmosphere while a ‘tunnel’ refers to two remote oceanic regions connecting within the ocean.

Why does a cyclone’s position matter?

Some cyclones in the North Indian Ocean have had both positive and negative impacts on the onset of the monsoon. Since the circulation of winds around the cyclones is in the anticlockwise direction, the location of the cyclone is critical as far as the cyclone’s impact on the transition of the monsoon trough is concerned. (The monsoon trough is a low-pressure region that is a characteristic feature of the monsoons.) For example, if a cyclone lies further north in the Bay of Bengal, the back-winds blowing from the southwest to the northeast can pull the monsoon trough forward, and assist in the monsoon’s onset.

Earlier this year, the Bay of Bengal had Cyclone Mocha develop in the first half of May and intensify briefly into a ‘super cyclonic storm’, before weakening rapidly upon landfall. Mocha’s northwest to east trajectory over the Bay was the result of unusual anticyclones (which rotate clockwise) that have been parked over the Arabian Sea and the Bay of Bengal since March. Mocha dissipated on May 15 and the back-winds helped the monsoon set in on time over the Andaman and Nicobar Islands.

One severe consequence of the anomalous anticyclones since March is that both the Arabian Sea and the Bay of Bengal have warmed by more than 1º  C in the pre-monsoon season. The late-season cyclone Biparjoy is still chugging along in the warm Arabian Sea and may well rapidly intensify – i.e., have its wind speeds increase by 55 kmph within 24 hours – before making landfall.

Mawar, Biparjoy, and Guchol

Cyclone Biparjoy is not interacting much with the monsoon trough at this time. However, its late birth as well as the late onset of the monsoon are both closely related to typhoons in the northwestern Pacific Ocean. On May 19, Typhoon Mawar was born and dissipated away by June 3. Mawar qualified as a ‘super typhoon’ and is thus far the strongest typhoon to have taken shape in May. It is also the strongest cyclone of 2023 so far. Tropical storm Guchol is now active just to the east of the Philippines and is likely to continue northwest before veering off to the northeast. These powerful typhoons are thirsty beasts and demand moisture from far and wide. 

Mawar pulled winds across the equator into the North Indian Ocean, setting up southwesterly winds over parts of the Arabian Sea and the Bay of Bengal. ‘Southwesterly’ means blowing from the southwest.

Southwesterly winds over the Arabian Sea are welcome news: they bring large quantities of moisture onto the Indian subcontinent. On the other hand, southwesterly winds over the Bay of Bengal are bad news for the monsoon. The monsoon winds over the southern Bay of Bengal sweep in from the southwest and west, but they turn around and head northwest towards India from the southeast.

Winds were southwesterly over the entire Bay when Mawar was active. This continues to be the case now due to Guchol, which has become a ‘severe tropical storm’ now. Winds have been blowing strongly towards the northeastward over the Bay, a key reason why the monsoon trough has been struggling to reach Kerala.

Little car on a highway

The strong southwesterly winds over the Bay of Bengal can be imagined to be a very large highway with heavy traffic heading from the southwest, over southern peninsular India and Sri Lanka, towards the South China Sea and the northwestern Pacific Ocean, feeding the monstrous typhoons there. The monsoon trough in the meantime is like a little car trying to cross this busy and wide highway from the Andaman Nicobar Islands to India across the Bay of Bengal.

This complicated dance of global warming affecting cyclogenesis over the Pacific and North Indian Oceans, the warming over the North Indian Ocean and the late pre-monsoon cyclones and typhoons is together just another monkey wrench in the dynamics of the monsoons – and in the predictions of the monsoon’s onset and its evolution through the season. Once seen as a very reliable system, with its annual migration northwestward and the withdrawal southeastward, the monsoon trough is now being kicked around in the game of climate-change football.

Fortunately, a late monsoon onset does not necessarily indicate a monsoon deficit. Then again, this year is unique, with an impending El Niño. So the nation waits and watches for the arrival of the monsoon – as always hoping for the best and preparing for the worst.

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

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