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Tuesday, October 26, 2021

The Atlantic Niño’s role in India’s erratic monsoon

Raghu Murtugudde writes: IITM Pune researchers have shown that the IMD prediction system is deficient in predicting the Atlantic Niño and, therefore, its effect on the Indian monsoon. Monsoon 2021 is a clear example of this missed link.

Written by Raghu Murtugudde |
Updated: September 30, 2021 7:30:29 am
A boy swims in a waterlogged street during monsoon rains in Kolkata (AP)

Last month, farmers from Madhya Pradesh threatened to take IMD to court for the inaccurate monsoon forecast this year. A question was also raised in Parliament about whether the Arctic warming had led to an erratic monsoon this year.

The onset of monsoon 2021 began on June 3, almost on time but subsequently, rainfall deficit of up to 30 per cent were seen in Kerala, Gujarat, Jammu and Kashmir, the Northeast and Odisha. The rest of the country is barely normal with deficit being less than 20 per cent.

There is no El Niño brewing in the Pacific. Instead, a return of the La Niña is forecasted by most models for later this year. Considering that 2020 was also a La Niña year, one would expect monsoon 2021 to be above normal. The Arctic can affect late-season rainfall and September has seen slightly above normal rain across India. But what can explain the deficits thus far this season?

It is El Niño’s little cousin in the Atlantic, known as the Atlantic Niño, or the Atlantic Zonal Mode. Every few years, from June to August, there is a warming in the eastern equatorial Atlantic, which does not get as much attention as its big brother El Niño. In 2021, Atlantic Niño has made an appearance. Sea surface temperatures in the eastern Atlantic have remained more than a degree higher than normal this summer.

Its impact on the monsoon has been known since 2014 when a study led by INCOIS showed that the number of low-pressure systems is greatly reduced by the Atlantic Niño, leading to deficit monsoons. IITM Pune researchers have shown that the IMD prediction system is deficient in predicting the Atlantic Niño and, therefore, its effect on the Indian monsoon. Monsoon 2021 is a clear example of this missed link.

This year has seen a sharply lower number of low-pressure systems, which contribute up to 60 per cent of the seasonal total rainfall over the core monsoon zone.

Monsoon predictions are a monumental challenge, especially when it comes to the spatial distribution and the northward migration of the monsoon trough.

Forecast models tend to rely heavily on El Niño for monsoon predictions. But only about 50 per cent of the dry years are explained by El Niño. How can monsoons be predicted during non-El Niño years? Clearly, Atlantic Niño is a significant player in monsoon evolution and models and forecasters must pay attention to this Atlantic teleconnection.

Low-pressure systems or LPSs originate in the northern Bay of Bengal and are three-10 times more in number during the active period of the monsoon.

The Atlantic and Indian Oceans are not directly connected in the tropics via the ocean. The Atlantic Niño affects the monsoon by producing atmospheric waves, which propagate into the Indian Ocean. These waves affect air temperatures over the Indian Ocean and influence the land-ocean thermal contrast as well as LPSs. The biggest rainfall deficits from the Atlantic Niño tend to occur over the Western Ghats and the core monsoon zone. The deficit patterns are a tell-tale sign of the Atlantic Niño influence.

Overall, monsoon prediction skill has gone up in the IMD but even a 70 per cent accuracy means the forecasts will be wrong 30 per cent of the time.

Many of the Atlantic Niños occur during non-El Niño years and this offers a window of opportunity to increase forecast skills based on the accurate prediction of the Atlantic Niño. Indian scientists from INCOIS have argued that the Atlantic Niño is in fact predictable up to three months in advance.

The next version of the forecast system will hopefully be able to capture this predictability.

No forecasts will ever be 100 per cent accurate. Farmers are well aware of that and will continue to brave the risks every single cropping season. Climate scientists are also aware of the monsoon prediction challenge and they will continue to try to improve monsoon forecasts.

This column first appeared in the print edition on September 29, 2021 under the title ‘The Atlantic Niño effect’. The writer is professor, CMNS-Atmospheric & Oceanic Science, University of Maryland

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