The role of the Atlantic Niño in India’s erratic monsoon
Last month, farmers in Madhya Pradesh threatened to sue IMD over the inaccurate monsoon forecast this year. A question has also been raised in Parliament as to whether the warming of the Arctic has led to an erratic monsoon this year.
The onset of the 2021 monsoon began on June 3, almost on time, but subsequently a precipitation deficit of up to 30% was observed in Kerala, Gujarat, Jammu and Kashmir, in the northeast and to Odisha. The rest of the country is barely normal with a deficit of less than 20 percent.
There is no El Niño brewing in the Pacific. Instead, a return of La Niña is expected by most models for later this year. Given that 2020 was also a La Niña year, one would expect the 2021 monsoon to be above normal. The Arctic may affect late season rainfall and September saw slightly above normal rains across India. But what can explain the deficits so far this season?
It is the little cousin of El Niño in the Atlantic, known as Atlantic Niño, or Atlantic Zonal Mode. Every few years, from June to August, there is a warming in the Eastern Equatorial Atlantic, which doesn’t get as much attention as its big brother El Niño. In 2021, Atlantic Niño made an appearance. Sea surface temperatures in the eastern Atlantic remained more than one degree warmer than normal this summer.
Its impact on the monsoon has been known since 2014 when a study conducted by INCOIS showed that the number of low pressure systems is greatly reduced by the Atlantic Niño, leading to monsoons in deficit. Researchers from IITM Pune 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 the number of low pressure systems has declined sharply, contributing up to 60% of the total seasonal precipitation over the central monsoon area.
Monsoon forecasting is a monumental challenge, especially with regard to the spatial distribution and northward migration of the monsoon trough.
Forecast models tend to rely heavily on El Niño for monsoon forecasts. But only about 50 percent of dry years are explained by El Niño. How to predict monsoons in years without El Niño? Clearly, Atlantic Niño is an important player in the evolution of the monsoon and models and forecasters need to pay attention to this Atlantic teleconnection.
Low pressure systems or LPS originate from the northern Bay of Bengal and are three to ten times more numerous during the active monsoon period.
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 that propagate in the Indian Ocean. These waves affect air temperatures over the Indian Ocean and influence land-ocean thermal contrast as well as LPS. The largest Atlantic Niño rainfall deficits tend to occur over the Western Ghats and the central monsoon area. The deficit patterns are a telltale sign of the influence of the Atlantic Niño.
Overall, the monsoon forecasting skill has increased in IMD, but even 70% accuracy means the forecast will be wrong 30% of the time.
Many Atlantic Niños occur in years other than El Niño, which provides a window of opportunity to increase forecasting skills based on the accurate prediction of the Atlantic Niño. Indian scientists at INCOIS have argued that the Atlantic Niño is in fact predictable up to three months in advance.
The next version of the forecasting system will hopefully be able to capture this predictability.
No forecast will ever be 100% accurate. Farmers are well aware of this and will continue to face the risks with every growing season. Climatologists are also aware of the challenge of monsoon forecasting and will continue to try to improve monsoon forecasting.
This column first appeared in the print edition on September 29, 2021 under the title “The Atlantic Niño effect”. The author is Professor, CMNS-Atmospheric & Oceanic Science, University of Maryland