India’s four-year run of munificent summer monsoon rainfall is likely to end this year, with the India Meteorological Department (IMD) forecasting a 4% shortfall in the coming monsoon season.
Though still categorised as ‘normal’, it is – at 96% of the Long Period Average (LPA) – at the lowest end of what the agency categorises as normal rainfall. Most recently, it was in 2017 that the IMD forecast 96% in April that year, for the monsoon, and India saw a 2.6% shortfall that year.
The key factor believed to be playing spoilsport this year is the development of ‘El Nino’, a cyclical phenomenon of warming in the central Pacific that in six out of ten years is linked to diminished rainfall in the country’s west, northwest and in western parts of central India.
Since 2019, India has been under the influence of the converse ‘La Nina’ or a cooling in those regions, and therefore, getting substantial monsoon rains.
In 2019 and 2020, for instance, India saw monsoon rains over 10% and 11% of the 87 cm LPA (a 50-year average from 1971-2022). Last year saw 6% more rains than what is usual. ‘Normal’ monsoon rainfall over India during June-September is 87 cm (considered 100% of the LPA) though this involves wide spatial variability. Anywhere from 96-104% of the LPA is considered ‘normal’ with 90-95% considered ‘below normal’ and less than 90% marked as ‘deficient.’
On Monday, private weather agency Skymet forecast the coming monsoons to be ‘below normal’ or 94% of the LPA, again premised on the developing ‘El Nino’, with Punjab, Haryana, Rajasthan and Uttar Pradesh expected to be see diminished rains in August and September.
‘El Nino’ effect
From 1951-2022, there have been 15 ‘El Nino’ years, defined as a greater-than-half-degree Celsius rise in temperatures in the central, equatorial Pacific Ocean with nine of those years witnessing ‘below normal’ rains. In 2015, the last ‘strong’ El Nino year (>1.5 C rise), monsoon rains shrivel by 14%. A ‘weaker’ El Nino (a sub-1C rise) in 2018 saw a contraction of 7.4%.
Experts say that while ‘El Nino’ conditions are imminent, there are ameliorating factors that may blunt its impact. One, ‘El Nino’ is only likely to begin to take root in the second half of the monsoon season – August and September. The weather models also indicate the development of a ‘positive’ phase of the Indian Ocean Dipole (IOD, or warmer temperatures in the Arabian Sea and hence more moisture and rainfall over India) during these months and so, a somewhat reduced impact of the ‘El Nino’, M. Ravichandran, Secretary, Ministry of Earth Sciences, told The Hindu.
Another factor that could blunt the ‘El Nino’ is reduced snow cover in Eurasia. “This February and March, we have seen below normal snow cover in Eurasia and typically reduced snow cover in these months is favourable for monsoon,” said M. Mohapatra, Director-General, IMD at a press briefing on Tuesday.
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In 1997, despite a ‘strong’ ‘El Nino’, monsoon rainfall turned out to be 2% more than normal and this was due to favourable IOD conditions.
Cultivation concerns
India’s kharif sowing, which begins in June, is extremely dependent on monsoon rainfall. Details on the parts of India that are likely to be most affected by the ‘El Nino’ are officially expected in May. “Most of central India, a key agricultural belt, as well as southern India are expected to get normal or even slightly above normal rain, the models so far suggest,” Mr. Ravichandran added.
In recent years, the IMD has started to place greater emphasis on the ‘dynamical’ monsoon forecast techniques where global atmospheric and ocean conditions are simulated on powerful supercomputers to forecast climate conditions. This is different from the traditional, statistical approach where 8-10 meteorological factors, such as Eurasian snow cover and Arabian sea surface, are correlated to the monsoon rainfall in past years to forecast a coming year’s monsoon. “Both the statistical and dynamical approaches, as of now, suggest 96% of the LPA,” said Mr. Mohapatra.
A positive IOD can’t entirely compensate for an ‘El Nino’ as the latter has a stronger effect on Indian monsoon, said D. Sivananda Pai, Director, Institute for Climate Change Studies, Kottayam, Kerala and closely involved with previous forecasts.
“The development of ‘El Nino’ after so many years of [La Nina-linked] rainfall is inevitable. An ‘El Nino’ is stronger than an IOD; however, there are many more intra-seasonal factors that will emerge during the monsoon season,” he told The Hindu.
Published - April 11, 2023 01:56 pm IST
