Second half of primary rainfall season for India will witness impact of El Nino, says IMD
The Southwest Monsoon 2023 is likely going to be normal, according to the India Meteorological Department (IMD). This is despite IMD also stating that the second half of the primary rainfall season for India would witness the impact of the developing El Niño event in the equatorial Pacific Ocean, which generally dampens monsoon rainfall.
The IMD, in its first long-range monsoon forecast issued on April 11, predicted that India would receive 96 per cent of its long-period average rainfall (LPA) in the monsoon season, which usually begins around the first week of June. IMD calculates the chance of this happening at 35 per cent.
The LPA for the period between June and September is 87 centimetres and is calculated as the average rainfall between 1971-2020. This would mean there would be 83.5 cm of rainfall across India.
The weather agency categorises a monsoon season as normal if the rainfall received is between 96-104 per cent of the LPA. Therefore this year’s forecast lies at the lowermost end of the category with a model error of +/-5 per cent which could make it anywhere between 91 and 101 per cent. A season is categorised as below normal if the rainfall received is 90-95 per cent of the LPA.
Skymet Weather, a private weather forecasting agency based in Noida, Uttar Pradesh, had predicted a below-normal monsoon season on April 10. It had said that the total monsoon rainfall across India would be 94 per cent of the LPA.
On the other hand, the IMD also gives a 29 per cent chance of the season being a below-normal rainfall season and a 22 per cent chance of it being a deficit monsoon. There’s a 51 per cent chance of monsoon being below normal or deficit this year.
The probability forecast map of the expected rainfall during the season shows below-normal rainfall in most parts of northwestern, western, central, some parts of the northeast and southern India. Above normal rainfall is restricted to the Union territories of Jammu and Kashmir, the easternmost parts of North East India and some parts of southern India.
The IMD said three large-scale climatic phenomena would be affecting the monsoon season rainfall. The first is the El Niño which is the warmer than normal phase of the El Niño Southern Oscillation (ENSO) phenomenon and generally decreases monsoon rainfall over India.
Currently, neutral ENSO conditions are prevailing in the equatorial Pacific Ocean, but these are likely to change to El Niño conditions during the monsoon season and may impact rainfall in the second half of the season.
“All El Niño years are not bad monsoon years. Between 1951-2022 40 per cent (six out of 15) of the years have been normal or above normal,” said Mrutyunjay Mohapatra, director general of the IMD, in a press conference.
The second large-scale climatic phenomenon is the Indian Ocean Dipole (IOD) which occurs due to differential warming of the western and eastern sides of the equatorial Indian Ocean.
When the western side is warmer, IOD is said to be in a positive phase and this enhances monsoon rainfall over India and when the eastern side is warmer, IOD is said to be in a negative phase and it decreases monsoon rainfall over India.
Currently, neutral IOD conditions are prevailing, but IMD predicts the conditions to change to the positive mode during the season, which would aid the rainfall, according to IMD.
“IOD occurs during September-November and I don’t think it has a robust link to the monsoon. IOD is not really predictable with high skill either,” said Raghu Murtugudde, a climate scientist from the University of Maryland and Indian Institute of Technology, Bombay.
The third large-scale climatic phenomenon mentioned by IMD is the snow cover over the northern Himalayas and the Eurasian landmass, which also has an impact on the Indian Monsoon through the differential heating of the landmass.
There has been lower-than-normal snowfall over the northern Himalayas and Eurasia during December 2022-March 2023, which is good for the monsoon, according to IMD.
“The historic relation says less snow is better for the Indian monsoon. But La Niña tends to increase snowfall over Eurasia although the teleconnections have been modified in recent years,” said Murtugudde. Eurasia had below-normal precipitation during 2020-2023, he added.
The El Niño impact on the seasonal amount should be higher but one has to gear up for the dry and wet extremes again. The Arabian and the Bay of Bengal have warmed so we can expect some heavy rain fireworks in the northwest India and Pakistan, said Murtugudde.
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