India Meteorological Department predicts intensity of monsoon rain could be between 96-104% of the long period average
The southwest monsoon from June to September over India may be normal, according to a long-range forecast issued by the India Meteorological Department (IMD) on April 15, 2020.
The IMD predicted the intensity of monsoon rainfall could be anywhere between 96 and 104 per cent of the long period average (LPA).
The LPA of the monsoon season rainfall over the country as a whole for 1961-2010 was 880 mm.
A good monsoon this year is crucial for farmers across India, after they were hit by losses due to excess rain in March and a nationwide lockdown to curb the spread of the novel coronavirus disease (COVID-19).
The neutral El Nino Southern Oscillation (ENSO) in the equatorial Pacific Ocean and Indian Ocean Dipole (IOD) conditions prevailed, with climate models suggesting that they would be the same for the entire monsoon, according to the IMD forecast.
Climate models also predicted the occurrence of a weak La Nina condition, defined by a cooling of sea waters in the equatorial Pacific Ocean, the forecast said.
This has a reverse impact of the El Nino phenomenon and leads to a basic cooling effect on worldwide climatic conditions.
Global climate model predictions prior to and during the spring season generally have noticeable uncertainty due to spring barrier in seasonal predictability, according to the IMD forecast.
Experts warned that this prediction of normal rainfall should be taken with a pinch of salt as it was too early to make accurate predictions at this stage.
“You get no credit for forecasting ‘normal’, according to some skill measures”, said Raghu Murtugudde, a climate scientist at the University of Maryland, United States.
The models used by the IMD also depended too heavily on ENSO and IOD, while ignoring the Atlantic Niño or the Atlantic Zone Mode, according to him.
This Atlantic mode was done wrong in the Coupled Forecasting System model used by the IMD, affecting forecasts, he said, citing a paper by the Indian Institute of Tropical Meteorology in Pune.
A warmer Atlantic led to fewer monsoon depressions bringing more rain, while a colder Atlantic led to more monsoon depressions, according to a paper published in the Journal of Geophysical Research in 2014.
Depressions during the warm Atlantic mode had longer tracks, leading to better distribution of rain over the Indian subcontinent.
The paper also showed a tele-connection link between the conditions in the Atlantic and Indian Oceans, crucial for understanding and forecasting monsoon behaviour.
This called for an integration of Atlantic Zone Mode impacts in India’s forecasting models.
The tropical Atlantic was warmer, indicating a move towards the warm Atlantic mode, according to a National Oceanic and Atmospheric Administration (NOAA)-generated temperature map from January to March, 2020.
The warming of the land surface — especially in the northern part of the country — was another important factor to be taken into account.
This was essential for attracting monsoon winds into the country due to the temperature contrast between the land and ocean.
“The NOAA map showed that between January and March, the northern part of India was strangely cooler than usual. April till now has not been as hot as it should be,” said Murtugudde.
The reason for this was a flurry of western disturbances and the rainfall that they brought. The rainfall in March was exceptionally high with a countrywide excess of 46 per cent between March 1 and April 1.
It was good to take the IMD forecast as a good guess and wait for the one that will come later in May, said Murtugudde, adding that models tended to be better as they got closer to June.
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