Normal monsoon forecast brings some cheer to India
Monsoon rains are likely to be normal this year at 100% of the long-period average (LPA), the India Meteorological Department (IMD) said on Wednesday, bringing some cheer to farmers reeling under the impact of the ongoing Covid-19 lockdown that has disrupted agricultural supply chains across India.
India receives about 70% of its annual rainfall during the monsoon season that generally begins in June and starts to retreat by September; the monsoon is key to the prospects of agriculture in the country.
For the first time in its monsoon forecast history, IMD on Wednesday also released new monsoon onset and withdrawal dates, factoring in the likely impact of climate change, which has altered the way the south-west monsoon progresses over India.
IMD’s new onset dates are based on an analysis of monsoon data from 1961 to 2019, and withdrawal dates are based on data from 1971 to 2019 by scientists in IMD, Pune.
The dates show that farmers may have to make some changes in their sowing schedule. The new onset dates are delayed by three to seven days in many parts of central, west and east India while withdrawal is delayed by a week to two weeks from northwest India.
But the onset date over Kerala continues to remain June 1 and withdrawal from the south peninsula October 15.
Until 2019, the onset and withdrawal dates were determined based on records of 149 meteorological stations between 1901 and 1940. “The data we were using for monsoon onset and withdrawal was very old and based on only a few stations. It’s better to use latest data as much as possible. We don’t know if the changes in monsoon dates are a result of climate change. It is a possibility. It could also be a normal change in monsoon cycle or what we call natural variability. These things will have to be studied further,” said M Rajeevan, secretary, ministry of earth sciences.
“Clearly taking into account data from the recent period makes more sense. Whether it is human-induced climate change or changes due to natural variability, taking data from so long ago (1901 to 1940) cannot be representative. What IMD has done is logical -- instead of farmers and others complaining about rains not arriving on time. The new data indicates a gap of one week for onset in some places and two weeks for withdrawal at some places, which is quite big,” said Krishna Achuta Rao, a professor of atmospheric sciences at the Indian Institute of Technology, Delhi
IMD said in its briefing on Wednesday that it expects a normal monsoon this year because prevailing neutral El Nino and Indian Ocean Dipole (IOD) conditions. El Nino is a climate pattern characterised by high sea surface temperatures in the tropical Pacific Ocean which is neutral now. El Nino years in India are linked to below normal monsoon rains and higher-than-normal frequency of heat waves. Last monsoon, weak El Nino conditions prevailed, which led to a delayed onset of monsoon rains, according to scientists.
IOD is characterised by warmer sea surface temperature in the equatorial Indian Ocean; positive IOD conditions are usually associated with normal or above normal monsoon rains.
But some global climate models are indicating the possibility of development of weak La Niña conditions over the Pacific Ocean during the second half of the monsoon season. La Niña is characterised by unusually cold ocean temperatures in the Equatorial Pacific, just the opposite of El Nino.
“La Nina is good for our monsoon. It is linked to normal or above-average rains in India,” said Rajeevan.
South-west monsoon rainfall is crucial to rice, wheat, sugarcane and oilseeds cultivation in the country, where farming accounts for about 15% of the economy and employs over half of its people. The farm-to-fork supply chain has been disrupted by the Covid-19 outbreak, subsequent lockdown, shortage of labour and transport bottlenecks.
Roxy Mathew Koll, a climate scientist at Pune’s Indian Institute of Tropical Meteorology, said global agencies too unanimously indicate normal monsoon rains for June to September, which is good news. “This is largely based on the favourable conditions in the Pacific as there is no coherent sign of an El Niño developing during the early stage of the monsoon.”
The rain in the pre-monsoon period has also been above average, which experts say will help improve and retain soil moisture and help with sowing in some parts of the country. From March 1 to April 15, central India received 150% and northwest India 52% excess rain even as east and northeast India are 46% deficient, according to IMD.
Ocean temperatures in the equatorial Indian Ocean are forecast to be warmer than normal, which has the potential to reduce monsoon rains but that is often not factored in by monsoon models, Koll said.
This monsoon also offers unique conditions for meteorologists to study this link because air pollution levels have dropped dramatically across the country since the lockdown imposed on March 25. The lockdown will continue till May 3. Several studies in the past have linked high air pollution levels in India with weakening of the south-west monsoon. Light-absorbing particles like black carbon (BC) scatter and absorb solar radiation which then affects the clouds. The particles also act as surfaces on which water vapour condenses, thereby impacting rainfall and monsoon wind pattern.
“I do not know if the monsoon models are factoring in this new scenario with less pollution. This has not happened before and hence can give us important insights,” added Koll.
As per IMD’s new analysis, the monsoon’s onset over Kerala will continue to be on June 1. But monsoon advance dates over Maharashtra, Gujarat, Madhya Pradesh, Chhattisgarh, Telangana, Andhra Pradesh, Odisha, Jharkhand, Bihar and parts of Uttar Pradesh are delayed by three to seven days compared to existing dates. Over northwest India, the monsoon will arrive a little earlier, on July 8 instead of July 15.
The monsoon will now withdraw from northwest India almost 7-14 days later but there is no change in the final withdrawal date over south India, i.e. October 15.
The zone-wise monsoon forecast will be released by IMD in the last week of May or the first week of June, IMD officials said on Wednesday.
IMD’s forecasts are prepared using the Statistical Ensemble Forecasting System (SEFS). Since 2012, IMD has also used the dynamical global climate forecasting system (CFS) model.