IMD’s ‘normal’ monsoon forecasts hide more than they reveal
New Delhi: Over the past 30 years, the government’s weather modellers have predicted a “normal” monsoon in all except five occasions. Sticking to that trend, ahead of an important election year, the India Meteorological Department’s (IMD) early forecast has pegged the upcoming monsoon as “normal”.
But these predictions of a “normal” monsoon, based on an estimate of the total volume of rain expected to fall on the Indian landmass, have begun to hide more than they reveal.
Between 1948 and 2015, the total quantum of rain that falls on the Indian landmass in the 122-day span of the monsoon has seen a small decline. Yet, the number of days of extreme rain (over 150mm a day) has increased significantly, particularly over central India, where it jumped three-fold.
The unevenness of the Indian monsoon is no longer a cautious rider to the overall national figures, but a near certainty. There is now substantial scientific evidence to show that the Indian monsoon is changing in fundamental ways.
“We are just carrying on with an IMD legacy,” said Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology. “The all-India forecast no longer gives too much meaningful information,” he said.
In 2017, when India had a healthy monsoon, Saurashtra and western Rajasthan received 40% excess rain while Uttar Pradesh and Vidarbha received 30% less. The national average hid these deep disparities.
The geographies affected in 2017 also seem to be part of a pattern as emerging evidence from the past six decades of rainfall data shows extreme monsoon events are concentrated along a band running through central India from Saurashtra-southern Rajasthan all the way up to Assam.
The final indication of a changing monsoon pattern is that the window of the rainy season itself is shrinking, at least since the mid-1970s.
The IMD has made attempts to move beyond national and broad regional forecasts to state-specific estimates. “Experimentally, we have started sharing state-level monsoon forecasts since 2017,” said M. Rajeevan, secretary, ministry of earth sciences.
A lot would ride on India’s ability to refine these sub-national forecasts to adapt to an increasingly erratic monsoon.
Because even in a “normal” monsoon year, extreme events, which are increasing over time, can reduce farm yields by 12-15%, according to the recent economic survey.
“The all India forecast matches very well with roughly about 20% of India’s landmass,” Koll said. “We don’t really know what could happen in the remaining 80% because the monsoon is getting more erratic,” he said.