New Delhi: Even as finance minister Pranab Mukherjee and agriculture minister Sharad Pawar on Friday described the monsoon situation as “grim", data from the India Meteorological Department (IMD) shows that low rainfall had been pretty much on the cards this year, judging from the monsoon’s behaviour in the past decade.

Rainfall in the decade between 1999 and 2009 was the lowest in the last eight decades. Summer monsoon months between June 2000 and 21 August 2009 registered only 8,124mm of rainfall, 5.5% short of the decadal normal of 8,900mm. The 1960s and 1970s, each of which saw three all-India drought years, posted 8,927.5mm and 8,850.9mm of rainfall respectively, only 0.2% and 0.5% away from the normal.

Graphics: Ahmed Raza Khan / Mint

The past decade has been different by another yardstick. Typically, scientists say, even drought-riddled decades have at least one compensatory year, in which rainfall is excess, defined as 10% more than the yearly monsoon normal of 89cm.

“Not a single year posted excess rainfall this decade. And that’s very unusual...perhaps a first in the last nine decades," said Madhavan Rajeevan, a meteorologist with the Indian Space Research Organisation, and formerly a top forecaster with IMD.

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Meteorologists add that monsoon rainfall this century broadly followed a 30-year cycle. The 1900s to 1930 saw severe droughts and rainfall years were usually below normal. The 1930s to the 1960’s saw good rainfall and few drought years; the 1960’s to the 1990s, again saw more droughts and depressed rainfall.

“Therefore, the 1990s to 2020 should have been characterized by good rainfall. The 90s were promising—there were no droughts," said Rajeevan. “But this decade has completely overturned that trend. The last time we had excess rainfall was 1994."

Why these 10 years saw exceptionally low rainfall is still a matter of research and debate, unlikely to be resolved anytime soon.

Some scientists blame a resurgent El Nino, a weather anomaly characterized by a 0.5-1 degree Celsius temperature increase on specific regions of the Pacific Ocean that, in a domino-effect of sorts, blows away rain-bearing monsoon clouds.

“If you just look at the numbers, slightly less than half of All-India drought years correspond to El Nino years. In the 1980s and 1990’s people thought El Ninos no longer had a bearing on India monsoons, because there were strong El Nino years and normal rainfalls. 2002 and 2004 changed all that," said D.S. Pai, one of the key officials involved with preparing the country’s monsoon forecasts.

The year 2002 was India’s last drought year, with monsoon rainfall at only 81% of the normal and 29% of the country affected. As per IMD’s official definition, 2004, in spite of seeing rainfall deficient by 13% of the normal, wasn’t a drought year. That’s because only 18.5% of the country’s area had deficient rainfall, a whisker short of the qualifying 20%.

Its failure to predict deficient rainfall forced the weather agency to abandon its workhorse forecasting models and adopt new weather models.

Other scientists say clues to the depressed rainfall this decade may lie closer home, in the Indian Ocean.

“Temperatures in the Indian Ocean have increased nearly 0.4 C over the last 30 years. Therefore, they may be drawing out moisture from the monsoon trough (a hulk of rain-bearing clouds that hover over the country between June and September)," said K. Krishnakumar, senior scientist at the Indian Institute of Tropical Meteorology (IITM), who studies the effect of changes in global climate on the monsoon.

Other veteran meteorologists, such as D.R. Sikka, a former director of IITM who’s chaired several IMD committees, theorize that the haze of black carbon particles, and other aerosols has the effect of absorbing moisture from clouds that contribute to the monsoon.

Sikka has worked with Veerabhadran Ramanathan of the Scripps Institute of Oceanography, University of California, San Diego. Ramanathan was among the first scientists to link the effects of black carbon particles and global warming in the 1970s.

But all of the meteorologists concede that their theories are still, at best, just that—theories. “There are several basic things not understood. For instance, the monsoon has a strange, unexplained way of compensating for itself. One bad monsoon month is usually followed by one month of excess rainfall," said Sikka. “That hasn’t happened this year. In fact, if all monsoon months this year post rain below their monthly normals, it will be a first since the drought year of 1972. Extremely unique."

Graphics by Paras Jain / Mint