Monsoon has strengthened over north central India in the last 15 years, researchers from the prestigious Massachusetts Institute of Technology (MIT) have said, indicating a reversal in the general perception that the region has dried up in over a decade.

Chien Wang, a senior research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, the Center for Global Change Science, co-authored the study with Qinjian Jin, a postdoctoral researcher in the Joint Programme for the Science and Policy of Global Change, on Indian Monsoon was published yesterday in Nature Climate Change. “The Indian monsoon is considered a textbook, clearly defined phenomenon, and we think we know a lot about it, but we don’t. Here, we identify a phenomenon that was mostly overlooked,” said Chien Wang.

The study said heightened monsoon activity has reversed a 50-year drying period during which the monsoon season brought relatively little rain to northern and central India. Since 2002, the researchers have found, this drying trend has given way to a much wetter pattern, with stronger monsoons supplying much-needed rain, along with powerful, damaging floods, to the populous north central region of India.

A shift in India’s land and sea temperatures may partially explain this increase in monsoon rainfall. The researchers note that starting in 2002, nearly the entire Indian subcontinent has experienced very strong warming, reaching between 0.1 and 1 degree Celsius per year. Meanwhile, a rise in temperatures over the Indian Ocean has slowed significantly, it said.

Indian Ocean warming slows

“Climatologically, India went through a sudden, drastic warming, while the Indian Ocean, which used to be warm, all of a sudden slowed its warming,” Wang said. “This may have been from a combination of natural variability and anthropogenic influences, and we are still trying to get to the bottom of the physical processes that caused this reversal,” he said in a statement.

Scientists had previously observed that, since the 1950s, the Indian monsoons were bringing less rain to north central India - a drying period that didn’t seem to let up, compared to a similar monsoon system over Africa and East Asia, which appeared to reverse its drying trend in the 1980s.

“There’s this idea in people’s minds that India is going to dry up. The Indian monsoon season is undergoing a longer drying than all other systems, and this created a hypothesis that, since India is heavily polluted by manmade aerosols and is also heavily deforested, these may be factors that cause this drying. Modelling studies also projected that this drying would continue to this century,” Wang wrote.

Reversal of dry spell

In their research, Wang and Jin found that India has already begun to reverse its dry spell. The team tracked India’s average daily monsoon rainfall from 1950 to the present day, using six global precipitation datasets, each of which aggregate measurements from the thousands of rain gauges in India, as well as measurements of rainfall and temperature from satellites monitoring land and sea surfaces.

Between 1950 and 2002, they found that north central India experienced a decrease in daily rainfall average, of 0.18 millimetres per decade, during the monsoon season. To their surprise, they discovered that since 2002, precipitation in the region has revived, increasing daily rainfall average by 1.34 millimetres per decade.

Wang and Jin believe the current strong monsoon trend is a result of higher land temperatures in combination with lower ocean temperatures. Wang noted that ocean cooling could be a result of the natural ebb and flow of long-term sea temperatures. India’s land warming on the other hand, could trace back to reduced cloud cover — particularly at low altitudes — in recent years. The possible reason for the reduced cloud cover could be in response to an increase in anthropogenic aerosols such as black carbon or soot, which can simultaneously absorb and heat the surrounding air preventing the formation of clouds. However, Wang is doubtful about the same, stating, “These aerosols have been around even during the drying period, so there must be something else at work. This raises a lot more questions than answers, and that’s why we’re so excited to figure this out”.