Indian monsoons have strengthened over past 15 years, study shows

July 24, 2017, Massachusetts Institute of Technology
Hervey clouds formed during storm from north-west to south-east, at monsoon, over Salt Lake, Calcutta. Credit: Biswarup Ganguly/Wikipedia

An MIT study published in Nature Climate Change finds that the Indian summer monsoons, which bring rainfall to the country each year between June and September, have strengthened in the last 15 years over north central India.

This heightened 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.

Chien Wang, a senior research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences, the Center for Global Change Science, and the Joint Program for the Science and Policy of Global Change, says this sharp gradient in temperatures—high over land, and low over surrounding waters—is a perfect recipe for whipping up stronger monsoons.

"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 says. "This may have been from a combination of natural variability and anthropogenic influences, and we're still trying to get to the bottom of the physical processes that caused this reversal."

Wang's co-author is Qinjian Jin, a postdoc in the Joint Program for the Science and Policy of Global Change.

A theory drying up

The Indian monsoon phenomenon is the longest recorded monsoon system in meteorology. Measurements of its rainfall date back to the late 18th century, when British colonists established the country's first weather observatories to record the seasonal phenomenon. Since then, the Indian government has set up several thousand rain gauges across the country to record precipitation levels during the monsoon season, which can bring little or no rain to some areas while deluging other parts of the country.

From these yearly measurements, scientists had 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," Wang says. "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. Modeling studies also projected that this drying would continue to this century."

A persistent revival

However, Wang and Jin found that India has already begun to reverse its dry spell. The team tracked India's average daily 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 millimeters 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 millimeters per decade.

"The Indian monsoon is considered a textbook, clearly defined phenomenon, and we think we know a lot about it, but we don't," Wang says. "Here, we identify a phenomenon that was mostly overlooked."

The researchers did note a brief drying period during the 2015 that caused widespread droughts throughout the subcontinent. They attribute this blip in the trend to a severe El Niño season, where ocean temperatures temporarily rise, causing a shift in atmospheric circulation, leading to decreased rainfall in India and elsewhere.

"But even counting that dry year, the long-term [wetting] trend is still pretty steady," Wang says.

More questions ahead

The team believes the current strong monsoon trend is a result of higher land temperatures in combination with lower ocean temperatures. While it's unclear what is causing India to heat up while its oceans cool down, the researchers have some guesses.

For example, Wang says 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. Normally, clouds act to reflect incoming sunlight. But Wang and others have observed that in recent years, India has experienced a reduction in low clouds, perhaps in response to an increase in anthropogenic aerosols such as black carbon or soot, which can simultaneously absorb and heat the surrounding air, and prevent clouds from forming.

"But these aerosols have been around even during the drying period, so there must be something else at work," Wang says. "This raises a lot more questions than answers, and that's why we're so excited to figure this out."

Explore further: Manmade aerosols can alter rainfall in the world's most populous region

More information: A revival of Indian summer monsoon rainfall since 2002, Nature Climate Change (2017). nature.com/articles/doi:10.1038/nclimate3348

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julianpenrod
1 / 5 (6) Jul 24, 2017
A demonstration of the tendency of "science" to claim only what it wants to claim.
An anomalous progress of monsoon seasons. The original model, or "model", based on precepts like pollution accepted as necessarily valid, especially since they promoted political agendas. But, then, they fail to account for what actually happens. In the same way, the claim that aerosols like black carbon, again related to the politically popular "fossil fuels" issue, being blamed for their keeping clouds from forming. But, then, it's described that the levels of soot and such were the same in the past when such unnatural warming in India did not occur. Incidentally, is it that the amount of soot in the air failed to disperse clouds, then, or, for some reason, Indian air is warmer than it should be. The fact is, something other than the standard, politically motivated, causes are at work. It's chemtrails that are causing climate change.
Shootist
1 / 5 (5) Jul 24, 2017
as if anything less than a thousand years is enough time to develop a usable climate trend.
PTTG
4.8 / 5 (4) Jul 24, 2017
Julian, you need to speak with a councilor. It is possible to find help for your paranoia. You can be happier and more rational, but you need to reach out to someone with the experience and knowledge to help you.
Parsec
5 / 5 (3) Jul 25, 2017
It never ceases to amaze me that no matter what is reported, the luddite change deniers claim that its evidence that climate change is a hoax, or is somehow invalid.

This article is reporting actual data, ground level measurements. As the authors suggest, it is quite unclear what part of the changes (if any) can be attributed to AGW, and which part to the kinds of decadal oceanic oscillations that tend to overwhelm climate changes due to AGW over period of up to a decade or two.

It is literally impossible to know for sure.

Climate denial is certainly politically and ideologically motivated, but AGW science rests on a firm foundation of dozens of repeatedly verified mountains of data which are largely independent of each other. In other words, we not only have a video of the guilty party repeatedly shooting the victim, we also have the murder weapon with the shooters fingerprints, a clear motive, and dozens of eyewitnesses.

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