MOTHER NATURE Mother Nature September 2017

Mother Nature Aug /Sep 2017 31 Monsoon have strengthened over north central India Monsoon have strengthened over north central India in the last 15 years, researchers from the prestigious Mas- sachusetts Institute of Technology have said, indicating a reversal in the general perception that the region has dried up in over a decade. The study said heightened monsoon activity has reversed a 50-year drying period during which the monsoon sea- son brought relatively little rain to northern and central India. "The Indian monsoon is considered a textbook, clearly defined phenomenon, and we think we know a lot about it, but we dont. Here, we identify a phenomenon that was mostly overlooked," said Chien Wang, a senior research scientist in MITs Department of Earth, Atmospheric and Planetary Sciences, the Center for Global Change Sci- ence. 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 pow- erful, damaging floods, to the populous north central re- gion of India. A shift in Indias land and sea temperatures may partially explain this increase in monsoon rainfall. "Theres this idea in peoples minds that India is going to dry up. The Indian mon- soon season is undergoing a longer dry- ing than all other systems, and this cre- ated a hypothesis that, since India is heavily polluted by manmade aerosols and is also heavily deforested, these may be factors that cause this drying. Model- ing studies also projected that this drying would continue to this century, but India has already begun to reverse its dry spell.” Wang wrote. The researchers note that starting in 2002, nearly the entire Indian subcontinent has experienced very strong warming, reach- ing between 0.1 and 1 degree Celsius per year. Meanwhile, a rise in temperatures over the Indian Ocean has slowed significantly. "Climatologically, India went through a sudden, drastic warming, while the Indian Ocean, which used to be warm, all of a sudden slowed its warming," They discovered that since 2002, pre- cipitation in the region has revived, increasing daily rainfall average by 1.34 millimetres per decade. The current strong monsoon trend is a result of higher land temperatures in combination with lower ocean temperatures. "This may have been from a combination of natural vari- ability and anthropogenic influences, and we are still try- ing to get to the bottom of the physical processes that caused this reversal," Wang said. Scientists had previously observed that, since the 1950s, the Indian monsoons were bringing less rain to north cen- tral India ? a drying period that didnt seem to let up, com- pared to a similar monsoon system over Africa and East Asia, which appeared to reverse its drying trend in the 1980s. The team tracked Indias average daily monsoon rain- fall from 1950 to the present day, using six global pre- cipitation datasets, each of which aggregate measure- ments 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. Wang notes that ocean cooling could be a result of the natural ebb and flow of long-term sea temperatures. Indias land warming on the other hand, could trace back to reduced cloud cover, particularly at low altitudes Normally, clouds act to reflect incoming sunlight. They observed that in recent years, In- dia has experienced a reduction in low clouds, perhaps in response to an in- crease in anthropogenic aerosols such as black carbon or soot, which can si- multaneously absorb and heat the sur- rounding air, and prevent clouds from forming.

MOTHER NATURE Mother Nature September 2017 | Page 32