According to a new NASA study, aerosols are responsible with an increase of rainfall amount during the monsoon season over Northern India and Tibetan Plateau.

Aerosols are small dust particles that blow in from deserts and collect in the atmosphere over the plateau's slopes early during the monsoon. A monsoon is a seasonal change in wind direction that alternately brings very wet and then very dry seasons to India, Southeast and Eastern Asia. 60 percent of the world's population is dependent on monsoon's rain for food.

A NASA team found aerosols can heat the air by absorbing the sun's radiation, altering the Asian monsoon water cycle, using computer models. Carbon particles from human activity (industry, fuel burning and forest fires) can add to this warming effect. Black carbon amplifies the heating effect because it absorbs solar radiation more efficiently than dust.

Little was known till now about how aerosols interact with the atmosphere to influence monsoons. Computer simulations indicate that heat absorbing-emitting aerosols, when mixed together with warm air currents and moisture, cause a heating effect in the air, triggering the rainy season earlier than usual and lengthening the wet monsoon season in Asia. "Traditionally, aerosols have been seen as only a local environmental problem. Until very recently, aerosols have not been viewed as an intervening presence in the atmosphere that could affect monsoon rains," said William Lau research scientist at NASA's Goddard Space Flight Center, Greenbelt, Md.

"This study is the first to link dust aerosols to monsoon rainfall changes and to claim a specific physical mechanism in the atmosphere, whereby the tiny dust particles interact with the monsoon heat and moisture." The process was named "elevated heat pump".

Dust aerosols blowing in from deserts of western China and the Middle East coupled with black carbon emissions from northern India accumulate in the pre-monsoon late spring in the atmosphere over the slopes of the Tibetan Plateau.

As the air warms and moves upward, new moist air from the Indian Ocean is drawn in to take its place, which is also warmed - creating a process like a pump that pulls heated air upwards. This creates more rainfall in Northern India by starting prematurely the rainy season and lengthening it. The "elevated heat pump" effect will shift the monsoon's path towards the foothills of the Himalayas, meaning that, as a result, more rain will fall earlier in the season (in May) in northern India, and less over the Indian Ocean to the south.

The combination of warming and more rain may increase glacier melt and erosion in Himalaya. "Understanding the relationship between aerosols and the cycle of rainfall has a potential impact on water resources all over the globe." said Lau.

The findings will be checked with observations from satellites and from NASA's Aerosol Robotic Network (AERONET), a global network of ground-level aerosol sensors.

The scientists are now examining the "elevated heat pump" effect over South America and West Africa.