Indian Monsoons Affected by Dust Heating the Air in Africa

Dust generated in North and West Africa can have a big impact on monsoons in India, according to new research published in Nature Geoscience.

Phil Rasch of the US Department of Energy's Pacific Northwest National Laboratory carried out the research with his colleagues, finding that as dust in the air absorbs sunlight west of India, it warms the air and strengthens the winds that carry moisture eastward. This results in more intense monsoon rainfall in India about a week later, according to the researchers, who suggest that the role of dust in the atmosphere could be used to explain changes in climate.

Summer monsoons are critical for life in India, and the new study highlights ways in which the monsoon season can be affected by what's going on in other parts of the world.

"The difference between a monsoon flood year or a dry year is about 10 percent of the average summer rainfall in central India. Variations driven by dust may be strong enough to explain some of that year-to-year variation," Rasch said.

Using satellite data, Rasch and his collaborators, including V. Vinoj of the Indian Institute of Technology in Bhubaneswar, India, established that a higher concentration of dust particles in the air above North Africa, West Asia, and the Arabian Sea seemed to be connected to stronger rainfall over India around the same time.

The went on to run experiments to test their theory, finding that dust was indeed a key ingredient that causes changes in Indian monsoons.

Using computer models, the researchers established that the atmospheric dust to the east led to relatively quick changes to monsoon intensity in India.

The researchers contend that this happens because dust can absorb sunlight that would normally reach the surface, warming the air instead. This warmer, dust-laden air pulls in moisture from the northern tropics, creating stronger winds that move moisture from the Arabian Sea to the India, where it lands as rainfall.

Rasch said that while African dust does influence monsoons, it does not overpower the other processes that affect the Indian monsoons, including temperature differences between land and ocean, pollution in India, global warming, and others.

"The strength of monsoons have been declining for the last 50 years," he said. "The dust effect is unlikely to explain the systematic decline, but it may contribute."