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Dust over North Africa boosts Indian monsoon

Swirling desert dust in North Africa and West Asia stimulates critical monsoon rains over India, according to a new study.

The study shows that dust in the air absorbs sunlight west of India, warming the air and strengthening the winds carrying moisture eastward.

This results in more monsoon rainfall about a week later in India.

The results explain one way that dust can affect the climate, filling in previously unknown details about the Earth system.

The study also shows that natural airborne particles can influence rainfall in unexpected ways, with changes in one location rapidly affecting weather thousands of miles away.

The researchers analysed satellite data and performed computer modelling of the region to tease out the role of dust on the Indian monsoon.

India relies heavily on its summer monsoon rains.

Phil from the Pacific Northwest National Laboratory and V Vinoj of the Indian Institute of Technology (IIT) Bhubaneswar and colleagues wanted to explore whether higher amounts of small particles called aerosols over North Africa, West Asia, and the Arabian Sea are connected to stronger rainfall over India around the same time.

The team used a computer model called CAM5 and focused on the area. The model included man-made aerosols from pollution, and natural sea salt and dust aerosols.

They ran the model and noted a similar connection: more aerosols in the west meant more rainfall in the east. Then they systematically turned off the contribution of each aerosol type and looked to see if the connection remained.

Dust turned out to be the necessary ingredient. The condition that re-created stronger rainfall in India was the rise of dust in North Africa and the Arabian peninsula.

To see how quickly dust worked, they ran short computer simulations with and without dust emissions.

Without dust emissions, the atmospheric dust disappeared within a week compared to the simulation with dust emissions. Rainfall declined in central India as well. This indicated the effect happens over a short period of time.

The researchers noted that dust can absorb sunlight that would normally reach the surface, warming the air instead.

This warmer dust-laden air draws moist air from the tropics northward, and strengthens the prevailing winds that move moisture from the Arabian Sea into India, where it falls as rain.

Although dust plays a role in strengthening monsoons, this natural phenomenon does not overpower many other processes that also influence monsoons, said Rasch.

The study was published in the journal Nature Geoscience.

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