A new analysis of data from satellite has revealed a link between dust in North Africa and West Asia and stronger monsoons in India.
Rasch, V. Vinoj of the Indian Institute of Technology Bhubaneswar, India, and their coauthors wanted to explore a correlation that appeared in satellite records: higher amounts of small particles called aerosols over North Africa, West Asia, and the Arabian Sea seemed to be connected to stronger rainfall over India around the same time. The team wanted to see if they could verify this and determine how those particles might affect rainfall.
To explore the connection, the team used a computer model called CAM5 and focused on the area. The model included manmade aerosols from pollution, and natural sea salt and dust aerosols.
First, the team 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 and rainfall declined in central India as well. This indicated the effect happens over a short period of time.
But there was one more mystery, how did dust do this to rainfall? To explore possibilities, the team zoomed in on the regional conditions such as air temperature and water transport through the air.
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Their likeliest possibility focused on the fact 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.
The study has been published in the journal Nature Geoscience.