Dust over Africa, Gulf linked to Indian monsoon

Increased dust in the air over northern Africa and West Asia directly gives the monsoon in India a boost, a new study has found.

The study, which analysed satellite images, shows that dust in the air towards the west of India absorbs sunlight, thus warming the air and strengthening the winds carrying moisture eastward.

This results in more monsoon rainfall about a week later over the Indian subcontinent, especially its central parts, says the study published in the latest issue of Nature Geoscience.

As part of the study, researchers -- including V. Vinoj of the Indian Institute of Technology (IIT), Bhubaneswar -- analysed satellite data and performed computer modelling of the region to look at the role of dust on the monsoon rains -- the lifeline for the entire subcontinent.

"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," climate scientist Phil Rasch of the Pacific Northwest National Laboratory was quoted as saying.

Rasch and Vinoj, along with other researchers, sought 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.

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.

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, especially in northern Africa and the Arabian peninsula, turned out to be a key ingredient that resulted in stronger monsoon rainfall.

To study how quickly dust worked, the team ran short computer simulations with and without dust emissions.

Without dust emissions, rainfall declined in central India. It indicated that the effect happened over a short period of time.

So, how did dust affect rainfall?

The likeliest answer: the 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 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."