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Atmospheric aerosols affect the Indian monsoon remotely

Aerosols are extremely small solid or liquid particles that remain suspended in air. Examples of such aerosols include dust, smoke and deodorant sprays. Apart from causing local air pollution, these particles reflect and absorb radiation from the sun and hence affect both local and global climate. A recent study published in the International Journal of Climatology has investigated how soot aerosols accumulating in other parts of Asia influence the Indian summer monsoon.

The presence of carbon-based aerosols (like soot from the burning of coal) in the atmosphere affects the climate in two very different ways. The particles absorb solar radiation and hence heat the atmosphere. They reduce the solar radiation reaching the surface and hence cool the surface.

Most carbon aerosols are found in the lower atmosphere – in the troposphere, the layer of atmosphere closest to the earth. Heating of the lower troposphere and the land surface cooling caused by aerosols can alter the circulation pattern of winds and rainfall, including the Indian monsoon. Also, aerosols change properties of clouds and modify rainfall patterns. These effects can alter the water cycle over different parts of the world.

J Srinivasan, lead author of the paper, said that different studies have found conflicting results on how aerosols affect the Indian monsoon. “One group has argued that soot in the atmosphere will cool the land surface and hence reduce monsoon rainfall. The other group has argued that soot will heat the atmosphere and hence increase rainfall in May and June”, he said.

Until now, studies have focused on the “local” effects of aerosol accumulation – they have studied how rainfall in India will be affected when aerosols are released from within the country. In this study, the researchers focused on how the presence of aerosols over other parts of Asia affects the Indian monsoon. “The Indian summer monsoon is a complex phenomenon that is influenced by many factors such as variations in patterns of sea surface temperature, mountains, water vapor, carbon dioxide and aerosols ”, said Srinivasan. “In this study we , we have looked only at the direct effect of aerosols.”

Using a mathematical model called the “General Circulation Model”, the researchers looked at aerosol levels and rainfall in South and East Asia. Based on their simulations, they found that when there was aerosol heating over East Asia (and not anywhere else), rainfall over northern Arabian Sea and Bay of Bengal increased. On the other hand, when there was no aerosol heating over East Asia, rainfall over the Indian region reduced substantially.

“When all other factors affecting the monsoon are kept constant, aerosol concentration over East Asia affects rainfall over India. Decrease in aerosols over East Asia can actually decrease the rainfall received during the Indian monsoon”, said Srinivasan. “This means, aerosols affect the climate not just locally but also remotely.”

Link to the paper: http://onlinelibrary.wiley.com/doi/10.1002/joc.3826/abstract

About the authors:

Arindam Chakraborty: Assistant Professor, Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science. Ravi S. Nanjundiah and J. Srinivasan are both Professors at the same centre. All three are also affiliated with the Divecha Centre for Climate Change at the Indian Institute of Science.

Contatct:

Prof J Srinivasan: http://caos.iisc.ernet.in/faculty/jayes.html; email: jayes@caos.iisc.ernet.in