Geophys. Res. Lett. http://doi.org/m6p (2013)

Credit: © THINKSTOCK/HIGHSTOCK

Changes in the South Asian monsoon can severely impact vulnerable regions and affect a large number of people. May–June precipitation in central and northern India has increased for the period 1950–1999, resulting in the onset of the monsoon occurring 10–20 days earlier.

Massimo Bollasina, from Princeton University, USA, and colleagues used a climate model with fully interactive aerosols and chemistry to investigate the interaction of increased aerosol emissions with the monsoon for the period 1860–2005.

They find that strong aerosol forcing over the Bay of Bengal and Indochina in the spring increases atmospheric stability and cools the sea surface temperature, preventing a northward shift of precipitation. This leads to thermodynamical changes, such as increased surface temperature, in northwest India. These changes result in monsoon precipitation arriving into the region in June. The aerosol–precipitation–circulation interactions reported here can help to explain the early arrival of the monsoon.