Monsoon model indicates potential for abrupt transitions

Science Daily: A self-amplifying effect presently sustains monsoon winds, but it could also disrupt the circulation over land and sea. The periodical rainfall could stop from one season to another or for months within seasons. High air pollution could lead to the disruption, researchers of the Potsdam Institute for Climate Impact Research report in the Proceedings of the National Academy of Sciences, Online Early Edition. Global warming increases the risk of abrupt monsoon transitions from high-precipitation to dry periods.

"The agricultural food supply for around two billion people in Asia and Africa depends on the eponymous regularity of monsoon rainfall," says the lead author Anders Levermann. The name "monsoon" stems from the Arabic word "mausim" for "season." However, months with extremely scarce precipitation have been observed within monsoon seasons, as in India in 2002, causing economic and humanitarian problems in the affected regions. During the past 11,000 years rainfall in monsoon regions has undergone strong and abrupt changes repeatedly.

The researchers investigated how such irregularities or disruptions of monsoon circulations can come about. "Our analysis shows on the basis of observations that there could be two stable states for monsoon systems and the possibility of abrupt transitions from one to the other," says Levermann.

The driving force of monsoon systems depends on different air temperatures. In spring, the air over land is warmed up more rapidly than the air over the sea. The warmer air rises and moist and cooler air from the ocean flows landwards and yields precipitation, which has two effects: the rainfall cools the land surface, but also releases latent heat, when water vapour condenses to raindrops. The more moist air is transported landwards, the more latent heat is released through rainfall and the more moist air is drawn towards land. This self amplification, named moisture-advection feedback, sustains the temperature difference and the entire circulation. However, the self-amplification is vulnerable and may lead to abrupt changes in response to relatively weak external perturbations, the authors state….

A sunset view on monsoon clouds over the Bay of Bengal, from a beach in Tamil Nadu, India. Shot by mmj171188, Wikimedia Commons via Flickr, under the Creative Commons Attribution ShareAlike 2.0 License.