Climate change upsets monsoon

New monsoon model points to the possibility of abrupt changes

Sketch of the model concept and the feedback mechanism [W = wind, P =
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A self-reinforcing effect currently maintains monsoon winds, but could also disrupt airflow over land and sea. The regular rainfall of the monsoon would then be missing from one year to another or months in a season. Heavy air pollution could bring about such an abrupt end. Researchers now report this in the current issue of the journal "Proceedings of the National Academy of Sciences" (PNAS).

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Global warming is also increasing the risk of abrupt transitions between monsoon-rich and dry phases, say the scientists at the Potsdam Institute for Climate Impact Research (PIK) in their new study.

Two billion people are dependent on the monsoon

"The eponymous regularity of monsoon precipitation depends on the agricultural food supply of about two billion people in Asia and Africa, " says Anders Levermann. The name "Monsoon" was derived from the Arabic word "Mausim" for season. Sometimes, however, extremely dry months are observed within the monsoon seasons, such as in India in 2002, leading to economic and humanitarian problems in the affected regions. Over the past 11, 000 years, rainfall in monsoon areas has fluctuated dramatically over extended periods.

The team around Levermann now went to the question of how such fluctuations or crashes of the monsoon circulations can come about. "Our analysis shows, based on observations, that monsoon systems could have two stable states, " says Levermann. Between both states abrupt transitions are possible. display

Different air temperatures as a driving force

The driving force of monsoon circulations are different air temperatures. In spring, the air is heated more over land than over the sea. The warm air rises and draws moist and cooler air from the sea. The precipitation that occurs over land has two effects: on the one hand they cool the land surface, on the other hand latent heat is released when water vapor condenses into raindrops. The more moist air is transported to the country and raining there, the more heat is released and the more humid air is absorbed by the sea.

This self-amplification, the so-called moisture advection feedback, maintains the temperature difference and circulation. However, the mechanism is vulnerable and could cause abrupt changes even with small disturbances, the researchers report.

You have now developed a conceptual monsoon model that maps the feedback. The underlying equations show that solar radiation must exceed a threshold to initiate monsoon circulation. If this threshold is not reached, for example due to heavy air pollution, no circulation starts. Above the critical value, two stable states are possible: one with and one without circulation.

Two options for sudden transitions

This gives rise to two possibilities for abrupt transitions between the stable states: Climatic changes could lead the circulation system beyond the threshold value. The transition would then take place from one to the other monsoon season and the new state would last as long as the climate change. The second possibility for an abrupt transition is between the two stable states above the threshold - the area where the monsoon systems are located today. Within one season, a weakening of the monsoon winds and the release of latent heat over land could cause the temperature difference between land and sea to be too low and the circulation to break off.

Using the model and observational data from the past 60 years, researchers narrowed thresholds for monsoon systems in India, China, Bangladesh, West Africa, North America, and Australia. "In the future, we want to be able to make more precise statements about the susceptibility of monsoon systems, " says Jacob Schewe, a co-author of the study.

Many uncertainties

The provisional estimates are still subject to great uncertainty. As global warming increases in precipitation, increasing air pollution, for example in India and China, may limit the stability of monsoon systems there.

"Switching between rainy and extremely rainy months could overwhelm the adaptability of people in the affected regions, " says Schewe. The researchers therefore want to investigate further in further studies, what is the risk of abrupt transitions in the various monsoon regions.

(idw - Potsdam Institute for Climate Impact Research, 20.10.2009 - DLO)