Monsoon: dancing to a common tune

A monsoonal climate is characterized by strong seasonality: the wind direction reverses annually and heavy rainfall occurs primarily during local summer. Monsoonal climate exists in various low-latitude regions of the world. The region stretching from southern to eastern Asia provides a well-known example, but monsoons also occur in Australia, Africa and the Americas. The regional monsoons have been recently proposed to be parts of an interconnected planetary-scale circulation system. A new synthesis by the International Geosphere-Biosphere Programme (IGBP)’s Past Global Changes (PAGES) project examines the evidence for a global monsoon, today as well as in the geological past, and discusses how it might vary in the future in response to climate change.

The researchers sought to confirm the global nature of the modern monsoon by re-assessing instrumental and satellite data for the past several decades. To uncover evidence of a coherent global monsoon in the past they turned to proxies – for example methane concentrations in ice core air bubbles for a globally averaged signal or the oxygen-isotopic composition of stalagmite calcite to capture the amount of local rainfall. They found that in modern as well as in ancient times the regional monsoon systems seem to have acted as a single global phenomenon over annual, millennial and longer timescales.

The monsoon is crucial to the economies of many regions of the world. In India, for example, agriculture is strongly dependent on monsoonal rainfall. In that context it is important to understand how the global monsoon will vary in the future. Recent research as summarised in the PAGES synthesis points to a future with increasingly frequent heat waves in the regions experiencing a monsoonal climate. Precipitation is also expected to increase significantly in a warmer climate, particularly during the summer monsoon season and in the Northern Hemisphere, and floods and droughts are expected to occur more frequently.

A key uncertainty in projecting future climate change is climate sensitivity – the response of global average surface temperature to the doubling of the atmospheric carbon-dioxide concentration. The PAGES researchers suggest that improving the representation of the global monsoon in climate models may help us to better understand the role of clouds and thereby to better constrain climate sensitivity.

Reference:

Wang, P.X., Wang, B., Cheng, H., Fasullo, J., Guo, Z.T., Kiefer, T., Liu, Z.Y., 2014. The global monsoon across timescales: coherent variability of regional monsoons. Climate of the Past 10, 2007-2052, doi:10.5194/cp-10-2007-2014, 2014.