Global Monsoon and Low-Latitude Processes: Evolution and Variability

Thursday, 13 January 2011 13:48

The term monsoon generally refers to the seasonal reversal in atmospheric low-level circulation, especially the surface winds and the associated precipitation (wet summer and dry winter). These changes, which occur in the tropical and subtropical regions, arise from seasonal migration of the Intertropical Convergence Zone and the reversal in northern and southern hemispheric heating and temperature gradients between continent and ocean, resulting in alternation of wet and dry seasons. The concept of “global monsoon”, therefore, is introduced as a global-scale persistent overturning of the atmosphere throughout the tropics and subtropics that varies with the time of year (Trenberth et al., 2000; 2006). In the modern world, six monsoon systems are recognized including the African, South Asian, East Asian, Australian, North American and South American monsoons (Fig. 1; Wang and Ding, 2006 and 2008), although the latter two have not been clearly identified with wind reversals (Webster et al., 1998).

Fig. 1: Global Monsoon domains. Color shading shows the annual range of precipitation (summer minus winter normalized to annual mean). Black lines outline the Global monsoon precipitation domain, defined by an annual range greater than 2 mm/day and 70% of the annual mean precipitation. Red (Blue) line indicates the ITCZ position in Jan-Feb (July-August), estimated from the maximum precipitation rate (Figure from Wang and Ding 2008).

Since the regional monsoons are brought about by the same annual cycle of solar heating, but differ from each other because of their geographic position and orographic features, changes of the global monsoon in general are now studied as well as regional monsoon systems. For example, the precipitation-based “Global Monsoon Index” was found to be declining from 1948 to 2003 over the land monsoon regions, primarily due to a weakening of the summer monsoon rainfall in the Northern Hemisphere, but increasing from 1979-2003 over the oceanic monsoon regions (including marginal seas) (Fig. 2;Wang and Ding, 2006).

This PAGES Working Group was initiated in 2007 in an effort to bring this holistic approach to monsoon research, to the paleocommunity.

Fig. 2: Global land monsoon precipitation AD 1948-2003 from 4 different global rainfall datasets. Downward trend due to weakening of the summer monsoon in Northern Hemisphere. (a) Northern Hemisphere averaged June-July-August precipitation, (b) the Southern Hemisphere-averaged December-January-February precipitation, and (c) the global monsoon index (GMI), or the sum of Figures 1a and 1b (Wang and Ding, 2006).

References

Trenberth, K. E., Stepaniak, D. P., and Caron, J. M. (2000). The global monsoon as seen through the divergent atmospheric circulation. Journal of Climate 13, 3969-3993.
Wang, B., and Ding, Q. (2006). Changes in global monsoon precipitation over the past 56 years. Geophysical Research Letters 33, L06711. doi 10.1029/2005GL025347
Wang, B., and Ding, Q. (2008). Global monsoon: Dominant mode of annual variation in the tropics. Dynamics of Atmospheres and Oceans 44, 165-183. doi 10.1016/j.dynatmoce.2007.05.002
Webster, P. J., V.O., M., Palmer, T. N., Shukla, J., Tomas, R. A., Yanai, M., and Yasunari, T. (1998). Monsoons: processes, predictability and the prospects for prediction. Journal of Geophysical Research 103, 14451-14510.