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The monsoon is a continental-size convective system with a large area of monthly mean precipitation. In the northern hemisphere the monsoon has a characteristic monthly mean southwesterly flow at the low levels and a reverse flow at the high levels. In the traditional interpretation, which dates back to the 17th century, the monsoon is attributed to land-sea contrast on the continental-scale. This textbook interpretation is now challenged by a new theory in a paper by Winston Chao and Baode Chen. In the new interpretation the monsoon is equated with an intertropical convergence zone (ITCZ, a zonal band of convective systems) located substantially away (more than ten degrees) from the equator and the existence of the ITCZ does not have to rely on land-sea contrast. Land-sea contrast can provide a favorable longitudinal location for the ITCZ, but this role can be replaced by sea surface temperature contrast in the longitudinal direction. Thus, this new interpretation of the monsoon differs from the long-held fundamental belief that its basic cause is land-sea thermal contrast on the continental scale in the sense that the existence of landmass is not considered as a necessary condition for monsoons. Experiments using the Goddard Earth Observing System general circulation model (GEOS-GCM) provided support for this new interpretation. The Asian and Australian summer monsoon systems are largely intact in an experiment in which Asia, maritime continent, and Australia are replaced by ocean with sea surface temperature taken from that of the surrounding oceans (see the above figure.) Thus in these areas land-sea contrast is not a necessary condition for monsoons. This also happens to the Central American summer monsoon. The same thing can also be said about the African and South American summer monsoons, if these continents are replaced by ocean of sufficiently high SST. It is also shown that in the Asian monsoon the change resulting from such replacement is due more to the removal of topography than to the removal of land-sea contrast. In the Asian and Australian winter monsoons land-sea contrast also plays only a minor role.

The origin of the ITCZ and its latitudinal location have been interpreted in a previous study. The circulation associated with an off-equator ITCZ, previously interpreted through the modified Gill solution explains the monsoon circulation. The longitudinal location of the ITCZ is determined by the distribution of surface conditions. ITCZ favors locations of high SST as in western Pacific and Indian Ocean, or tropical landmass, due to land-sea contrast, as in tropical Africa and South America. Thus, the role of landmass, when it is important, in the origin of monsoons can be replaced by ocean of sufficient high SST.

This work, published in Journal of Atmospheric Sciences in Nov. 2001, created quite a stir in the monsoon research community. Soon after, Prof. Yanai of UCLA organized an international email discussion to examine this work. After two months of discussion by monsoon experts this new theory remained standing. The record of the email exchanges is archived at http://epswww.unm.edu/facstaff/gutzler/ (see email 5.1). Winston Chao won a division-level award for this work in 2002. An article about this work is being prepared for the NASA Earth Observatory website.