The role of continental-scale landmass in monsoons—A GCM investigation
Winston Chao, NASA/GSFC, Greenbelt, MD
It was argued by Chao and Chen (2001) that land-sea thermal contrast on the continental scale is not a necessary condition for monsoons and that a monsoon is an ITCZ that have moved into the subtropics in its annual cycle of latitudinal movement. Chao and Chen supported their contention by GCM experiments in which they replaced landmass by ocean and were able to generate monsoons. However, land-sea thermal contrast does exist and must play a role in monsoonal rainfall distribution. Land-sea thermal contrast is one facet of continental-scale landmass. The other important characteristic of landmass is its topography. In this article the roles of landmass in monsoonal rainfall distribution and in middle latitude storm tracks are examined through GCM experiments. Comparison of a set of two GCM experiments in which the sea surface temperature (SST) from observations is prescribed from observations with and without a six-month delay reveals the role of land-sea thermal contrast. Another set of experiments, which repeats the first set but with topography of all landmass reduced to zero, reveals the role of topography of landmass. These experiments confirm that land-sea thermal contrast is not a necessary condition for monsoons and that a monsoon should be viewed as an ITCZ displaced into the subtropics, instead of a continent-sized giant sea breeze. However, land-sea thermal contrast does have influence on the distribution of monsoonal rainfall. The temperature rise over south Asia as the season moves into summer helps the Asian monsoon to start early. However, this role is not the same as that of the land-sea thermal contrast as in the conventional explanation for the cause of monsoon. The heated landmass in summer contributes to the displacement of ITCZ into the subtropics. Also, the heated landmass in summer, by drawing moisture toward itself, limits the range of the summer storm tracks in the middle latitude oceans. On the other hand, in winter the landmass does not present a competition for rainfall and thus allow middle latitude storm tracks to expand over the ocean.
Session 1A, Year of Tropical Convection
Monday, 12 January 2009, 10:45 AM-12:00 PM, Room 129A
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