Abstracts for the 6th International GAME Conf.
3-5 December 2004
Kyoto Japan
Characteristics, Evolution and Mechanisms of the Asian Summer Monsoon Onset over the Southeast Asia
Zuqiang Zhang (1), Johnny C. L. Chan (2), Yihui Ding (1)
(1) National Climate Center, China Meteorological Administration, Beijing, China
(2) Department of Physics and Materials Science,City University of Hong Kong, Hong Kong, China
Based on the 1979-1995 mean pentad reanalysis data, the climatological characteristics and physical mechanism of the Asian summer monsoon (ASM) onset are investigated. A special focus is whether the ASM onset first commences over the Indochina Peninsula and why the ASM is established earlier over Southeast Asia than over India. The thermal forcing of the landmass in the lower latitudes of Asian continent on the seasonal transition is also investigated.
An examination of thermodynamic and dynamic quantities confirms the previous result that the ASM onset commences first over the Indochina Peninsula. The characteristics not only include the earliest active convection and rainfall, but also the earliest reversal of meridional gradient of temperature throughout the entire troposphere and the corresponding establishment of an easterly vertical wind shear. The convergence of southwesterly flow from the Bay of Bengal (BOB) vortex and easterly winds associated with the subtropical anticyclone over the South China Sea (SCS) accounts for the first characteristic. The last two features are responsible of the upper-level warming due to the vertical transport of latent heat through cumulus- and turbulence-induced eddies in convection.
A new criterion for the tropical ASM onset is proposed by considering both the rainfall and the reversal of the part of planetary-scale circulation. It appears that the ASM onset occurs first over the southwestern Indochina Peninsula in early May, and then advances to the north on May 16-20. Over the SCS, the onset starts in the fourth pentad of May and the monsoon covers the entire region by the end of May. A similar onset process is found to commence over the eastern Arabian Sea, India and western BOB in the last pentad of May, and the complete establishment of ASM over India is accomplished in mid June. In the process of the onset of each component of the ASM, the overturning of upper-level planetary-scale circulation strongly depends on the reversal of the meridional temperature gradient. Over the Indochina Peninsula, the seasonal transition of upper-level temperature results from convection¨Cinduced diabatic heating, while over western Asia it is attributed to the subsidence warming induced by the active ascending motion over the former region.
The steady increase in surface sensible heating over the Indian subcontinent and the latent heating over the tropical Indian Ocean in April to early May appear to be the major impetus for the development of the cyclonic vortex over the BOB. A similar enhancement over the Arabian Peninsula and the surrounding regions is also identified to be crucial to the development of the cyclone over the Arabian Sea, and then ultimately to the ASM onset over India. Due to its later seasonal transition versus the tropical region, the thermal forcing of the Tibetan Plateau appears to have little influence on the ASM onset over the Indochina Peninsula in early May.
Submittal Information
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National Climate Center, China Meteorological Administration | |
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No.46 Zhongguancun South Ave., Haidian, Beijing | |
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