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Utilizing the NECP/NCAR reanalysis data, the annual atmospheric circulation over East Asia from 1981 to 2000 is
investigated. It is discovered that a zonal positive vorticity belt maintains to the south of Tibetan Plateau, due to the
interaction of the plateau boundary layer and its neighboring free atmosphere. Particularly, there is an obvious
topographic trough related to the positive vorticity near 90°E. According to this phenomenon, a Tibetan Plateau
Topographic Trough Index (TPTTI) is defined in the paper over the key areas (80-90°E, 25°N). The index is proved to be
effective in distinguishing between the characteristic of the Tibetan Plateau topographic trough (TPTT) and that of the
Bay of Bengal Trough (BOBT). The annual variation of the TPTT is closely related to the plateau heating source, and the
former's significant abrupt changes during April and June might be primarily induced by the seasonal sudden jump of the
latter. In winter, the low-level anticyclone caused by the Tibetan plateau cooling is strengthened and superimposes the
westerly wind that should have been strengthened by dynamic effect, which weakens the TPTT. However, in summer, the
low-level cyclone resulting from the Tibetan plateau heating strengthens the circumferential westerly and deepens the
TPTT. Further investigations indicate that there is a considerable relationship between the South China Sea summer
monsoon onset and the evolution of the TPTT and the BOBT. The TPTT propagates southward and the vortex near Sir
Lanka moves northward continuously, till they meet and interact over the Bay of Bengal. This is the direct process of the
subtropical high belt splitting initially over Bay of Bengal and the establishment of the BOBT. Subsequently, the
southwesterly wind becomes stronger and promotes the eastward retreat of subtropical high, causing the South China Sea
summer monsoon bursts over the whole South China Sea.
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