Characteristics of Tibetan Plateau Topographic Trough and Bay of Bengal Trough and Their Relationship with the South China Sea Summer Monsoon Onset

摘要

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.