Onset of the regional monsoon over Southeast Asia

Summary ?This is an observational study in which regional features of the different summer monsoon components over Asia especially the South China Sea (SCS) are examined. The authors use various data sets including satellite measurements to understand the onset, maintenance, and retreat of monsoon and explain the connection and independence among the variabilities in the monsoon components. It is shown that while outgoing longwave radiation (OLR) data can only measure tropical convection, upper-tropospheric water vapor band brightness temperature (BT) represents appropriately convective precipitation in both the tropics and the extratropics. The authors define criteria for measuring the SCS monsoon using precipitation, BT, OLR, and lower-tropospheric winds and suggest that multi-variables should be considered to depict regional monsoon features adequately. Under the criteria defined in this study, the SCS summer monsoon is considered as an expansion of deep convection from the tropics. The onset of the monsoon occurs in mid-May, with its precursory signal found over the Indochina peninsula. It is characterized by an abrupt establishment, especially over the central SCS. Although the role of convection over the southern SCS in the monsoon onset is unclear, the early precipitation over the northern SCS and South China, resulted from the effect of subtropical fronts, is separated from the tropical monsoon rainfall. The relative independence from one monsoon component to another is explained by the effects from local topography and land-sea thermal contrast. Received November 5, 1999/Revised April 13, 2000