Mechanisms responsible for the maintenance of the 1998 South China Sea Summer Monsoon

This paper first presents results of a preliminary study of the dynamic and thermodynamic characteristics associated with the onset of the South China Sea (SCS) summer monsoon in 1998. A frontal cyclone that developed at the northeastern part of the Tibetan Plateau and moved equatorward into the SCS appears to be the mechanism responsible for the onset. The onset day is defined based on the switch of the 850-hPa zonal winds from easterly to westerly, and a sudden increase in the observed daily rainfall at Xisha (112.33°E, 16.83°N) and Yongshujiao (112.88°E, 9.53°N), which for the entire SCS is 25 May. Further, analyses of the outgoing-longwave radiation (OLR) dataset (from May to August, 1998) in the SCS (110-120°E, 5-20°N) show that the convective variance appears to be associated with the Madden-Julian Oscillation (MJO), which modulates the maintenance of the monsoon. Three active periods can be identified. The mechanisms that maintain the active monsoon appear to be: 1) conduits transporting moisture to the SCS, and the subsequent release of a large amount of convective available potential energy, and 2) strong convergence and divergence in the lower and upper levels over the SCS, respectively. When one of these mechanisms decays, the monsoon changed from active to inactive.