Dynamic and thermodynamic characteristics associated with the onset of the 1998 South China Sea summer monsoon

The South China Sea (SCS) Monsoon Experiment was conducted in May and June 1998 to study various aspects of the SCS summer monsoon (SCSSM). This paper presents results of a preliminary study of the dynamic and thermodynamic characteristics associated with the SCSSM onset. The objective is to determine the mechanism that triggered the onset, which is defined based on the shift of the 850-hPa zonal winds from easterly to westerly, and a sudden increase in the observed daily rainfall over the SCS. Based on those criteria, the onset day of the 1998 SCSSM is May 25. The results show that the summertime meridional temperature gradient, the meridional circulation as well as high relative humidity were already established in early May over the SCS. On the other hand, reversals of the meridional mean-sea-level pressure gradient and the zonal wind circulation occurred only near the onset. One day before the onset, the vertical gradient of the regional mean saturated equivalent potential temperature between 850 and 500 hPa reached a (negative) maximum over the entire SCS, which implies that the atmosphere was very unstable. A tropical cyclone in the Bay of Bengal apparently provided a conduit for the transport of moisture to the SCS. In addition, the subtropical high over the northwest Pacific withdrew eastward, and a cross-equatorial flow developed at 105 degrees E in association with an equatorial buffer zone between 105 and 125 degrees E. These resulted in the setting in of a low-level westerly flow. All of the above processes provided the necessary conditions for the SCSSM onset. When a frontal cyclone generated at the northeastern part of the Tibetan Plateau moved equatorward into the SCS, rising motion was strongly enhanced in situ and led to-the development of heavy rainfall. This was when the monsoon onset occurred over the entire SCS.