Maintenance of the East Asian summer monsoon over South China

Following the onset and an active period (maintenance) of the summer monsoon over South China (SC), an inactive period (break) occurs which corresponds to a significant reduction of monsoon rainfall. In order to have a better understanding of the summer monsoon circulation system, changes in the atmospheric circulation during the maintenance period from 1985-1990 are studied. Firstly, the breaks are defined by using the 5-day weighted running total rainfall over Hong Kong. Then the daily dynamic and thermodynamic parameters, and the blackbody temperature are composited by using the ECMWF data set and the GMS T-bb data respectively. It is found that the atmospheric circulation during a break seems to be opposite to that occurring during the onset. The characteristics include: 1) a southward shift of the high pressure system over the western Pacific accompanied by the arrival of a ridge from the northeast to the SC coastal regions; 2) a westward shift of the centre of the 850-hPa jet, with a weakening of the southwesterly wind over SC, and finally this southerly wind switching to easterly over the region 20-25 degrees N, 115-120 degrees E; 3) low-level divergence and upper-level convergence over the SC region and; 4) a relatively cloud-free region moving from the western Pacific to the SC.