Precipitation and moisture balance of the Asian summer monsoon in 1991 - Part I: Precipitation and major circulation systems

The large-scale features of the moisture transport and the water balance of the Asian summer monsoon in 1991 were studied by utilizing the 24-hour prediction by a T106 spectral global model. In part I of the present study, we will describe the seasonal change of the precipitation in relation to the change of the major circulation systems over the Asian monsoon region. The most remarkable change in the onset period of the Asian summer rainy season over the Indian Ocean and the Southeastern Asia is the rapid displacement of the major precipitation areas from the equatorial zone to the Indochina Peninsula and Indian subcontinent. The notable change in the onset phase of the rainy season over East Asia is the formation of a precipitation belt, which corresponds to the Meiyu and Baiu frontal zone. In order to realize the relation between the variations of the precipitation with the those of the low-level circulation, several major circulation systems (CSs) in the lower troposphere are defined. Among them, the variations of the clockwise circulation centered over the Indian Ocean (CS-3) and a cyclonic circulation over the northern part of the Indian subcontinent (CS-6) are closely related with variations of the Indian monsoon rainfalls, while the cross-equatorial flow from the Australia anticyclone (CS-4) and the circulation around the North Pacific subtropical anticyclone (CS-5) are the major systems related with the East Asia rainfalls. The variation of the mixing ratio of water vapor, equivalent potential temperature, vertical stability and the vertical motion are also described in relation with the variations of the precipitation and CSs. During the Asian summer monsoon season, the gradual decrease of the mixing ratio and the increase of the vertical stability, in association with the subsidence, are significant features over the western tropical Indian Ocean, while the increase of the specific humidity and the vertical instability are the notable features over the Indian sub continent, Indochina Peninsula and the southern part of China.