Moisture Balance over China and the South China Sea during the Summer Monsoon in 1991 in Relation to the Intense Rainfalls over China

Abstract

The moisture balance over the central part of China in May, June and July (MJJ) of 1991 is studied by utilizing the 24-hour prediction data of a global weather prediction model of JMA, in relation with the heavy rainfalls over China.
The moisture balance over a domain bounded by 20-40°N and 110-125°E (Area-C, main part of China) indicates that the moisture flux convergence within this domain is mainly due to the southerly moisture inflow across the southern boundary. The precipitation in this domain increases in the early June with the abrupt increase of the southerly moisture inflow. The precipitation within this domain is highly correlated with the southerly moisture inflow crossing 20°N.
To relate the southerly moisture flow into Area-C with the moisture balance in the tropical zone, the moisture balance in a domain bounded by 0-20°N and 110-125°E (Area-S, the South China Sea including the monsoon trough region) is further studied. The variation of the cross-equatorial moisture inflow in Area-S is significantly smaller than that of the moisture efflux across the northern boundary of Area-S. In Area-S, the strong correlation is found between the zonal moisture flux convergence and the southerly moisture flux across the 20°N latitude circle. This zonal moisture flux convergence is seen within the monsoon trough region between the monsoon westerly and the easterly flux in the southern rim of the Pacific subtropical anticyclone. The monsoon trough region plays the role of the channel to transport the moisture from 0-20°N zone toward north, although Area-S itself is not a moisture source region. The westeast intraseasonal oscillation of the monsoon trough, monsoon westerly and the North Pacific subtropical anticyclone have strong influence on the precipitation and westerly moisture transport over China.
The features of precipitation over China in early July are characterized by a narrow intense rainfall zone over the Yangtze River Valley. The moisture flux field during this period indicates the strong moisture flux convergence within the precipitation zone.