A decomposition study of moisture transport divergence for inter-decadal change in East Asia summer rainfall during 1958-2001

In this paper, we report on the results of an investigation into inter-decadal changes in moisture transport and divergence in East Asia for the two periods 1980-2001 and 1958-1979. The aim is to explore the mechanism of summer rainfall change in the region after abrupt changes. The relevant changes are calculated using ERA-40 daily reanalysis datasets. The results show that both stationary and transient eddy moisture transports to the Chinese mainland have declined since the abrupt change in atmospheric general circulation in the late 1970s, leading to more rainfall in South China and less in the North. The anomalous rainfall pattern coincides well with anomalous large-scale moisture divergence in the troposphere, of which stationary-wave or monsoon transport is dominant, in comparison with the contribution of the transient eddies. Furthermore, their divergences are found to be in opposite phases. In addition, meridional divergence is more important than its zonal counterpart, with an opposite phase in East Asia. Abnormal zonal moisture convergences appear in northwestern and northeastern parts of China, and are related to the excess rainfalls in these regions. An increase in transient eddy activity is one of the major mechanisms for excess rainfall in northern Xinjiang. Consequently, the anomalous rainfall pattern in East Asia results from a decline of the East Asian monsoon after the abrupt change, while the rainfall increase in northwestern China involves anomalies of both stationary waves and transient eddies on boreal westerly over the mid- and high latitudes.