Interannual relationship between intensity of rainfall intraseasonal oscillation and summer-mean rainfall over Yangtze River Basin in eastern China

The analysis of observational rainfall shows that the intensity of rainfall intraseasonal oscillation (ISO) and the summer-mean rainfall over the middle-lower reaches of the Yangtze River Basin (YRB) exhibit a significant positive correlation during 1979–2007. A stronger (weaker) ISO variability is often associated with wet (dry) summer in the YRB. The composite ISOs in both the wet and dry summers are further analyzed. In the wet summers, the rainfall ISO in YRB is primarily associated with the northward propagation of a low-level cyclone-anticyclone pair from the tropics. Cyclonic vorticity and associated boundary layer convergence strengthen the rainfall in situ. In contrast, the rainfall ISO in YRB in the dry summers is primarily associated with the westward propagation of an anomalous anticyclone. Southerly flow to the west of the anomalous anticyclone enhances rainfall in YRB through anomalous moisture advection. In addition to the difference in ISO propagation, the background mean state also shows a marked difference. The diagnosis of water vapor flux budget shows that the convergence and advection of seasonal mean moisture play a critical role in maintenance of the intraseasonal rainfall in the YRB. A greater mean ascending motion and associated higher mean moisture in YRB in the wet summers favor greater intraseasonal rainfall variability in situ. The mean state difference is responsible for distinctive vertical structures of boundary layer vertical velocity. A possible feedback of the ISO to the summer-mean rainfall over the YRB is also discussed.