Interdecadal variability of summer extreme rainfall in the Huai River basin of China associated with the Asian-Pacific Oscillation

Using the rainfall data at rain gauge stations in China and the European Center for Medium-Range Weather Forecast (ECMWF) reanalysis, the daily Asian-Pacific Oscillation (APO) index, a tropospheric temperature thermal contrast between Asia and the North Pacific, is constructed to investigate an interdecadal relationship between summer (June-July) APO and rainfall in the Huai River basin (HRB) of central-eastern China. The daily APO index averaged over June 22-July 12 shows an interdecadal shift around the late 1990s. This shift is characterized by a negative phase of the daily APO index during 1987-1996 and a subsequent positive phase during 1997-2006. Accompanying this shift, the occurrence frequency, intensity and amount of extreme rainfall events remarkably increase in the HRB region. The increased rainfall can be reasonably explained by the atmospheric circulation anomalies. Corresponding to the decadal change in the summer daily APO from the negative phase to the positive one, the subtropical high pressure over the western North Pacific moves toward the north and easterly wind anomalies in the middle and lower layers of the troposphere prevail over extratropical East Asia, which strengthens water vapour convergence, the instability of atmospheric stratification and upward motion anomalies in the HRB region.