Interannual variations of early summer monsoon rainfall over South China under different PDO backgrounds

摘要

The interannual variations of the early summer South China monsoon rainfall (SCMR) in relation to different Pacific Decadal Oscillation (PDO) backgrounds are examined using station rainfall data and the European Centre for Medium-range Weather Forecasts (ECMWF) 40-year reanalysis (ERA-40) data. The objective of this study is to investigate the atmospheric circulation patterns responsible for such variations, thereby understanding the interdecadal modulation and the cause for the extreme wet and dry SCMR. The interannual SCMR variance shows an interdecadal change before and after the late 1970s, with significant differences in sea-surface temperature (SST) and atmospheric circulation anomalies between the negative (1958-1976) and positive (1980-1998) PDO epochs. The dominant atmospheric teleconnection patterns associated with extreme wet and dry SCMR are remarkably different in these two epochs. During 1958-1976, an anomalous wet (dry) SCMR is characterized by a significantly anomalous anticyclone (cyclone) over the South China Sea (SCS) and western Pacific in the lower troposphere in the form of a meridional wave-like coupling along the coast of western North Pacific in the early summer, and preceded by a monopole circulation anomaly over the Ural Mountain in the preceding winter. During 1980-1998, however, an anomalous wet (dry) SCMR features a relatively weak anomalous anticyclone (cyclone) over the SCS in the low level without a significantly meridional juxtaposition of anomalous circulations in the early summer, and it follows a remarkable coherent long wave train pattern in middle-high latitudes in the preceding winter. Such interdecadal differences in anomalous circulation structures suggest that the relative roles of internal atmospheric variability and external forcing may be different from different epochs. But for both epochs, the circulation anomaly over the Ural Mountain shows a highly negative correlation with anomalous SCMR, suggesting that the winter circulation anomaly around the Ural Mountain is of great significance for improving the seasonal prediction of SCMR.