Mechanism of Regional Subseasonal Precipitation in the Strongest and Weakest East Asian Summer Monsoon Subseasonal Variation Years

Using the National Center for Environment Prediction Climate Forecast System Reanalysis coupled dataset during 1979–2010, we selected four subseasonal indexes from the 16 East Asian Summer Monsoon (EASM) indexes to characterize the subseasonal variability of the entire EASM system. The strongest (1996) and weakest (1998) years of the subseasonal variation were revealed based on these subseasonal EASM indexes. Furthermore, three rainfall concentration areas were defined in East Asia, and these areas were dissected by the atmospheric midlatitude jet stream axis and the position of the Western North Pacific Subtropical High (WNPSH). Then, the subseasonal effects of the WNPSH, the South Asian High (SAH), the Mongolian Cyclone (MC), and the Boreal Summer Intraseasonal Oscillation (BSISO) on each rainfall concentration area were studied in the strongest and weakest subseasonal variation years of the EASM. During the summer of 1998, the WNPSH and the SAH were stable in the more southern region, which not only blocked the northward progression of the BSISO but also caused the MC to advance southward. Therefore, the summer of 1998 was the weakest subseasonal variability of the EASM, but with significant subseasonal precipitation episodes in the northern and central rainfall areas. However, in 1996, the BSISO repeatedly spread northward in the south rainfall area because of the weak intensities and northern positions of the WNPSH and the SAH, which caused significant subseasonal precipitation episodes. In addition, MC was blocked to the north of approximately 42°N with a weak subseasonal rainfall.