Interannual Variation of Northern Edge of Summer Monsoon in Eastern China: Zonal Discrepancies and Impact Factors

Based on monthly reanalysis data from the JRA-55 (Japanese 55-year Reanalysis) during the period 1958-2016, the northern edge of the East Asian Summer Monsoon (EASM) is defined by the Precipitable Water (PW) in summer. In this study, the authors used harmonic decomposition, regression analysis, composite analysis, wave activity flux analysis, and other analytical methods to study the interannual variation of the displacement of the northern edge of the EASM. The results show a dramatic west-to-east discrepancy with independent variations over North China (107.5°-115°E) and Northeast China (122.5°-130°E), which are related to an anticyclonic moisture circulation anomaly to the east. The discrepancy related to the location of northern edge in North China lies over the Yellow Sea, and that of Northeast China lies over the northern Sea of Japan. As a result, these two regions experience enhanced transport of southerly moisture and moisture convergence. The abnormal anticyclonic circulation influencing North China is related to the propagation of wave trains in the upper-troposphere mid-high latitudes from the east of Europe. The abnormal anticyclonic circulation affecting Northeast China is due to the asymmetry between the southward-displaced and northward-displaced monsoon edge conditions. The former might be related to the northward propagation of abnormal signals from low latitudes via the East Asia-Pacific teleconnection, whereas the latter might be related to the positive geopotential height anomaly caused by local warming over northeastern Asia.