Chinese Academy of Meteorological Sciences
Owing to the complicated change of East Asian summer monsoon and chaotic influence, the accurate forecast of summer rainfall anomalies in North China is a great challenge. To understand the origin of summer rainfall anomalies in this region, we applied the daily observed rainfall with CRA-40 atmospheric reanalysis, harmonic and MV-EOF analysis to examine the impact of annual cycle of East Asian subtropical summer monsoon (EASSM) on the summer rainfall anomalies in North China during 1979-2020. Our results show that, in climatology, the rainfall related seasonal cycle of winds at 850hPa in North China exhibits two dominant modes, characterized by the southwestly and southeastly, which reaches their peak in early and late July, respectively. Although the rainfall in North China revealed by the first two modes exhibits consistent changes on interannual scale, it is successively affected by southwestly wind and southeastly wind anomalies. Based on the analysis of the onset date (P1), peak date (P2), retreat date (P3), duration (D) and amplitude (A) of the annual cycle modes during the rainy season and the summer monsoon rainfall (from June to August) anomalies in North China, We found that the onset and retreat date, and the amplitude of annual cycle modes dominated by southeast wind are significantly positively correlated with the summer rainfall anomalies in North China, while the peak date and duration are negatively correlated with the summer rainfall anomalies in North China. Its phases (P1, P2, P3) variation are related to the intensity of southwest wind in summer, while the amplitude (A) variation mainly depends on the intensity of southeast wind. The peak date, retreat date and amplitude of the second annual cycle mode, dominated by southwestly wind shows a significantly positive correlations with summer rainfall anomaly in North China. Since the summer rainfall dominated by the southeast wind starts from April to May, the establishment of the EASSM related to the rainfall in North China provides a new index for the seasonal forecast of summer rainfall in North China.