Interannual Variation in the Seasonal Cycle of the East Asian Subtropical Monsoon and Its Impact on Summer Rainfall in North China

Owing to the complicated changes in the East Asian summer monsoon and its chaotic influence, the accurate forecast of summer rainfall anomalies in North China is challenging. To understand the origin of summer rainfall anomalies in this region, we applied the daily observed rainfall data, CRA-40 atmospheric reanalysis, harmonic analysis, and MV-EOF analysis for examining the impact of the seasonal cycle of the 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 850 hPa in North China exhibits two dominant modes: southwesterly wind and southeasterly wind, which peak in early and late July, respectively. On interannual time scale, although the rainfall of first two modes exhibits consistent change, it is successively affected by southwest and southeast wind anomalies. Based on the analysis of the onset date (P1), peak date (P2), retreat date (P3), duration (Dur), and amplitude (Amp) of the seasonal cycle modes during the rainy season and the summer monsoon rainfall (from June to August) anomalies in North China, we found that the P1, P3 and Amp of the southeast wind mode have a significant positive correlation with the summer rainfall anomalies in North China, while P2 and Dur are negatively correlated with the summer rainfall anomalies in North China. The P2, P3, and Amp of the southwest wind mode show significantly positive correlations with the summer rainfall anomalies in North China. The phase (P1, P2, and P3) variations of southeast wind are related to the intensity of the southwest wind in summer, whereas the variation of Amp mainly depends on the intensity of the southeast wind. 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 forecasts of summer rainfall in North China.