Changes in the East Asian Summer Monsoon and Rainy Season in China and Critical Impacts of Regional Air and Sea Surface Temperature Anomalies

Precipitation patterns in China are shaped by a combination of westerly wind and monsoon circulation, with both exhibiting significant annual cycle characteristics. Herein, we analyzed the trends of the East Asian Summer Monsoon (EASM) onset, withdrawal, duration, and rainy season in China from 1961 to 2020. We used daily precipitation data from CN05.1 and JRA55, CRU, and HadISST reanalyzed datasets. Our findings indicate a shift in the EASM timing. The onset has advanced by 3.54 days, and the withdrawal has been delayed by 1.64 days. Consequently, the monsoon duration has increased by 5.18 days per decade since 1961. The rainy season in China has also experienced changes, particularly after 1999, with noticeable spatial differences. Recently (1999−2020), the onset of the rainy season has advanced mainly in the eastern part of Northeast China and the Qinghai–Xizang Plateau, as well as the north of Northwest China. In these areas, the onset of rain was approximately 5 days earlier, exceeding 20 days in some areas. Conversely, the withdrawal of the rainy season has been delayed by over 10 days in the northeast of the Qinghai–Xizang Plateau, the north of the Yangtze River, and the western China. These shifts in onset and withdrawal have resulted in a substantial increase in the rainy season’s duration in the northeast of the Qinghai–Xizang Plateau, the north of the Yangtze River, and the southeastern part of Northeast China. Here, the rainy period has extended by over 15 days and even surpassed 25 days in some areas. We identified several key factors driving these changes. The warming surface air temperature around Lake Baikal and enhanced upper-level anticyclone in April are pivotal for the onset advance of the EASM and rainy season in China. Similarly, the warming of the northwest Pacific sea surface temperature in October and enhanced subtropical anticyclones in the northwest Pacific are crucial factors leading to the delayed withdrawal of the EASM and a longer rainy season in northern China.