The recent increase in central eastern China summer rainfall and its possible mechanism

This study investigated interdecadal variations in summer rainfall in central eastern China before and after the mid-1990s. The mean difference in rainfall between the two epochs (average rainfall in the 1981–1995 period: 5.0 mm day⁻¹; average rainfall in the 1996–2010 period: 6.0 mm day⁻¹) was significant at the 95% confidence level. This study explored the cause of the recent increase in summer rainfall in central eastern China by analyzing the difference in average rainfall between 1996–2010 (hereafter, Epoch II) and 1981–1995 (hereafter, Epoch I). Analysis of rainfall revealed that positive anomalies were observed in a southwest–northeast direction across the south-central region of eastern China, Korea, and Japan, while negative anomalies were observed at latitudes of 40–50°N and in most areas in the western North Pacific region. These findings suggest that summer rainfall in central eastern China is part of East Asian summer monsoons and have an out-of-phase relationship with western North Pacific monsoons. To identify the cause of the recent increase in summer rainfall in central eastern China, the difference in 850 hPa stream functions between Epoch II and Epoch I was analyzed. This study found that anomalous huge anticyclonic circulations were strengthened in the western North Pacific region and anomalous cyclonic circulations were strengthened in the south-central region of eastern China. As a result, anomalous northeasterlies originating from anomalous cyclones centered on Lake Baikal and anomalous southerlies originating from anomalous cyclones in the south-central area of eastern China joined in central eastern China. The typical anomalous atmospheric circulations in the negative Pacific Decadal Oscillation (PDO) phase and the average PDO index and summer rainfall in central eastern China from June to August exhibited a high negative correlation, which was presumed to have been present since the previous winter.