Unstable relationship between the Pacific Decadal Oscillation and eastern China summer precipitation: Insights from the Medieval Climate Anomaly and Little Ice Age

摘要

The eastern China summer precipitation is related to the Pacific Decadal Oscillation (PDO) on interdecadal time scales in modern times, but it remains unclear whether such a relationship holds prior to the instrumental period. We examine this relationship during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). According to a composite analysis using simulations of the HadCM3 model, which is selected from nine available climate models, the PDO–eastern China summer precipitation relationship varies with climatic background. The modern relationship features deficient precipitation over North and South China and excessive precipitation in the Yangtze–Huai River Valley in positive PDO phases compared with negative phases. In contrast, there is more precipitation over South and North China but less in the Yangtze–Huai River Valley during the MCA and widespread below-normal summer precipitation over eastern China during the LIA. Such different PDO-related precipitation patterns between the MCA and LIA are closely linked to distinct changes in local atmospheric circulation. Compared with negative PDO phases, positive phases during the MCA show an anomalous Pacific–Japan/East Asia–Pacific pattern over East Asia and strengthened high-level westerlies centering on 120°E and 25–30°N, which lead to the triple pattern in the precipitation anomaly. During the LIA, a cyclonic anomaly occurs over the South China Sea–Philippine Sea in the lower and middle troposphere, and two upper-level low trough anomalies occur over East Asia, causing the anomalous precipitation deficit. The different PDO-related local circulations are found to be relevant to the thermodynamic effect of low-latitude sea surface temperature and summer precipitation over India, as well as the propagation of upstream wave trains.