Weakening of the Summer Monsoon Over the Past 150 Years Shown by a Tree-Ring Record From Shandong, Eastern China, and the Potential Role of North Atlantic Climate

The causes of the decreased intensity of the East Asian summer monsoon (EASM) over the past 150 years are still not fully understood, although several studies have linked the monsoon weakening to the warming of tropical oceans. Here, we use pine tree-rings to reconstruct the precipitation total for April-August from 1810 to 2018, in south-central Shandong Province, in the EASM region. The reconstruction accounts for 41.8% of the instrumental precipitation variance during 1965-2018. The EASM precipitation reconstruction shows extreme pluvial conditions in 1832, 1833, 1886, and 1998, and extreme droughts in 1878, 1901, and 1910, which correspond precisely to extreme climatic events recorded in historical documents. The reconstructed precipitation reveals a drying trend since the 1870s, which matches well with the decreasing trend of the EASM inferred from stalagmite oxygen isotope (d(18)O) records and climate simulations. The trend of decreasing precipitation since the 1870s, indicated by our reconstruction, is significantly correlated with the spring sea surface temperature (SST) of the North Atlantic Ocean, which suggests that the EASM weakening was linked to North Atlantic SST variations during the past 150 years. This potential role of North Atlantic SST variability is supported by climate sensitivity simulations of the Community Earth System Model. North Atlantic SST variability induces two teleconnections of Rossby-like wave propagation from the North Atlantic into East Asia, resulting in anomalous precipitation in this region.