Variations in Summer Extreme Hot-Humid Events over Eastern China and the Possible Associated Mechanisms

This study aimed to explore the changing characteristics of summer, compound, extreme hot-humid events over eastern China in recent decades and the associated underlying mechanisms. Results showed that the summer extreme hot-humid events over eastern China have tended to increase but with strong interannual variability in recent decades. Fur-ther analyses indicated that the summer sea surface temperature (SST) anomalies over the northwest Pacific (NW Pacific) and western tropical Pacific, and the May Arctic sea ice concentration (SIC) anomalies are the key elements that are closely connected to the variations in extreme hot-humid events over eastern China. The positive SST anomalies over the NW Pacific in summer, on the one hand, could strengthen the local circulation patterns that lead to the anomalous anticyclonic cir-culation over eastern China. On the other hand, the positive SST anomalies over the western tropical Pacific excite an anom-aly pattern similar to the Pacific-Japan pattern that weakens the westerly jet and enhances the anticyclone circulation anomalies over the upper level of eastern China. Both of them provide favorable conditions for extreme hot-humid events over eastern China with increased moisture and temperatures. The sensitivity experiments by the Community Atmosphere Model, version 5 (CAM5), with SST forcing yield similar influencing processes to the observations. Additionally, the effect of SIC may be potentially responsible for the hot conditions in extreme hot-humid events over eastern China. The signal of declining May SIC could persist to summer and then influence the SST anomalies to its south, which could also pro-duce a beneficial background for extreme hot-humid events to increase over eastern China by exciting the Rossby wave train propagating southeastward across the Mongolia-Lake Baikal region.