Maintenance mechanism for the summertime + EAP/-SR combination pattern

The co-occurrence of the East Asia–Pacific (EAP) and the Silk-Road (SR) modes is found to have a more pronounced impact on summer rainfall over southern China compared to any single mode, causing huge economic losses. Based on daily ERA5 reanalysis data, this study identifies 25 typical positive EAP (+ EAP) and negative SR (-SR) combination pattern events during the summer of 1979–2023. The + EAP/-SR combination pattern is accompanied by significantly enhanced rainfall over the Yangtze-Huaihe River Basin (YHRB) compared to the other three combination patterns. The coupling of the + EAP and -SR modes contributes to a strengthened western Pacific subtropical high and a deep cyclonic system over northeastern Asia, favoring the maintenance of the Mei-yu front and enhanced rainfall over the YHRB. The energy budget and dynamical mechanisms of the + EAP/-SR combination pattern are further analyzed. The + EAP/-SR combination pattern can efficiently extract available potential energy through baroclinic energy conversion. In addition, the co-occurrence of suppressed convection over the Philippine Sea (SCPS) and enhanced rainfall over southern Europe (ERSE) is conducive to the + EAP/-SR combination pattern. On the one hand, the SCPS and the associated diabatic cooling effect could generate a poleward Rossby wave train, promoting the formation of the + EAP mode. On the other hand, the ERSE enhances divergence anomalies and negative Rossby wave source in the upper troposphere, contributing to the propagation of the -SR mode along the midlatitude westerly jet. The development of the -SR mode also strengthens the anomaly center of the + EAP mode over the Okhotsk region. A linear baroclinic model is applied to verify the above-mentioned physical mechanisms. Model results support the crucial roles of the SCPS and the ERSE and also confirm the contribution of the -SR mode to the + EAP mode over the Okhotsk region. These findings deepen our understanding of the + EAP/-SR combination pattern and have important implications for subseasonal prediction of summer rainfall over the YHRB.