Interdecadal shift in North Indian Ocean-South China Sea influence on the evolution of the East Asian summer monsoon from July to August

The East Asian Summer Monsoon (EASM) shape China's summer climate. The transition from July to August marks a critical shift between the Meiyu season and the North/Northeast China rainy season, yet the underlying drivers of the inter-monthly evolution in precipitation during this period remains unclear. This study investigates the relationship between sea surface temperature anomalies (SSTAs) in the northern Indian Ocean (NIO)-South China Sea (NIO-SCS) and the inter-monthly precipitation evolution in eastern China during July–August. A significant interdecadal shift in the relationship between the NIO-SCS SSTAs and eastern China's precipitation around 1995/96. Before 1995, warm NIO-SCS SSTAs consistently enhanced precipitation in the Huaihe-Yangtze River Basin (HYRB) throughout July and August, primarily altered by the Pacific-Japan (PJ) teleconnection pattern. However, after 1996, a distinct pattern emerged: warm NIO-SCS SSTAs increased precipitation over the HYRB and simultaneously reduced precipitation in North/Northeast China in July. In contrast, this pattern is reversed in August. Based on observations and numerical experiments, the interdecadal shift is linked to a weakening of Indian Ocean SSTAs after 1996 compared to before. During the post-1996, cooler Indian Ocean SSTAs in July weaken the Kelvin wave, shifting the PJ pattern westward and broadly influencing precipitation across eastern China. In August, the cooler Indian Ocean SSTAs induce westward-shifted Walker circulation anomalies, with a descending branch near the Philippine Sea extending to 30°N, leading to decreased precipitation in the HYRB and increased precipitation over North/Northeast China. Furthermore, the reduction in NIO-SCS SSTAs' amplitude after 1996 may be attributed to a weakened spring ENSO amplitude, potentially linked to the positive phase of the Atlantic Multidecadal Oscillation.