A novel index for depicting the northern boundary of western pacific warm pool

The Western Pacific Warm Pool (WPWP) serves as a major thermal driver of global atmospheric circulation. However, its relationship with summer precipitation in China is unstable when using the conventional strength or area indices, as these indices do not necessarily correspond to the northward expanding/southward shrinking of the WPWP itself that further determines its northward expansion influences on China. Here, this study introduces a new index, Western Pacific Warm Pool Northern Boundary Index (WPNBI), to depict the northward extensions of the warm pool. It can effectively capture the long-term warming trend and interannual variability of the WPWP. Furthermore, precipitation regression based on the WPNBI reveals a tripole pattern over eastern China, with the most pronounced positive anomaly in the middle and lower reaches of the Yangtze River. The WPNBI also reflects key climate-mode-like circulation patterns influencing precipitation over China, including the Silk Road teleconnection, the Pacific-Japan teleconnection, and the North Atlantic Oscillation. These patterns exhibit highly consistent spatial structures across datasets and demonstrate the robustness and effectiveness of the WPNBI. Moreover, this study reveals the significant impact of an independent northward expansion of the warm pool northern boundary on the rainfall band over China, by triggering an anomalous Hadley circulation which shifts its ascending branch northward to around 35°N, thereby driving the large-scale rainfall band northward. Consequently, a tripole precipitation pattern emerges over eastern China, highlighted by a significant rainfall increase in the middle and lower reaches of the Yangtze River.