Mechanisms linking spring SST anomalies in key regions to interdecadal shifts in summer heavy precipitation frequency over the Yangtze River Delta

The Yangtze River Delta (YRD) in eastern China is among the regions most vulnerable to frequent summer heavy precipitation and severe flood events. Observational data analysis reveals that the frequency of summer heavy precipitation over the YRD has exhibited a significant interdecadal increase since 1988, making it a concentrated high-incidence area for heavy precipitation in eastern China. This change is significantly associated with preceding spring sea surface temperature (SST) anomalies in the southeastern Indian Ocean (SEIO). During high-frequency precipitation years, the atmospheric circulation exhibits a pronounced anomalous structure, accompanied by moisture flux convergence, thereby providing favorable conditions for the occurrence of heavy precipitation. This study reveals the key SST precursor signals influencing the frequency of heavy precipitation over the YRD and identifies signal transition around the late 1980s. After this transition, the frequency of heavy precipitation shows a significant positive correlation with SST anomalies in the SEIO. The aforementioned interdecadal changes in heavy precipitation frequency are closely related to the warming of the SEIO and the associated anomalous atmospheric circulation structures. The CESM model reasonably simulates the meridional circulation, the upper- and lower-level circulation configuration, and the moisture convergence characteristics identified in the observational analysis. This study elucidates spring SEIO SST anomalies as a key precursory signal governing interdecadal shifts in YRD summer heavy precipitation and clarifies the influencing mechanism of SST-associated anomalous circulations, offering new understanding for regional climate prediction improvement.