A manipulator of the extreme precipitation in South China behind the tropical sea surface temperature: the polar front jet

During April-May 2024, South China experienced an unprecedented extreme precipitation event, leading to substantial socioeconomic losses and human casualties. The primary driver of this event was an exceptionally strong moisture convergence linked to a local low-level horizontal trough. This trough was passively induced by two meridionally-oriented anomalous anticyclones located over the tropical western North Pacific and Northeast Asia. The tropical anticyclone facilitated the advection of abundant moisture towards southern China, while the Northeast Asian anticyclone impeded northward moisture export, jointly resulting in the observed extreme precipitation. The tropical anticyclone represents a typical Kelvin wave response to convection anomalies over the tropical Indian Ocean, which were forced by localized positive sea surface temperature (SST) anomalies. In contrast, the Northeast Asian anticyclone was a node of a mid-to-high latitude barotropic Rossby wave train. This Rossby wave train, initiated by the tropical Atlantic convection, was guided towards Northeast Asia by a transient eddy-driven polar front jet. Although the European Centre for Medium-Range Weather Forecasts showed high skill in predicting tropical Atlantic and Indian Ocean SST and associated convection anomalies, its ability to predict the April-May 2024 South China precipitation extreme was limited, primarily owing to difficulties in accurately predicting the strength of polar front jet. Overall, this study highlights the critical role of extratropical mean flow in modulating climate extremes that are responsive to tropical forcing.