Polar front jet regulates tropical sea surface temperature impacts on South China extreme precipitation

Extreme precipitation over South China is commonly attributed to tropical sea surface temperature forcing, yet the role of extratropical circulation remains unclear. Using observational and reanalysis data, Rossby wave propagation diagnostics and numerical experiments, we investigate record-breaking April–May 2024 rainfall. Here we show that a trough between anomalous anticyclones over the tropical western North Pacific and Northeast Asia drives moisture convergence over South China. The tropical anticyclone enhanced moisture transport into South China, whereas the Northeast Asian anticyclone suppressed moisture export, favoring moisture accumulation. While the tropical anticyclone was forced by Indian Ocean sea surface temperature anomalies, the Northeast Asian anticyclone developed within a midlatitude Rossby wave train initiated by tropical Atlantic convection and guided by a transient eddy-driven polar front jet. European Center for Medium-Range Weather Forecasts model reproduced tropical forcing but underestimated precipitation because of the polar front jet biases. These findings highlight the importance of extratropical flow for predicting tropical-forced extremes. Record precipitation over South China in April–May 2024 resulted from low-level moisture convergence associated with a trough between two anomalous anticyclones, according to statistical diagnostics and model simulations.