A synoptic clustering-based definition of South China Sea summer monsoon onset and application to seasonal prediction

Accurate prediction of the South China Sea summer monsoon (SCSSM) onset date is important for water resources, agriculture, and disaster preparedness across East Asia, yet reliable seasonal prediction remains challenging. Here, we propose a synoptic clustering-based onset definition that identifies monsoon onset as a persistent transition into large-scale monsoon circulation regimes derived from clustering of low-level atmospheric fields. Using ECMWF SEAS5 seasonal forecasts, we compare predictions based on this regime-based framework with those based on the conventional area-averaged 850 hPa zonal-wind criterion of Wang et al. (2004). The clustering-based definition yields improved deterministic prediction skill, more favorable ratio of predictable component values, and generally higher categorical and probabilistic forecast skill during both the dependent training period and an independent forecast period. Additional diagnostics show that the identified regime transition is accompanied by coherent convective and thermodynamic changes, supporting its physical consistency with monsoon establishment. Overall, the results suggest that circulation-regime-based onset definitions provide a physically meaningful and practically useful alternative to conventional threshold-based approaches for seasonal prediction of SCSSM onset.