Initial atmospheric constraints limit the sub-seasonal prediction skill of the South China Sea Summer Monsoon onset

The onset time of the South China Sea summer monsoon (SCSSM) is widely recognized as being closely related to the distribution of summer rainfall belts in East Asia. This study examines the sub-seasonal prediction bias of SCSSM onset and investigates the associated oceanic and atmospheric processes using the hindcasts from the ECMWF sub-seasonal-to-seasonal (S2S) forecasting system. The system can accurately predict the onset time two pentads in advance. As the lead time increases, the predicted monsoon onset time tends to be later than the observations. When the forecast lead time exceeds 15 days, the inaccuracies in the northward propagation of tropical convection result in the absence of initial atmospheric constraints. This lack of initial constraints, along with their influence on the boundary forcing over the Bay of Bengal, may lead to a late onset bias of the SCSSM in this forecasting system.