Extremely Late Onset of the South China Sea Monsoon in 2020

The South China Sea Monsoon (SCSM) typically onsets in May, marking the beginning of the East Asian rainy season. In 2020, however, the SCSM onset — defined by the establishment of 850hPa westerly wind anomalies over the South China Sea (SCS) — occurred exceptionally late on July 29, challenging our understanding of monsoon dynamics. Using reanalysis data and CMIP6 historical simulations, we demonstrate that tropical sea surface temperature (SST) anomalies statistically modulate the SCSM onset date. In May, low-level easterly anomalies over the SCS are strongly associated with warming in the Indian Ocean (IO), Niño3.4, and the North Tropical Atlantic (NTA). A broadly similar pattern characterizes June–July, except that easterly anomalies are instead associated with Niño3.4 cooling. Dynamically, IO warming drives SCS easterly anomalies via a Kelvin wave response, while NTA warming induces SCS easterly anomalies through a relay mechanism involving enhanced rainfall over the IO. In May, Niño3.4 warming strengthens the western North Pacific subtropical high, reinforcing SCS easterly anomalies, whereas in June–July, Niño3.4 warming promotes convection over the western North Pacific as the monsoon trough develops, generating SCS westerly anomalies instead. In 2020, strong IO and NTA warming outweighed the influence of Niño3.4 cooling in May, acting to delay the SCSM onset. During June–July, the combined effects of persistent IO and NTA warming and Niño3.4 cooling further postponed the onset, ultimately producing the latest SCSM onset on record. These findings highlight the complexity of multi-basin SST forcing on the SCSM and underscore the challenges faced by current prediction systems in a changing climate.