Impacts of early spring soil moisture over the Greater Mekong Subregion on the interannual variation of South China Sea summer monsoon onset

This study investigates the influence of early spring (March-April) soil moisture (SM) over the Greater Mekong Subregion (GMS) on the interannual variation of South China Sea summer monsoon (SCSSM) onset, using observational analyses and numerical experiments. It is found that when early spring SM over the GMS is wetter, westerly anomalies dominate the South China Sea, corresponding to an early onset of the SCSSM, and vice versa. The analyses of physical mechanism show that the positive anomalies of early spring SM decrease local surface air temperature by adjusting the surface latent heat flux and sensible heat flux. The persistence of anomalous ground cooling contributes to negative geopotential height anomalies in the middle troposphere, and further induces the eastward retreat of western North Pacific subtropical high in May. The anomalous cyclone over the South China Sea favors the early onset of SCSSM. The key physical processes linking the variability of early spring SM over the GMS and the following SCSSM onset are confirmed by the sensitivity experiments using a coupled atmosphere-land model (CAM6-CLM5). Specifically, the onset date of the SCSSM in the drier experiment is 11 days later than that in the wetter experiment. The present results have implications for the forecast of SCSSM onset.