Interannual variability of the onset timing of the South China Sea summer monsoon and its possible mechanisms

The East Asian Monsoon (EAM) system can be separated into summer and winter regimes. In this study, commence of the summer regime is represented by the withdrawal of the ridge of the Western Pacific Subtropical High (WPSH) from the South China Sea (SCS). Commence of the winter regime is represented by the withdrawal of the ridge of the WPSH from the Yellow Sea. Two indices, VOR850 and WND850, are respectively used to define onsets of the summer and winter monsoons of the EAM system. VOR850 is the 850hPa vorticity averaged over SCS. WND850 is defined as a vector angle of the vector of the combined 850hPa wind by the winds averaged over two key regions in northern China. Transition from winter to summer regime is determined according VOR850, while from summer to winter is according WND850. The onset of the SCS summer monsoon is recognized as the beginning of the summer EAM. Extremely early and late SCS monsoon onset years are identifies using 42-year NCEP/NCAR reanalysis data set. Differences between the extremely early and late onset years are analyzed from the aspects of surface temperature, tropical convection, low-level and upper-level circulation anomalies, extratropical-tropical interactions and interhemispheric connectivity. Two primary factors causing the onset time to be significantly different from the normal are identified. One is the pattern of the global SST anomalies. In springs (March and April) of the late onset years, the SST anomalies of the Pacific, in particular in the mid-latitudes, show zonal wavenumber one structure with positive anomalies in the east and negative in the west. The anomalies of the tropical Indian Ocean are mainly positive. In the springs of the early onset years, the Pacific SST anomalies are positive in the west and negative in the east. The tropical Indian Ocean is on the average colder than normal. The SST anomalies in the tropics result in east-west shift of the tropical convection. Accordingly, the direction of the abnormal upper-level equatorial zonal flow also changes. The other factor is the mid-latitude transient eddy activity, in particular over the coastal area of East Asia. The eddy activity is in close relationship with the land-sea contrast over the East Asia and its marginal seas. In springs of the early onset years, the transient eddies are more active than normal, while in late onset years they are much less active. A scenario for the possible mechanisms causing extremely early or late SCS summer monsoon onset is proposed. The patterns of the equatorial SST anomalies determine the signs of the anomalous upper-level equatorial flows. Interhemispheric interaction is less active over the Pacific when the equatorial upper-level anomalous flow is easterly. On the other hand, both mid-latitude baroclinity and the land-sea contrast near the East Asia coastal area determine the anomalies of the extratropical transient eddy activity. The winter EAM is weaker when the baroclinity is smaller and the land-sea contrast is less. If both the interhemispheric connectivity and the East Asia transient eddy activity are stronger (weaker) than normal in winter and particularly spring, the subsequent summer EAM onset is earlier (later). Our results suggest that the significant interannual fluctuations of the onset timing of the SCS summer monsoon is not just a local phenomenon, but a part of the fluctuations in the annual cycle of the tropical convection.