Two Types of Interannual Variability of South China Sea Summer Monsoon Onset Related to the SST Anomalies before and after 1993/94

An advance in the timing of the onset of the South China Sea (SCS) summer monsoon (SCSSM) during the period 1980–2014 can be detected after 1993/94. In the present study, the interannual variability of the SCSSM onset is classified into two types for the periods before and after 1993/94, based on their different characteristics of vertical coupling between the upper- and lower-tropospheric circulation and the differences in their related sea surface temperature anomalies (SSTAs). On the interannual time scale, type-I SCSSM onset is characterized by anomalous low-level circulation over the northern SCS during 1980–93, whereas type-II SCSSM onset is associated with anomalies of upper-level circulation in the tropics during 1994–2014. The upper-tropospheric thermodynamic field and circulation structures over the SCS are distinct between the two types of SCSSM onset, and this investigation shows the importance of the role played by the spring SSTAs in the southern Indian Ocean (SIO) and that of ENSO events in type-I and type-II SCSSM onset, respectively. In the early episode, the warming SIO SSTAs can induce an anomalous low-level anticyclone over the northern SCS that affects local monsoonal convection and rainfall over land to its north, demonstrating a high sensitivity of subtropical systems in type-I SCSSM onset. However, in type-II SCSSM onset during the later episode, the winter warm ENSO events and subsequent warming in the tropical Indian Ocean can influence the SCSSM onset by modulating the spring tropical temperature and upper-level pumping effect over the SCS.