Abstract: The South China Sea (SCS) summer monsoon is one of the major systems for the East Asian monsoon. It consists of multi-scale features. In addition to seasonal mean circulation, low-frequency (intraseasonal oscillation) and high-frequency (synoptic-scale) eddies are both active and demonstrate significant interannual variability. This study investigated the energy sources for the interannual variability of monsoon circulation and its interactions with eddies during 1979-2010 using two reanalysis datasets, i.e. the ERA-Interim and the NCEP/NCAR. Results show that during the years with enhanced convections in the SCS summer monsoon, the mean kinetic energy (MKE) strengthened over the southern SCS (south of 12°N) and Indochina. The increase in MKE is associated with the eastward extension of a strengthened westerly jet in the Indian summer monsoon region. In the northern SCS (north of 12°N) and western Pacific, an anomalous cyclonic circulation appeared, which is related to the deepening of monsoon trough. The enhanced MKE in the southern SCS is supported by interactions between eddy momentum fluxes and mean circulation. The monsoon mean flows lost less kinetic energy to eddies (i.e., mean flows retain more kinetic energy) during strong monsoon years. Interactions between the high-frequency (<10 d) and low-frequency (10-90 d) eddies with different components (divergent part, rotational part, and vertical wind shear) of mean flows were further examined. It is found that the interactions between <10 d eddies and monsoon divergent flows play a major role in producing positive MKE anomaly. The MKE in the monsoon trough over the northern SCS is maintained by the interaction between atmospheric heating and ascending motion anomaly. Meanwhile, the enhanced MKE is converted to eddy kinetic energy that is favorable for the growth of eddies. As a result, tropical cyclones and northward-propagating intraseasonal oscillation were both active over the northern SCS during strong monsoon years, which often lead to disastrous weather events in southern China.