This study focuses on the decadal variability of tropical cyclone (TC) activity over the South China Sea (SCS) since the 1970s and its possible cause.It is found that TC activity over the SCS experienced a significant decadal variation around mid-1990s.Compared to the period from the 1970s to mid-1990s,the number of TC formed in the SCS obviously increased from mid-1990s through the 2000s.In particular,this change of TC genesis is closely related to the decadal variation of summer monsoon intraseasonal oscillation (ISO) over the SCS.The increased TC frequency after mid-1990s is attributed to the enhanced convection during the active phase of SCS monsoon ISO.Additionally,the activities of TCs before mid-1990s are mostly confined within the SCS basin.However,more TCs generated over the SCS move northeastward since mid-1990s,and finally enter the East China Sea.Strong southwesterly anomaly over the northern SCS is found since mid-1990s,which is responsible for the northeastward movement of TCs.