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 behind. It is found that TC activity over the SCS experiences a significant decadal change around the mid-1990s. Compared to the period from the 1970s to the early 1990s, the number of TCs formed in the SCS remarkably increases from the mid-1990s through the 2000s. In particular, this change of TC genesis is closely related to a decadal shift in atmospheric intraseasonal variability (ISV) that occurred in 1994. The ISV on the 30–60 days time scale over the SCS has been increasing since the mid-1990s, and the increased TC frequency after 1994 is attributed primarily to the active convection induced by the enhancement of the SCS ISV. In addition, the TC activities before the mid-1990s are mostly confined within the SCS basin. However, more TCs form over the SCS and move northeastward since the mid-1990s and finally enter the East China Sea and the Philippine Sea. Anomalies of westerly over the northern SCS after 1994 are responsible for the northeastward moving of TCs.
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