During the period 1958–2001, the frequency of tropical cyclone (TC) genesis over the South China Sea (SCS) experienced an obvious interdecadal change around the mid-1970s. Compared to the period from late 1950s to early 1970s, the number of TCs is significantly reduced during mid-1970s through late 1990s. This interdecadal change in the TC frequency appears to relate to the increase in sea surface temperature (SST) of the tropical Indian Ocean. The difference of the circulation between the warm phase and the cold phase of the tropical Indian Ocean SST provides support for the influence of the tropical Indian Ocean warming on the variability of the TC frequency over the SCS. In the warm phase, lower-level convergent and ascending flows over the tropical Indian Ocean are accompanied by upper-level divergent flows, part of which go toward the SCS, leading to upper-level convergence and descent there. Consequently, two lower-level anticyclones develop and the TC genesis is suppressed over the SCS.