An interdecadal change of tropical cyclone activity in the South China Sea in the early 1990s

Tropical cyclone (TC) genesis in the South China Sea (SCS) during 1979–2008 underwent a decadal variation around 1993. A total of 55 TCs formed in the SCS from May to September during 1994–2008, about twice that during 1979–1993 (27). During the TC peak season (July–September, JAS), there were 43 TCs from 1994–2008, but only 17 during 1979–1993. For July in particular, 13 TCs formed from 1994–2008, but there were none during 1979–1993. The change in TC number is associated with changes of key environmental conditions in atmosphere and ocean. Compared to 1979–1993, the subtropical high was significantly weaker and was displaced more eastward during 1994–2008. In the former period, a stronger subtropical high induced downward flow, inhibiting TC formation. In the latter period, vertical wind shear and outgoing longwave radiation all weakened. Mid-level (850–500 hPa) humidity, and relative vorticity were higher. Sea surface temperature and upper layer heat content were also higher in the area. All these factors favor TC genesis during the latter period. The decadal change of TC genesis led to more landfalling TCs in Southern China during the period 1994–2008, which contributed to an abrupt increase in regional rainfall.