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This study investigates the impact of the intraseasonal oscillation (ISO) on tropical cyclone (TC) tracks in the South China Sea (SCS) during 1970-2010. About one third of TCs in the SCS move eastward, while the other two thirds move westward. In the TC genesis peak seasons of June-October (JJASO), the westward moving TCs are controlled by the background TC steering flow of easterly, and the eastward moving TCs by the TC steering flow induced by the ISO. The outgoing longwave radiation and wind fields show that the eastward moving TCs were mostly along the main axis of strong TC steering flow anomaly of westerly associated with the ISO, while the westward moving TCs were only weakly associated with the ISO. An experiment performed with a simple two-level model further confirmed the result. The interannual variation of TC tracks in the SCS is also discussed. It is found that the steering flow anomalies in the SCS mostly favor eastward moving TCs in central Pacific (CP) El Niño and eastern Pacific (EP) El Niño years. However, the eastward flow anomalies are too weak to have strong influence on the majority of the TCs. During La Niña years, TCs in the SCS tend to move westward, possibly related to the westward steering flow anomalies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]