Occurrence and impacts of tropical cyclones over the southern South China Sea

This study investigates the seasonal variation of tropical cyclone (TC) occurrences over the southern South China Sea (SCS) based on the Joint Typhoon Warning Center dataset during 1979-2016. In this region, TCs are most active from October to December (OND), rather than in the active TC season from July to October (JASO) in other western North Pacific (WNP). According to the location of their genesis, the southern SCS TCs are divided into two groups. One consists of TCs generating in the WNP, which are induced by a westward steering flow and move into the southern SCS in OND. The other group contains TCs generated locally in the SCS that move into the southern SCS, which peak in OND due to the positive relative vorticity and high genesis potential index (GPI). In this region, TC genesis is influenced by the El Nino-Southern Oscillation (ENSO). On an interannual scale, TCs tend to occur in La Nina years due to high GPI. In addition, the mid-level relative humidity and the low level vorticity play important roles in the contribution of the environmental factors of GPI during ENSO. Both composite analysis and a case study show the impact of TCs on precipitation. TC occurrence is accompanied by heavy rainfall, which is triple the amount of climatological rainfall in OND during 1979-2016, over 15 mm.day(-1). Nevertheless, the influence of locally generated TCs, rather than TCs generated in the WNP, contributes to more heavy rainfall in the southern SCS.