资源环境科技发展态势分析平台

Tropical cyclones (TCs) initially forming in the western North Pacific (WNP) and later moving into the South China Sea (SCS) are defined as entrance TCs. El Nino-Southern Oscillation (ENSO) and inter-annual variability of entrance TCs during fall exhibit four types of asymmetric relationships: La Nina-more, La Nina-less, El Nino-more, and El Nino-less. Large-scale modulation of the La Nina-more (La Nina-less) type features negative sea surface temperature (SST) anomalies in the tropical central-eastern Pacific with a maximum centre in the eastern (central) Pacific. The responding circulation pair extends northwestwards (northwards) with a cyclonic anomaly in the SCS and an anticyclonic anomaly in the North Pacific (NP). Above-normal TCs forming to the east/northeast of the Philippines are steered by anomalous southeasterly (southerly) flows to enter the SCS (NP), leading to more (less) entrance TCs. For the El Nino-less (El Nino-more) type, positive SST anomalies over the tropical Pacific exhibit a centre in the eastern (central) Pacific. A circulation pair characterizes an anticyclonic anomaly over the entire (southwestern) SCS and a cyclonic anomaly within the NP (displacing westwards into the northern SCS) inducing anomalous flows from the northern (central and southern) SCS into the tropical WNP. TC entrance in both El Nino-less and El Nino-more types is hindered by inter-annual flows. More entrance TCs in the El Nino-more type are facilitated by the intra-seasonal oscillation (ISO). Entrance TCs move into the SCS along a favourable environment provided by a northwest-southeasterly elongated 30-60-day cyclonic anomaly. Regarding the relative role of the inter-annual and intra-seasonal modes, ISO (ENSO) exerts major modulatory effects on entrance TCs in the El Nino-more (El Nino-less) type. Both ISO and ENSO jointly facilitate (suppress) entrance TCs in the La Nina-more (La Nina-less) type.