资源环境科技发展态势分析平台

This study investigates the tracks of category 4 and 5 typhoons (TYs) in the western North Pacific (WNP) in November-December during 1979-2016 and their relationships with the intraseasonal oscillation (ISO) including the 30-60-day and 12-25-day variations. It is shown that the impact of ISO on the TY tracks found in previous studies is mainly contributed by the quasi-biweekly oscillation (QBWO). The impact on the TY tracks is through influence on the steering flows over the WNP. Accordingly, TYs are separated into two types: moving westward (TYW) and northeastward (TYE). QBWO in the WNP is associated with an active convection originated from the equatorial WNP, which propagates northwestward and reaches the SCS and middle-high latitudes. TYW mostly occurs in the decline stage of the QBWO life cycle, while TYE appears in the mature stage of QBWO. For TYW, QBWO hardly changes the background large-scale circulation, which features an anticyclonic circulation covering the entire WNP and SCS, thus TYW moves westward. However, TYE appears in the mature phase of QBWO. The strong convection induces an intense cyclonic circulation in western part of the WNP, thus changes the steering flow. The steering flow features an anticyclonic circulation over the mid-eastern WNP, which is favorable for TYE to divert the direction and move northeastward.