Effect of ISO-SSE Interaction on the Rapid intensification of TS in the WNP Since the Late 1990s

This study addressed the abrupt increase in the development speed of Tropical storms (TSs) to severe TSs ( Category 3, referred to as STS) in the western North Pacific (WNP) during the late 1990s. Although the annual mean number of TSs in the WNP exhibited an abrupt decrease in the late 1990s, the annual mean number of STs did not exhibit significant change. This caused the ratio of annual mean number of STSs to the total TSs number displayed an abrupt increase in the late 1990s. Our observations indicate that the mean lifetime of a STS during the post 1990s period was approximately 70 hours shorter than that in the pre-1990s period because of the northwestward shift of the mean TS genesis location in response to the mega-La Niña-like mean state change in the late 1990s. This indicated that the TSs have developed into STSs with a faster speed since the late 1990s. The eddy kinetic energy budget of synoptic-scale eddy (SSE) indicated that the enhancement of scale interaction of intraseasonal oscillation (ISO)−SSE played a critical role in accelerating the TS–to–STS development. A further diagnosis revealed that the increase of ISO–SSE interaction was attributed to the mega–La Niña–like mean state change. The mega–La Niña–like associated anticyclone anomaly and warm oceanic condition in the WNP substantially modified the mean TS genesis location (northwestward shift), enhanced the ISO magnitude, and shifted the ISO propagation northward, thereby amplifying the ISO–SSE interaction in the WNP in the late 1990s.

Key words: Tropical storm, abrupt increase,  Intraseasonal oscillation (ISO), ISO–SSE interaction