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This study addressed the abrupt increase in the development speed of tropical storms (TSs) to severe TSs (>= category 3, referred to as STSs) in the western North Pacific (WNP) during the late 1990s. Our investigation indicated that the TSs 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 energy conversion from intraseasonal oscillation (ISO) to SSE played a critical role in accelerating the TS-to-STS development. The increase of ISO-SSE interaction was attributed to the mega La Nina-like mean state change. The anticyclone anomaly associated with mega La Nina and warm oceanic condition in the WNP substantially modified the mean TS genesis location (northwestward shift) and enhanced the ISO magnitude in the South China Sea and Philippine Sea, thereby amplifying the ISO-SSE interaction in the WNP in the late 1990s.

Plain Language Summary The number of severe tropical storms (>= category 3, referred to as STSs) could be expected to decrease in response to the shortening of the mean TS lifetime since the late 1990s. However, in this study, we demonstrated that the annual mean number of STSs after late 1990s was nearly unchanged and the ratio of TSs developing into STSs in the western North Pacific exhibited an abrupt increase in the late 1990s, meaning that the TSs have developed into STSs at a higher speed since the late 1990s. We report here that the enhanced intraseasonal oscillation-synoptic-scale eddy interaction plays a critical role in accelerating the TS-to-STS development.