The interdecadal shifts in subtropical intraseasonal oscillations and their impacts on tropical cyclones in the western North Pacific

Abstract This study investigates the interdecadal variability of intraseasonal oscillation (ISO) activity in the western North Pacific (WNP) during July-September using a dataset from 1979 to 2021. Analysis of the regime shift index identifies three distinct epochs: 1979–1993, 1994–2004, and 2005–2021. The middle epoch stands out for its pronounced westward extension of the subtropical anticyclone and peak activity of the westerly northward-propagating ISO. During this period, the interaction between the enhanced subtropical anticyclone and the strengthened ISO cyclonic anomalies intensifies southeasterly winds south of Japan. These winds act as a barrier to the formation and development of tropical cyclones (TCs) in the region east of the strong wind zone. Concurrently, the anticyclonic anomalies over the South China Sea hinder TC movement into this area. These combined effects lead to more clustered TC tracks between Taiwan and Japan, with increased TC frequency, intensity, and duration, supported by an active ISO background. The findings highlight a strong connection between interdecadal ISO variability and TC characteristics, underscoring the relevance of ISO behavior in long-term climate projections and TC forecasting. These insights are essential for improving prediction accuracy in the face of shifting climate patterns.