Triggering Effect of Indian Ocean Dipole on Subtropical Indian Ocean Dipole: Interdecadal Change and Modulation by Interdecadal Pacific Oscillation

ABSTRACT Based on observational and reanalysis datasets, this study examines the relationship between the tropical Indian Ocean Dipole (IOD) that matures in boreal autumn and the subsequent Subtropical Indian Ocean Dipole (SIOD) that occurs in the following boreal winter, as well as its interdecadal variability over 1960–2024. Results suggest that a significant correlation between IOD and SIOD only exists in 1979–2005. During this strong correlation period, an intense IOD can trigger a persistent meridional circulation and mid‐latitude Rossby wave train, maintaining an anomalous cyclone–anticyclone pair (CACP) over the subtropical Indian Ocean, with the cyclone in the west and the anticyclone in the east, ultimately leading to the development of SIOD via local air–sea coupling. In other periods, this relationship weakens and the described coupling pathway fails to establish. A key factor enabling this efficient cross‐seasonal linkage is the presence of sustained oceanic memory. Specifically, the sea surface temperature and subsurface heat anomalies associated with the IOD persist from boreal autumn into winter, providing a thermal precondition that interacts with the atmospheric circulation to eventually develop the SIOD. Further analysis indicates that this interdecadal variation is modulated by the Interdecadal Pacific Oscillation (IPO). Specifically, the positive phase of IPO strengthens the Indo–Pacific Walker circulation anomaly, reinforces the tropical–subtropical meridional pathway, and thereby enhances the IOD's triggering effect on SIOD. In contrast, the negative IPO phase suppresses this pathway. This study offers new mechanistic insights that advance our understanding of cross‐seasonal interactions in the Indian Ocean and contribute to improved climate predictions. Furthermore, it clarifies the importance of accounting for non‐stationarity in the climatic background for seasonal forecasting.