Interdecadal change in triggering effect of subtropical Indian Ocean dipole on tropical Indian Ocean dipole and the role of Southern Annular Mode

Based on atmosphere reanalysis data from ERA5 and sea surface temperature (SST) from Hadley Center, this study investigates the triggering effect of the subtropical Indian Ocean Dipole (SIOD) on the following tropical Indian Ocean Dipole (IOD), as well as its interdecadal change, during the period 1960–2024. Results show that from 1960 to the late 1980s (WEAK period), the correlation between SIOD and IOD is not significant; whereas, from the late 1980s onward (STRONG period), a stable positive correlation appears between them. During the STRONG period, the positive SST anomalies (SSTA) in the southwestern Indian Ocean as well as the overlying anomalous anticyclonic circulation (AC), associated with the positive SIOD, are sustained and shift eastward under the positive wind–evaporation–SST (WES) feedback. This SSTA drives a vertical motion of the above atmosphere through diabatic heating, subsequently promoting the development of the low-level AC and its equatorward expansion by exciting Rossby wave trains and local meridional circulation. Ultimately, the development of IOD is triggered and enhanced through air–sea coupling processes, including the Bjerknes feedback, Walker circulation, and oceanic Rossby waves. In contrast, during the WEAK period, the responses of aforementioned physical processes are weak, preventing an effectively influence of the SIOD on the IOD. Further analysis indicates that the intensity variation of the Southern Annular Mode (SAM) plays a significant role in the interdecadal shift of the SIOD–to–IOD relationship: during the STRONG period, SAM intensity is generally stronger and predominantly in a negative phase, with its background circulation favoring an enhanced WES feedback and linkages between the subtropics and tropics; and vice versa during the WEAK period. This paper provides new insights for understanding cross-latitude air–sea interactions and improving seasonal IOD prediction.