The Indian Ocean Tripole (IOT) is a key mode of Indo–Pacific climate variability, distinct from the Indian Ocean Dipole, whose driving mechanisms have changed in recent decades. We show that IOT events occurred nearly twice as often before 2000, reflecting a fundamental shift in their triggering processes. Prior to 2000, IOT variability was jointly triggered by the Australian winter monsoon and El Niño-Southern Oscillation (ENSO). After 2000, ENSO forcing weakened substantially, and IOT variability became primarily controlled by the Australian winter monsoon. This transition arises from background SST warming near northern Australia, cooling in the tropical eastern Pacific, and an increased frequency of late-developing Central-Pacific El Niño events. Warming near Australia enhances monsoon-induced convection and associated Gill-type atmospheric responses, whereas Pacific cooling suppresses ENSO-related convection and weakens the Walker circulation. These results demonstrate how mean-state shifts and ENSO diversity reshape Indo-Pacific air-sea coupling.