Revisiting the Roles of the Australian Winter Monsoon and ENSO in Indian Ocean Tripole Variability

The Indian Ocean Tripole (IOT) is a prominent mode of tropical Indian Ocean sea surface temperature (SST) variability. The mechanisms governing IOT variability and their long-term evolution remain poorly understood. Here, we show that the dominant drivers of IOT variability underwent a pronounced shift around 2000. Prior to 2000, IOT variability was jointly regulated by the Australian winter monsoon (AM) and El Niño-Southern Oscillation (ENSO). After 2000, however, the influence of ENSO weakened substantially, and IOT variability became increasingly dominated by the AM. This transition is associated with background SST warming near northern Australia, cooling in the tropical eastern Pacific, and an increased occurrence of late-developing Central-Pacific (CP) El Niño events. Warming near Australia strengthens monsoon-induced convection and the associated Gill-type atmospheric responses, enhancing the AM influence on the IOT. In contrast, the increased prevalence of CP El Niño events, together with La Niña-like cooling in the tropical eastern Pacific, weakens ENSO-induced atmospheric teleconnections to the Indian Ocean, thereby reducing the effectiveness of ENSO forcing. These results demonstrate how mean-state changes and ENSO diversity can reshape Indo-Pacific air-sea coupling.