Breakdown of the Relationship between Australian Summer Rainfall and ENSO Caused by Tropical Indian Ocean SST Warming

The relationship between El Niño–Southern Oscillation (ENSO) and Australian summer rainfall (ASR) during 1960–2015 experienced an interdecadal change around the mid-1980s. Before the mid-1980s, ASR was significantly correlated with tropical central Pacific (TCP) sea surface temperature (SST), whereas after that it was not. While El Niño was always independent from ASR, La Niña had a close relationship with ASR. However, this relationship was weakened after the mid-1980s. The Indian Ocean SST warming might contribute to the weakening relationship between La Niña and ASR. For La Niña events before the mid-1980s, the negative SSTA over TCP and the southern tropical Indian Ocean induced a large-scale lower-level cyclonic anomaly over Australia, leading to nearly uniform positive precipitation over Australia. In this manner, a significant relationship between ASR and La Niña was established. On the contrary, for the La Niña events after the mid-1980s, because of the Indian Ocean SST warming, the equatorial eastern Indian Ocean and Maritime Continent presented positive SSTAs and enhanced moisture, favoring enhanced rainfall anomalies over the equatorial Maritime Continent. This enhanced rainfall condensation heating induced a lower-level cyclonic anomaly to the west of Australia. The northerly anomalies at the eastern flank of this cyclonic anomaly counteracted the southerly anomalies at the western flank of the cyclonic anomaly over eastern Australia induced by the negative TCP SSTA, leading to insignificant circulation and rainfall anomalies over Australia. As such, being interfered with by the equatorial Maritime Continent heating, the relationship between ASR and La Niña was weakened.