<p>The Australian monsoon system plays a pivotal role in the tropical climate system by modulating the El Niño-Southern Oscillation (ENSO) development through multi-scale ocean-atmosphere interactions. This study identifies a significant decadal weakening of the Australian cross-equatorial flow intensity over the past two decades, attributed to the concurrent westward shift of the Australian High (AH) during austral winter. These decadal changes in the Australian monsoon reduce tropical Pacific atmospheric convection and the associated westerly wind anomalies over the central-to-western Pacific, which are crucial precursors for ENSO development. This process diminishes air-sea coupling feedback, including the thermocline feedback and the Ekman feedback, ultimately decreasing the strength of warm ENSO (El Niño) events. Using the Community Earth System Model, we confirm the close linkage between the Australian monsoon and ENSO on the decadal timescale. These findings provide new insights into the coupled relationship between ENSO and monsoon variability, offering a valuable framework for understanding ENSO’s long-term modulation and improving future climate predictions.</p>