An onset mechanism of the Australian summer monsoon that incorporates possible air-sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land-ocean thermal contrast during the premonsoon period results in a well-organized continental-scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600--700 hPa level. Intensified low-level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at ~700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large-scale disturbances with ascending motion (such as the Madden-Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics.