Dansgaard-Oeschger (D-O) cycles were the most prominent, abrupt climate events of the last glacial period whose impact was most strongly felt in the high latitudes of the North Atlantic region. The climate links between the North Atlantic, the Asian and American tropics, and Antarctica during these cycles are well documented. However, the potential role of the Indo-Pacific Warm Pool and Australasian monsoon system in propagating climate impacts across the hemispheres is still unclear. Here, we use tandem measurements of oxygen isotopes in calcite and fluid inclusions, as well as carbon-isotope ratios, from multiple stalagmites from Liang Luar Cave, Flores (southern Indonesia) to examine the monsoon response to D-O event number 21 (~87,000-84,000 years ago), the longest and warmest event recorded in Greenland ice cores. The record shows that there was a rapid decline in monsoon rainfall in Indonesia during D-O21 warming in Greenland and cooling in Antarctica. At around the same time, the East Asian monsoon was intensified, indicating that the intertropical convergence zone (ITCZ) shifted abruptly to the north during this event. Our record also shows that there was a 2-3 °C increase in local air temperature, which would have acted to increase primary productivity and promote the generation of soil carbon for methanogenesis. Therefore, our findings indicate that ITCZ positioning in tropical Australasia--through its influence on large-scale oceanic-atmospheric circulation--played a key role in transmitting the abrupt climate signal between the hemispheres, thereby facilitating the rapid rise of atmospheric CO2 and CH4 concentrations during D-O21 that ultimately led to global warming and the demise of the MIS5b stadial.