The Asian-Australian monsoon system (A-AuMS) is the world's most typical cross-equatorial coupled monsoon system. On a seasonal timescale, the summer monsoon in one hemisphere is usually linked to the winter monsoon in the other via outflows. However, robust evidence remains lacking as to whether such cross-equatorial monsoon coupling persists during orbital-scale paleoclimate evolution. A scarcity of high-resolution paleoclimatic records from the Northern Australian monsoon region in the Southern Hemisphere has limited a full understanding of the A-AuMS's dynamic mechanisms.

Previous studies have attempted to compare Chinese and South American speleothem records to decipher monsoon evolution in the two hemispheres, proposing that orbital-scale low-latitude monsoon changes are primarily controlled by solar insolation. However, the South American monsoon and the Asian monsoon do not belong to the same cross-equatorial monsoon system and thus lack direct dynamic linkages. Coupled with ongoing debates over the indicative significance of speleothem oxygen isotopes, this view remains uncertain.

To address this issue, a research team led by Prof. YAN Hong from the Institute of Earth Environment of the Chinese Academy of Sciences (IEECAS), in collaboration with Australian scholars, reconstructed Australian summer monsoon (AuSM) variability over the past 13.5 ka using a 5.13-m-long sediment core from Bromfield Swamp in the tropical monsoon region of northern Australia.

Their research found that the AuSM weakened during the early Holocene (approximately 11–7.8 ka), a trend inconsistent with increasing Southern Hemisphere summer insolation, implying that low-latitude insolation change is not the sole driver of tropical monsoon evolution.

Through comparative analysis with Northern Hemisphere paleoclimatic records, the team proposed that the retreat of Northern Hemisphere high-latitude ice sheets during the early Holocene weakened the East Asian winter monsoon, which in turn significantly modulated AuSM intensity by reducing cross-equatorial airflow. This finding provides evidence for cross-equatorial monsoon coupling between the two hemispheres at the orbital timescale.

Further analysis reveals that the Asian and Australian summer monsoons exhibited nearly opposite variations throughout the Holocene. This coupling is primarily regulated by interhemispheric thermal imbalance, which can be characterized by inter-hemispheric temperature gradients. From the early to middle Holocene, the retreat of Northern Hemisphere high-latitude ice sheets likely contributed to gradual Northern Hemisphere warming and a consequent reduction in the north–south temperature gradient. The "thermal equator" shifted northward, also driving a northward migration of the mean position of the Intertropical Convergence Zone (ITCZ), strengthening the East Asian summer monsoon (EASM) and weakening the AuSM. From the middle to late Holocene, after major ice-sheet retreat, the increased interhemispheric temperature gradient may be linked to opposing changes in summer insolation between the Southern and Northern Hemispheres, resulting in a southward shift of the "thermal equator" and ITCZ. This led to a weakened EASM and a strengthened AuSM.

This framework also applies to the Younger Dryas (YD) event. Integrated records show that the Asian-Australian monsoon region generally exhibited an opposing hydrological spatial pattern during the YD: the Asian monsoon region was dry, while the Australian monsoon region was wet. This corresponds to a southward shift of the "thermal equator" (and ITCZ) induced by Northern Hemisphere cooling associated with a weakened Atlantic Meridional Overturning Circulation (AMOC).

This study not only provides empirical evidence for orbital-scale coupling of monsoon changes between the Northern and Southern Hemispheres, but also explicitly identifies interhemispheric thermal imbalance as the primary driver of monsoon variability. Solar insolation is only one factor contributing to this thermal imbalance, not the sole determinant.

Published in Nature Communications, this work was jointly supported by the National Natural Science Foundation of China (NSFC), the National Key R&D Program of China, and the State Key Laboratory of Loess Science.