Influence of the tropics and southern westerlies on glacial interhemispheric asymmetry

During the last glacial period and deglaciation, climate shifts in the Northern and Southern Hemispheres occurred asynchronously, cooling in one coinciding with warming in the other(1-6). This asymmetry has been attributed to the phenomenon known as the bipolar seesaw(1), which, in turn, has been linked to latitudinal shifts of the southern westerly wind belt(2). The southern westerlies substantially determine the location of the oceanic Subtropical Front. A more poleward location of the Subtropical Front allows the Leeuwin Current, which carries warm tropical water along the west coast of Australia to extend at times as far as Tasmania. Here we use multiple proxies obtained from a sediment core off the southern coast of Australia to reconstruct sea-surface temperature and other environmental conditions, both marine and terrestrial, from 33,000 to 10,000 years ago. We find millennial-scale warm phases south of Australia and attribute them to the presence of the Leeuwin Current. The warm phases are synchronous with cool Northern Hemisphere Heinrich Stadials(7) and coincide with warm intervals south of Africa and in the western tropical Atlantic Ocean(6). We therefore suggest that the poleward displacement of the Subtropical Front during these intervals extended across the Indian Ocean, thus promoting the leakage of warm Indian Ocean water via the Agulhas Current(6) into the Atlantic Ocean.