Equatorward Southern Westerlies and Reduced Deep Ocean Ventilation During the Last Glacial

The midlatitude westerlies in the Southern Hemisphere play a crucial role for the climate system, because they ventilate the deep ocean when they are aligned with the Antarctic Circumpolar Current (ACC). As such, their latitudinal position directly influences the concentration of CO2 in the atmosphere. Earlier work has shown that the westerlies were poorly aligned with the ACC and not in a position to ventilate the deep ocean during the Last Glacial Maximum (LGM: ~20 ka) when the northern limit of the westerlies was well to the north of the ACC. The westerlies and the ACC became better aligned between 20 and 11 ka in a way that mirrors the increase in atmospheric CO2. Here we present surface exposure ages from the Valle Rucachoroi (~39°S), Southern Central Andes, which show that the most prominent preserved moraine dates back to ~40 ka, significantly older than the global LGM. Given the precipitation-sensitivity of the glacier mass-balance at the research site, this documents increased precipitation and an even more equatorward position of the westerlies at 40 ka compared to the LGM. Although yet undated in the Central Andes, we surmise that the westerlies were in an equatorward position as early as ~60 ka, based on published glacial chronologies from Australia. An equatorward position from ~60 to 20 ka would, of course, be consistent with reduced deep ocean ventilation and the low atmospheric CO2 concentrations observed in the Antarctic ice cores. It would seem from the glacial chronologies that the southern westerlies began shifting poleward very early with respect to the last deglaciation. So, what would make the westerlies begin to shift at this time? We suggest that the critical factor is the temperature difference between the hemispheres. SSTs near New Zealand and the air temperatures over and around Antarctica reached their glacial minimum levels at ~65 ka. The Northern Hemisphere, meanwhile, was relatively warm because the northern ice sheets were just beginning to grow. So, it would seem that the southern westerlies were in their most equatorward position when the Northern Hemisphere was warmest with respect to a cooler south. The subsequent growth of the northern ice sheets then slowly reduced the temperature difference and caused the westerlies to shift poleward gradually between 60 and 20 ka. The abrupt shift at the end was then brought about by increasing northern insolation and the initial melting of the northern ice sheets, which killed the Atlantic Meridional Overturning Circulation during Heinrich Event 1 and caused the Northern Hemisphere to cool even more. The southern westerlies reached their most poleward position at the point in time when northern winters were coldest and the warming in Antarctica and the Southern Ocean was underway. CO2 began venting from the deep ocean at this point and began warming both hemispheres until the northern ice sheets finally vanished.