Abstract

Palaeoceanographic reconstructions indicate that the distribution of global ocean water masses has undergone major rearrangements on glacial-interglacial time scales. Different rates of Antarctic sea-ice formation, which plays a key role in shaping the abyssal overturning circulation today, may have driven these past circulation changes. This mechanism is investigated in preindustrial and Last Glacial Maximum (LGM, ~20,000 years ago) fully-coupled climate simulations, which are compared to an idealised ocean model. The coupled simulations show substantial inter-model differences in their representation of glacial ocean circulation, which is often at odds with the geological evidence. Such inconsistencies are attributed to differing (and likely insufficient) Antarctic sea-ice formation, where discrepancies are further amplified by short integration times. The idealised model reproduces circulation patterns that are broadly consistent with LGM reconstructions. Here, the effect of Antarctic sea-ice expansion on the simulated glacial circulation and air-sea gas exchange also results in increased deep-ocean carbon storage. These physical changes alone can explain about half of the glacial-interglacial variations in atmospheric CO2 concentrations.