Zonally asymmetric changes in the Antarctic Circumpolar Current strength over the past million years.

The Antarctic Circumpolar Current (ACC) plays a central role in regulating the global ocean circulation, climate and Antarctic Ice Sheet dynamics. Yet the spatiotemporal variability of the ACC during the Pleistocene remains poorly constrained. Here we reconstruct ACC flow-speed variation using a meridional transect of sediment cores from the Indian sector of the Southern Ocean. Our results reveal zonally asymmetric changes in ACC strength across the Southern Ocean on orbital timescales over the past one million years; the ACC intensified in the South Indian Ocean but weakened in the South Pacific during glacial and low-obliquity periods, with the opposite pattern during interglacial and high-obliquity periods. These anti-phased changes probably reflect an integrated response to bathymetric constraints, shifts in the Southern Hemisphere westerlies, sea-ice extent, buoyancy forcing and current confluence. Such zonally asymmetric and anti-phased ACC dynamics persisted during warmer-than-present intervals of the Pleistocene, offering a potential analogue for future anthropogenic warming-albeit under fundamentally different boundary conditions.