Zonally asymmetric variability of the Antarctic Circumpolar Current strength on orbital timescales

The Antarctic Circumpolar Current (ACC), Earth's largest ocean current, regulates global ocean circulation, Antarctic Ice Sheet stability, and the carbon cycle. Previous investigations have typically assumed uniform ACC variability across the Southern Ocean over Pleistocene glacial-interglacial cycles, yet proxy records show conflicting responses on orbital timescales. Here, we reconstruct the spatiotemporal variability of ACC strength over the past one million years using sortable silt mean grain size from a transect of six sediment cores in the South Indian Ocean, complemented by a synthesis of existing records from all Southern Ocean sectors. Our results reveal a persistent zonal asymmetry in ACC strength on glacial-interglacial and obliquity timescales. The Indian and Pacific sectors of the Southern Ocean exhibited anti-phased changes on both glacial-interglacial and obliquity timescales: during glacial and low-obliquity intervals, the ACC intensified in the Indian sector but weakening in the Pacific sector, with the pattern reversing during interglacials and high-obliquity periods. Proxy-model integration indicates this asymmetry is likely driven by sector-specific responses to shifts and intensification of the Southern Hemisphere westerly winds, bathymetric steering, sea-ice extent and meridional density gradients. These findings link ACC dynamics to interbasin exchange, ice sheet variability and the carbon cycle, providing insights into past and ongoing climate change.