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The Antarctic Circumpolar Current (ACC) represents the world's largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability 1-3 . Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity 4 . Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial-interglacial cycles 5-8 , the long-term evolution of the ACC is poorly known. Here we document changes in ACC strength from sediment cores in the Pacific Southern Ocean. We find no linear long-term trend in ACC flow since 5.3 million years ago (Ma), in contrast to global cooling 9 and increasing global ice volume 10 . Instead, we observe a reversal on a million-year timescale, from increasing ACC strength during Pliocene global cooling to a subsequent decrease with further Early Pleistocene cooling. This shift in the ACC regime coincided with a Southern Ocean reconfiguration that altered the sensitivity of the ACC to atmospheric and oceanic forcings 11-13 . We find ACC strength changes to be closely linked to 400,000-year eccentricity cycles, probably originating from modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability 14 . A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO 2 during glacial periods first emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, providing evidence of potentially increasing ACC flow with future climate warming., (© 2024. The Author(s).)

We compiled modern and fossil relative abundance of coccolithophore species Florisphaera profunda from published and unpublished datasets, along with ocean environmental variable data from satellite remote sensing and physical measurements. The database includes relative abundances of F. profunda in sediment trap ( n  = 26) and core-top ( n  = 1258), and sediment core samples ( n  = 104). Downcore data covers the Last Glacial Maximum ( n  = 94, 24-19 ka) or the Mid-to-Late Holocene ( n  = 77, <6 ka). This database allows studying modern and past biogeography of F. profunda as a response to changing ocean and climate conditions, "Quantitative reconstruction of primary productivity in low latitudes during the last glacial maximum and the mid-to-late Holocene from a global Florisphaera. profunda calibration dataset" (Hernández-Almeida et al., 2018).