The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability^1,2,3. Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity⁴. Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial–interglacial cycles^5,6,7,8, 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 cooling⁹ and increasing global ice volume¹⁰. 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^11,12,13. 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 variability¹⁴. A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO₂ 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.

南极绕极流（ACC）代表世界上最大的洋流系统，影响全球海洋环流、气候和南极冰盖稳定性^1,2,3。如今，ACC 动力学由大气强迫、海洋密度梯度和涡流活动控制⁴。尽管古海洋重建显示更新世冰川-间冰期旋回中 ACC 位置和强度的区域异质性^5,6,7,8，但 ACC 的长期演化却鲜为人知。在这里，我们记录了南太平洋沉积物岩心 ACC 强度的变化。我们发现自 530 万年前（Ma）以来 ACC 流量没有线性长期趋势，这与全球变冷⁹和全球冰量增加¹⁰形成鲜明对比。相反，我们观察到百万年时间尺度上的逆转，从上新世全球冷却期间ACC强度增加到随后随着早更新世进一步冷却而减少。 ACC 状态的这种转变与南大洋的重新配置同时发生，南大洋的重新配置改变了 ACC 对大气和海洋强迫的敏感性^11,12,13。我们发现ACC强度的变化与40万年的偏心率周期密切相关，可能源于与热带太平洋温度变化相关的南太平洋急流的进动变化的调节¹⁴。冰川期ACC流减弱、蛋白石沉积向赤道移动和大气CO ₂减少之间的持续联系在中更新世过渡期(MPT)首次出现。最强的 ACC 流发生在上更新世比现在温暖的时期，这提供了随着未来气候变暖可能增加 ACC 流的证据。