The Antarctic Circumpolar Current (ACC), the largest oceanic current system on Earth, plays a pivotal role in global heat redistribution, the carbon cycle, and inter-ocean nutrient exchange. However, the response and evolutionary pathway of Southern Ocean circulation under global warming remain contentious. Investigating the natural variability of the Southern Ocean circulation during past warm analogues preserved in geological records is thus critically needed. This study reconstructs latitudinal migrations of ACC fronts and variations in upwelling intensity across key Pleistocene climatic transitions, based on sediment cores (U1537 and U1538) retrieved from 3000–4000 meters water depth in the Scotia Sea during IODP Expedition 382. Our findings further reveal a shift in the dominant periodicity of Southern Ocean frontal movements and upwelling from 41-thousand-year (obliquity cycle) to 100-thousand-year (eccentricity cycle) bands during the Mid-Pleistocene Transition (MPT). Moreover, records from the Last Interglacial (LIG) indicate that flow velocities at the southern boundary of the ACC were nearly three times higher than during the Holocene, suggesting an overall strengthening and ~5° (approximately 600 km) poleward shift of the ACC. Over multiple glacial-interglacial cycles, the northern and southern boundaries of the ACC migrated in anti-phase, closely correlated with Southern Ocean upwelling intensity and atmospheric CO₂ fluctuations. This study provides crucial insights into the mechanisms of the Southern Ocean carbon sink since the MPT and its potential evolution under future carbon-neutral scenarios.

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