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 activity⁴. 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 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^11–13. 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.

Surface ocean circulation in the western equatorial Atlantic is mainly wind driven and plays a major role for the transport of warm waters to the North Atlantic. Past changes in the strength and direction of the trade winds are well documented, but the response of the western equatorial Atlantic circulation and water column structure to these changes is unclear. Here we used the difference between the stable isotopic oxygen composition of two species of planktonic foraminifera (Globigerinoides ruber white and Neogloboquadrina dutertrei) from two sediment cores collected off northeastern Brazil to investigate millennial‐ and orbital‐scale changes in upper ocean stratification since the Last Interglacial. Our records indicate enhanced upper ocean stratification during several Heinrich stadials, partly due to a shoaling of the thermocline, which was linked to a decrease in the strength of southeast trades winds. In addition, we show that a decrease in wind zonality induced by increases in Northern Hemisphere low‐latitude summer insolation causes a shoaling of the thermocline in the western equatorial Atlantic. These ocean‐atmosphere changes contributed to a reduction in the cross‐equatorial transport of warm waters, particularly during Heinrich stadials and Marine Isotope Stage 4.

The Brazil Current (BC) is a relevant feature in the Atlantic Meridional Overturning Circulation (AMOC). Its behavior during slowdown or intense AMOC remains poorly known because of the lack of paleoceanographic records, especially for the Holocene. Here, we investigate changes in a western boundary upwelling system (Cabo Frio, off Southeastern Brazil) which are driven by variations in the BC and NE winds during the last 9 kyr. To assess the variability of the BC, we used δ¹⁸O, Mg/Ca, and assemblages of planktonic foraminifera. Our results indicate five oceanographic phases during the last 9 kyr. During Phase I (from 9.0 to 7.0 cal kyr BP), the BC diverged offshore from the modern upwelling area because of the low sea level, increasing the influence of shelf waters and coastal upwelling plumes on foraminifera assemblages. Phase II (7.0–5.0 kyr BP) was marked by the approach of the internal front of the BC with low intensity and episodes of strong productivity that were linked primarily to the upwelling of the South Atlantic Central Water (SACW) and/or Subpolar Shelf Waters (SPSWs) (cold). Phase III (5.0–3.5 kyr BP) was a transition, marking a large oceanographic and climatic change from the weakening of the AMOC. The internal front of the BC became warm and subsurface SACW upwelling was stronger. In Phase IV (3.5–2.5 kyr BP), the BC acquired its modern dynamics, but weak NE winds weakened the SACW’s contribution to upwelling events. Finally, in Phase V (last 2.5 kyr BP), the NE winds reintensified, promoting frequent episodes of upwelling and intrusion by SPSWs during the Medieval Climate Anomaly.