The tumultuous life of the Antarctic Circumpolar Current over 5,3 million years, including focus on the glacial-interglacial forcing!

29 April 2024

To reconstruct the strength of the Antarctic Circumpolar Current (ACC) and shifts of the Southern Ocean frontal system over the past roughly 5.3 Myr, the authors used sediment records – and sortable silt as proxy for near-bottom water-velocity variations^_from the pelagic central and remote South Pacific Ocean.

Over the Pleistocene scale, they compared down-core records north–south across the ACC over the past three glacial cycles (0–350 ka). The data revealed that, relative to the Holocene mean, glacial ACC strength was reduced by about 30–50% in the Subantarctic Zone, about 20–30% in the Polar Frontal Zone (PFZ) and at the Polar Front (PF) and about 20% in the Antarctic Zone (AZ), whereas ACC strength during interglacial Marine Isotope Stage (MIS) 5 and MIS 7 slightly exceeded the Holocene levels. The authors also discuss the opal content fluctuations in the same cores, characterized by strongly increased opal contents across the Subantarctic Front (SAF) and the PF and reduced opal deposition in the AZ during glacials compared with interglacials.

On a longer timescale, over the past 5.3 Myr, the sediment records document large variations in ACC strength, between roughly 50% and 180% of the mean Holocene ACC flow. Surprisingly, they show that after an ACC strength maximum in the Late Pliocene (about 3.0 Ma), ACC strength broadly declines, in opposition to expectations from continued Early Pleistocene cooling and ice-volume expansion. These contrasting trends indicate that the ACC responded to fundamentally different forcings in the Pliocene versus the Early Pleistocene…to read and understand more, see the figures below…

**Figure 1: Today´s Antarctic Circumpolar Current (ACC).** Shown is the simulated ocean velocity at 100 m water depth (blue=weak; white=strong) showing the very eddy-reach structure, with a lot of detail. Eddies in the Southern Ocean are important in shaping details of the ocean transport and exchange with the atmosphere. Red lines show location of oceanic fronts (e.g. the Polar Front (PF) where ACC transport in frontal jets is particularly strong, isopycnals are tilted, and the deep ocean connects to the atmosphere. (Figure from Lamy et al., 2024).

**Figure 2:** **Schematic illustrating key atmospheric and oceanic processes influencing million-year trends in ACC strength.** The schematics depict an idealized north-south transect from Antarctica across the Pacific (at ~125°W; north of 20°S out of scale). We illustrate major atmosphere-ocean mechanisms influencing long-term changes in the ACC relative to the early Pliocene. a, the early Pliocene, b, the late Pliocene before the iNHG, and c, theEarly Pleistocene (1.5Ma) situation following the Southern Ocean reconfiguration connected to the iNHG. U1540/U1541 = location of IODP sites, ACC = Antarctic Circumpolar Current, AIS = Antarctic ice-sheet, EASM = East Asian Summer monsoon, NB = North Boundary, SAF = Subantarctic Front, PF = Polar Front, PMOC = Pacific Meridional Overturning Circulation, DT = temperature gradients as in Fig. 4c, SWW = Southern Westerly Wind belt (Figure from Lamy et al., 2024).

Reference:

Lamy, F., Winckler, G., Arz, H.W. et al. Five million years of Antarctic Circumpolar Current strength variability. Nature 627, 789–796 (2024). Access the paper: 10.1038/s41586-024-07143-3

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