Der Antarktische Zirkumpolarstrom ist die größte Ozeanströmung der Welt, der immer im Uhrzeigersinn um die Antarktis herumströmt. Er transportiert Wärme sowie Nährstoffe und übernimmt dadurch eine wichtige Rolle für unser Klima. Trotz dieser Bedeutung wissen wir noch relativ wenig über sein Verhalten. Um dem Verständnis der Strömung einen Schritt näher zu kommen, haben Forschende des Alfred-Wegener-Instituts und des Leibniz-Instituts für Ostseeforschung Warnemünde aus einem Sedimentdatensatz rekonstruiert, wie der Antarktische Zirkumpolarstrom in den letzten 790.000 Jahren auf Klimaänderungen reagiert hat. Das Ergebnis: Die rekonstruierte Strömung und die Temperatur der Meeresoberfläche haben in der Vergangenheit gemeinsam reagiert. Je mehr die Temperaturen stiegen, desto stärker wurde der Antarktische Zirkumpolarstrom. Abkühlungsereignisse im nördlichen Ozean scheinen ebenfalls miteinander verbunden zu sein. Ihre Erkenntnisse veröffentlichten die Forschenden in der Fachzeitschrift Nature Communications (https://doi.org/10.1038/s41467-025-58458-2).
The Antarctic Circumpolar Current (ACC) connects the Pacific, Atlantic and Indian Oceans thus influences oceanic heat transport and nutriments around the world. Fluctuations in the strength and position of the ACC therefore have a significant impact on various elements of our world: our climate, the ocean's carbon cycle and the global meridional overturning circulation. "However, we don't yet fully understand exactly how these fluctuations affect us, partly due to the scarcity of high-resolution sediment records covering multiple glacial-interglacial cycles," says Dr Vincent Rigalleau from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). This palaeodata is urgently needed to reconstruct the behavior of the ACC in the past and to improve climate models.
That's why the AWI geologist, together with colleagues from the AWI and the Leibniz Institute for Baltic Sea Research Warnemünde (IOW), analyzed sediment cores from Chile more closely. These provide insights into the current strength and sea surface temperature of the Cape Horn Current as part of the subantarctic Antarctic Circumpolar Current system. They contain data from several glacial cycles over the last 790,000 years in the Southern Hemisphere. The researchers have reconstructed sea surface temperature and current strength variability at a site on the Chilean margin, in order to understand how the ACC in southern Chile has reacted to persistent climate variability during the last eight ice ages. "We were able to see that there is a significant correlation between the strength of the current ant the sea surface temperatures during Antarctic warming periods," explains Vincent Rigalleau. The warmer the sea surface was, the stronger the ACC current became.
An important part of the study compares the CO2 concentration in the atmosphere, as the Southern Ocean modulates the exchange of CO2 between the deep sea and the atmosphere. This is because the Antarctic circumpolar current influence how much carbon the biological carbon pump transports from the sea surface to the depths of the ocean and how much carbon comes from the depths to the surface and enters the atmosphere as CO2.
The data also show that short-term climate fluctuations in the Southern Hemisphere have the opposite effect to the Northern Hemisphere. This is the concept of the "bipolar seesaw": It states that warm phases in the Southern Ocean are accompanied by cold phases in the north - and vice versa. So while the ocean in the south was heating up, the North Atlantic became colder, which is presumably associated with changes in the Atlantic thermohaline circulation, the global conveyor belt. "Our findings align with contemporary observations of a warming and accelerating Southern Ocean, in conjunction with the widely anticipated weakening of the Atlantic Meridional Overturning Circulation in response to anthropogenic forcing." So it appears to be an interhemispheric teleconnection between Southern Ocean conditions, North Atlantic circulation, and atmospheric CO₂ concentrations.
This new data ´provides a 'global picture' of the strong atmosphere-ocean interactions that have prevailed over the past 800,000 years. "During this period, the climate has undergone major shifts shorter-term millennial-scale climate changes," summarizes Vincent Rigalleau. " These, high amplitude and partly abrupt climate fluctuations provide important information about possible climate changes in the Southern Ocean, an area which is only beginning to be affected by anthropogenic global warming and is crucial for potential future Antarctic ice-sheet instability."
Besides AWI and IOW, the following institutions were also involved in the study: MARUM Centre for Marine Environmental Sciences, Cardiff University, Universidade Federal Fluminense and Columbia University.
Original publication
Rigalleau, V., Lamy, F., Ruggieri, N. et al. 790,000 years of millennial-scale Cape Horn Current variability and interhemispheric linkages. Nat Commun 16, 3105 (2025). https://doi.org/10.1038/s41467-025-58458-2