Like a conveyor belt, the Atlantic Meridional Overturning Circulation (AMOC) transports warm water from the tropics northwards at the ocean surface and cold water southwards in the deep sea. Through this heat distribution, it plays a central role in the global climate system. However, increasing global warming as a result of climate change is altering the circulation patterns of the AMOC. In a recent study, researchers from the Alfred Wegener Institute found that this change has led to asynchronous poleward shifts of the Atlantic subtropical gyre in response to the “bipolar seesaw” theory since the last ice age. On top of the relatively short-term changes in the AMOC, the natural global rise in CO2 induces a long-term response of the gyres to drift poleward on glacial-interglacial timescale. Some of the regional drifts reached an amplitude of more than 6°, implying a long-way journey as if the gyre migrated from Bremerhaven to Munich and it seems they will continue its journey. This has an impact on the climate and marine ecosystems. The researchers published their findings in the journal Geophysical Research Letters.
Ocean gyres come in different sizes. Their diameter ranges from a few meters to several kilometers. Depending on their size, they have different influences on our planet's climate. They influence how oceans absorb and transport heat and carbon from the atmosphere and how nutrients are distributed. Subtropical gyres are huge ocean circulation systems connecting the tropics and the high latitudes. “Satellite observations show that the Earth's large ocean gyres are drifting poleward at a rate of about 0.1 degrees per decade. If the CO2 content in the atmosphere doubles, they could even reach 1 degree,” says Tainã Pinho, lead author of the study from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).
In order to fully understand the effects of the changes in subtropical gyres on the climate, our society and marine ecosystems, AWI-researchers used model simulations to investigate how the subtropical gyres in the North and South Atlantic have shifted since the last ice age. “Based on the relative abundance of a tiny planktonic single-celled organisms (foraminifer) together with numerical modelling results, we were able to see that the subtropical gyres have shifted poleward since the last ice age until today,” says Tainã Pinho. “The increase in atmospheric CO2 has caused this long-term shift in the past because it led to changes in oceanic and atmospheric circulation patterns.” However, the gyres have not moved poleward at the same time, instead they have drifted separately towards high latitudes over the last 22,000 years. This asymmetry in poleward migration is closely linked to the “bipolar seesaw” concept, which defines an opposite thermal response between the hemispheres. Our findings indicate that during the transition from the last ice age to the Holocene, the subtropical gyre in the South Atlantic may have begun warming Antarctica via atmospheric bridges before the North Atlantic subtropical gyre initiated similar warming of the Arctic.
The opposite thermal response pattern is known as a “bipolar seesaw” and is thought to be associated with changes in the strength of the AMOC. “We found that during the last ice age, the subtropical gyres moved towards the pole depending on the strength of the AMOC and the bipolar seesaw,” explains Tainã Pinho. In the South Atlantic, the gyre starts shifting when it is warming in the Southern Hemisphere and the AMOC is weakened. In the North Atlantic, on the other hand, the gyre began drifting polewards when the AMOC strengthened and the northern hemisphere started warming up, which is 1,500 years later than the shift in the South Atlantic. These results from the past indicate that the natural shift of the gyres in the present day is influenced by human activities. “The position of subtropical ocean gyres shapes heat transport in the Atlantic. For example, the poleward shift contributes to warming high latitudes in the context of polar amplification.”
Original publication
Pinho, T.M.L., Yang, H., Lohmann, G., Portilho‐Ramos, R.C., Chiessi, C.M., Bahr, A., Nürnberg, D., Repschläger, J., Shi, X., Tiedemann, R., Mulitza, S., 2025. Asynchronous Poleward Migration of the Atlantic Subtropical Gyres Over the Past 22,000 years. Geophys. Res. Lett. 52. https://doi.org/10.1029/2024GL111497