Going Steady—Study Reveals North Atlantic’s Gulf Stream Remains Robust

A study by the University of Bern and the Woods Hole Oceanographic Institution in the USA concludes that the ocean circulation in the North Atlantic, which includes the Gulf Stream, has not weakened in the past 60 years. These results contradict previous assumptions.

We owe Europe’s mild climate to the ocean circulation in the North Atlantic, which not only transports heat northwards from the equator, but also distributes oxygen and nutrients in the ocean. The collapse of this central element of the climate system would have serious consequences and has been identified by the Intergovernmental Panel on Climate Change (IPCC) as one of 15 so-called tipping elements. Once tipping points are exceeded, the system enters a new stable state and the consequences are potentially irreversible. It cannot be said with certainty whether the Atlantic Meridional Overturning Circulation (AMOC) has already weakened as a result of climate change, as direct observations have only been available for 20 years. This is why the climate research community is debating this issue intensively.

A study that has just been published in the journal Nature Communications makes a new contribution to this debate. Led by Dr. Led by Prof. Dr. Jens Terhaar from the Department of Climate and Environmental Physics at the Physics Institute of the University of Bern, the research team has developed a new methodological approach using 24 Earth system models and observations of the heat flow between the ocean and the atmosphere in the North Atlantic, and has come to surprising results.

Previous studies put into perspective – but no all-clear

“We asked ourselves how stable the AMOC is and whether it has already weakened,” says lead author Terhaar. The current study shows that there is no evidence of a weakening of the AMOC between 1963 and 2017. “Our reconstructions show considerable variability, but no clear trend can be identified,” explains the Bern-based ocean modeling specialist, who is also a member of the Oeschger Center for Climate Research at the University of Bern.

This finding puts studies that have been widely cited in the media recently into perspective, according to which the Atlantic circulation has weakened in recent decades. With regard to future climate change and its consequences, however, there is no reason to sound the all-clear, says Terhaar. Because the AMOC has been stable up to now, it is less likely that the ocean circulation will tip in the near future, but the AMOC will certainly be weakened by climate change. “However, it is still highly uncertain how great this weakening will be and what consequences can be expected in the future.”

Previous reconstructions of the strength of the Atlantic circulation in the past were based primarily on anomalies in the sea surface temperature in the North Atlantic. However, the new models now show that the AMOC cannot be reliably reconstructed using temperature anomalies. This means that conclusions drawn from reconstructions created in this way are not robust either. This is because, as other studies have already shown, temperature anomalies in the North Atlantic are influenced not only by the AMOC, but also by other processes in the ocean and atmosphere.

Improved methodology

The new reconstruction developed at the University of Bern and the Woods Hole Oceanographic Institution (WHOI) uses anomalies in the heat flow between air and sea as an indicator of changes in the AMOC instead of temperature anomalies in the North Atlantic. As the study by Terhaar and his colleagues Linus Vogt and Nicholas Foukal from the WHOI shows, these deviations in the heat flow between air and sea are closely linked to those of the AMOC. As the AMOC strengthens, it transports more heat northward, which the ocean then releases into the atmosphere. However, as the AMOC weakens, less heat is transported northward and the ocean absorbs more heat. The relationship between the two anomalies is based on the concept of heat conservation in the North Atlantic. To reconstruct the decadally averaged AMOC anomaly since 1963, the authors then combined this relationship with observation-based estimates of air-sea heat fluxes.

Although reconstructions of the Atlantic circulation based on the new method are more robust than previous ones, they too are burdened with “limitations and caveats,” the study authors emphasize. Most importantly, there are uncertainties related to estimating air-sea heat fluxes from observations and the fact that climate models do not capture all the processes that influence the AMOC. For example, the influence of fresh water that enters the ocean as a result of the melting of the Greenland and Antarctic ice sheets. This is why the uncertainty in the reconstructed circulation changes is relatively large. “A decline in the AMOC in the last 60 years,” the study concludes, “seems very unlikely.”

Expert Contact
Dr. Jens Terhaar
Institute of Physics, Department of Climate and Environmental Physics and Oeschger Centre for Climate Research, University of Bern
Phone Number: +41 31 684 85 64
Email ID: jens.terhaar@unibe.ch

Original Source: https://mediarelations.unibe.ch/medienmitteilungen/2025/medienmitteilungen_2025/atlantikzirkulation_seit_jahrzehnten_stabil/index_ger.html

Original Publication
Jens Terhaar, Linus Vogt & Nicholas P. Foukal
Journal: Nature Communications
Article Title: Atlantic overturning inferred from air-sea heat fluxes indicates no decline since the 1960s
Article Publication Date: 15 January 2025
DOI: 10.1038/s41467-024-55297-5

Media Contact
Nathalie Matter
Media/PR Editor
Phone Number: +41-31-631 45 80
Fax: +41-31-631 45 62
Email ID: nathalie.matter@kommunikation.unibe.ch

Source: IDW