A study by the University of Bern and the Woods Hole Oceanographic Institution in the USA concludes that ocean circulation in the North Atlantic, which includes the Gulf Stream, has not weakened over the past 60 years. These results contradict previous assumptions.
We owe the mild climate in Europe 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. If tipping points are exceeded, the system enters a new stable state and the consequences are potentially irreversible. It is not possible to say 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 discussing the issue intensively.
A study that has just been published in the journal Nature Communications provides a new contribution to this debate. Led by 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 atmosphere in the North Atlantic and has come to some surprising conclusions.
Earlier 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. ’Although our reconstructions show considerable variability, there is no clear trend,’ explains the Bernese specialist in ocean modeling, who is also a member of the Oeschger Center for Climate Research at the University of Bern.
This finding puts into perspective studies that have recently been widely quoted by the media, according to which the Atlantic circulation has weakened in recent decades. With a view to future climate change and its consequences, however, the all-clear is not appropriate, says Terhaar. The fact that the AMOC has been stable up to now makes it less likely that the ocean circulation will tilt in the near future, but the AMOC will be influenced by climate change.
security weakened. ’However, it is still highly uncertain how great this weakening will be and what consequences must be expected in the future.
Previous reconstructions of the strength of the Atlantic circulation in the past were mainly based on sea surface temperature anomalies in the North Atlantic. However, the new modeling now shows that the AMOC cannot be reliably reconstructed with the help of temperature anomalies. This means that conclusions drawn from such reconstructions are also not robust. This is because, as other studies have already shown, temperature anomalies in the North Atlantic are not only influenced 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 flux between air and sea instead of temperature anomalies in the North Atlantic as an indicator of changes in the AMOC. 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. When the AMOC becomes stronger, it transports more heat northwards, which the ocean then releases into the atmosphere. However, if the AMOC weakens, less heat is transported northwards 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 observational estimates of air-sea heat fluxes.
Although reconstructions of the Atlantic circulation based on the new method are more robust than the previous ones, they are also subject to ’limitations and caveats’, as the study authors emphasize. The most important of these are the uncertainties regarding the estimation of heat fluxes between air and sea based on observations and the fact that the climate models do not represent all processes that influence the AMOC. For example, the influence of freshwater that enters the ocean due to the melting of the Greenland and Antarctic ice sheets. The uncertainty in the reconstructed circulation changes is therefore relatively high. ’However, a decline in the AMOC in the last 60 years’, the study concludes, ’seems very unlikely’.
