The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents in the North Atlantic with a major impact on climate. Its evolution during the industrial era is poorly known owing to the lack of direct current measurements.
Through climate modelling and sea surface temperature (SST) measurements, this study provides evidence for a 15 % weakening of the AMOC since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal SST fingerprint, consisting of a cooling in the subpolar gyre region due to reduced heat transport, and a warming in the Gulf Stream region due to a north-ward shift of the Gulf Stream.
The authors state that although long-term natural variations cannot be ruled out entirely, the AMOC decline since the 1950s is very likely to be largely anthropogenic, given that it is a feature predicted by climate models in response to CO2 levels.
It is highlighted that the AMOC weakening may already have an impact on weather in Europe and could become the main cause of future west European atmospheric circulation changes and potentially lead to increased storminess.
It is stated that continued global warming is likely to further weaken the AMOC in the long term. The authors conclude that given that the AMOC is one of the well documented tipping elements of the climate system, it is of considerable concern that the proximity of the Atlantic to this threshold is still poorly known.
Caesar L, Rahmstorf S, Robinson A, Feulner G, Saba V (2018) Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature 556:191-196.
Note: Another paper on the same topic was published in the same volume of the scientific journal “Nature” (Thornalley et al. 2018). It is titled: “Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years”. It shows further palaeo-oceanographic evidence that the Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years, compared with the preceding 1,500 years. (https://www.nature.com/articles/s41586-018-0007-4)