Despite global warming, a region in the North Atlantic ocean has been observed to cool, a phenomenon known as the warming hole. Its emergence has been linked to a slowdown of the Atlantic meridional overturning circulation, which leads to a reduced ocean heat transport into the warming hole region. Here we show that, in addition to the reduced low-latitude heat import, increased ocean heat transport out of the region into higher latitudes and a shortwave cloud feedback dominate the formation and temporal evolution of the warming hole under greenhouse gas forcing. In climate model simulations of the historical period, the low-latitude Atlantic meridional overturning circulation decline does not emerge from natural variability, whereas the accelerating heat transport to higher latitudes is clearly attributable to anthropogenic forcing. Both the overturning and the gyre circulation contribute to the increased high-latitude ocean heat transport, and therefore are critical to understand the past and future evolutions of the warming hole.
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HadCRUT4 data were provided by the UK Met Office Hadley Centre (http://www.met-office.gov.uk/hadobs/hadcrut4/), as well as HadISST data (https://www.metoffice.gov.uk/hadobs/hadisst/). Data from the RAPID-WATCH MOC monitoring project are freely available from www.rapid.ac.uk/rapidmoc funded by the Natural Environment Research Council. The Grand Ensemble is publicly available at ESGF (https://esgf-data.dkrz.de/projects/esgf-dkrz/). The special simulations are available on request from the corresponding author.
An archive with the bash, python and NCL scripts used to conduct the calculations that underlie this study and reproduce the figures is archived by the Max Planck Institute for Meteorology and can be accessed from the public repository of the Max Planck Society, https://pure.mpg.de/pubman/faces/ViewItemOverviewPage.jsp?itemId=item_3213979.
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The study benefited from comments by B. Stevens, H. Haak and C. Li. T.M. received funding from the European Research Council (ERC) Consolidator Grant 770765 and the European Union’s Horizon 2020 Program Grant agreement 820829. Computational resources were made available by Deutsches Klimarechenzentrum (DKRZ) through support from the Bundesministerium für Bildung und Forschung (BMBF) and by the Swiss National Supercomputing Centre (CSCS). J.J. acknowledges support through BMBF under grants 01LP1517B (MiKliP) and 03F0729D (RACE).
The authors declare no competing interests.
Peer review information Nature Climate Change thanks Melissa Gervais and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Keil, P., Mauritsen, T., Jungclaus, J. et al. Multiple drivers of the North Atlantic warming hole. Nat. Clim. Chang. 10, 667–671 (2020). https://doi.org/10.1038/s41558-020-0819-8