The efficiency with which the oceans take up heat has a significant influence on the rate of global warming. Warming of the ocean above 700 m over the past few decades has been well documented. However, most of the ocean lies below 700 m. Here we analyse observations of heat uptake into the deep North Atlantic. We find that the extratropical North Atlantic as a whole warmed by 1.45±0.5×1022 J between 1955 and 2005, but Lower North Atlantic Deep Water cooled, most likely as an adjustment from an early twentieth-century warm period. In contrast, the heat content of Upper North Atlantic Deep Water exhibited strong decadal variability. We demonstrate and quantify the importance of density-compensated temperature anomalies for long-term heat uptake into the deep North Atlantic. These anomalies form in the subpolar gyre and propagate equatorwards. High salinity in the subpolar gyre is a key requirement for this mechanism. In the past 50 years, suitable conditions have occurred only twice: first during the 1960s and again during the past decade. We conclude that heat uptake through density-compensated temperature anomalies will contribute to deep ocean heat uptake in the near term. In the longer term, the importance of this mechanism will be determined by competition between the multiple processes that influence subpolar gyre salinity in a changing climate.
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C.M. and A.M. acknowledge the support of the Norwegian Research Council through iAOOS-Norway (grant number 176096) and the European Union through its 6th Framework Programme Integrated Project DAMOCLES. R.T.S. acknowledges the support of the UK National Centre for Atmospheric Science and the Natural Environment Research Council. We thank J. Gregory and R. Tailleux for valuable comments on the manuscript. We thank R. G. Curry for providing the new Atlantic Ocean analyses, and for valuable assistance with the data analysis at the initial stages of the project.
The authors declare no competing financial interests.
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Mauritzen, C., Melsom, A. & Sutton, R. Importance of density-compensated temperature change for deep North Atlantic Ocean heat uptake. Nature Geosci 5, 905–910 (2012). https://doi.org/10.1038/ngeo1639
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