Importance of density-compensated temperature change for deep North Atlantic Ocean heat uptake

Abstract

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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Figure 1: Heat content, salt and mass anomalies in the extratropical North Atlantic between 1955 and 2005.
Figure 2: Property anomalies, averaged vertically and zonally, within the NADW as a function of time and latitude.
Figure 3: Time series of QHC changes (J) within the four main water masses of the STG.
Figure 4: Maps of depth-average temperature anomalies within UNADW and LNADW for selected 5-year periods.

Change history

  • 22 November 2012

    In the version of this Article originally published online, the y axis labels in Fig. 1a–d were incorrect. The labels are now correct in all versions of the Article.

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Acknowledgements

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.

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C.M. led the study, and performed with A.M. and R.T.S. the analysis and interpretation. A.M. formulated the density space approach and the error analysis, and wrote the Supplementary Information. C.M. and R.T.S. jointly wrote the paper. All authors discussed the results and made comments on the manuscript.

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Correspondence to C. Mauritzen.

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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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