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Global surface warming enhanced by weak Atlantic overturning circulation

Nature volume 559, pages 387–391 (2018)Cite this article

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Abstract

Evidence from palaeoclimatology suggests that abrupt Northern Hemisphere cold events are linked to weakening of the Atlantic Meridional Overturning Circulation (AMOC)1, potentially by excess inputs of fresh water2. But these insights—often derived from model runs under preindustrial conditions—may not apply to the modern era with our rapid emissions of greenhouse gases. If they do, then a weakened AMOC, as in 1975–1998, should have led to Northern Hemisphere cooling. Here we show that, instead, the AMOC minimum was a period of rapid surface warming. More generally, in the presence of greenhouse-gas heating, the AMOC’s dominant role changed from transporting surface heat northwards, warming Europe and North America, to storing heat in the deeper Atlantic, buffering surface warming for the planet as a whole. During an accelerating phase from the mid-1990s to the early 2000s, the AMOC stored about half of excess heat globally, contributing to the global-warming slowdown. By contrast, since mooring observations began3,4,5 in 2004, the AMOC and oceanic heat uptake have weakened. Our results, based on several independent indices, show that AMOC changes since the 1940s are best explained by multidecadal variability6, rather than an anthropogenically forced trend. Leading indicators in the subpolar North Atlantic today suggest that the current AMOC decline is ending. We expect a prolonged AMOC minimum, probably lasting about two decades. If prior patterns hold, the resulting low levels of oceanic heat uptake will manifest as a period of rapid global surface warming.

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Fig. 1: Quantifying the global heat budget and the partition among ocean basins in the two periods 2000–2004 and 2005–2014.
Fig. 2: The OHC linear trend in the Atlantic basin.
Fig. 3: AMOC and GSTA variations.
Fig. 4: Contrasting thermosteric SSH* patterns for increasing and decreasing AMOC.

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Acknowledgements

The research of K.-K.T. is supported by the National Science Foundation, under AGS-1262231 and by the Frederic and Julia Wan Endowed Professorship. X.C. was supported by the National Key Basic Research Program of China under grant 2015CB953900 and by the Natural Science Foundation of China under grants 41330960 and 41776032.

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Authors and Affiliations

  1. Physical Oceanography Laboratory, CIMST, Ocean University of China, and Qingdao National Laboratory of Marine Science and Technology, Qingdao, China

    Xianyao Chen

  2. Department of Applied Mathematics, University of Washington, Seattle, WA, USA

    Ka-Kit Tung

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Contributions

K.-K.T. and X.C. undertook the analysis of global ocean temperature and salinity profiles, RAPID observations, and satellite altimetry datasets. K.-K.T. led the draft of this manuscript. X.C. produced all figures. Both authors contributed substantially to the drafting and revision of this manuscript.

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Correspondence to Ka-Kit Tung.

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Extended data figures and tables

Extended Data Fig. 1 Unfiltered AMOC proxy time series in monthly resolution.

The thick solid lines are 13-month running means. The numbers to the right of each time series show the correlation coefficient with the unfiltered AMOC subsurface temperature fingerprint of Zhang. Data are taken from refs 20,21,22. All of the correlation coefficients are above 95% confidence level. The accumulated sea-level index is shifted to the right by 4.8 years in this figure. Without the time shift, its correlation with the AMOC proxy is practically zero (r = 0.06).

Extended Data Fig. 2 Error bars for the three salinity time series shown in Fig. 1.

The colour lines are monthly values of uncertainty, superimposed on the 13-month means of the time series. psu, practical salinity units.

Source_Data

Extended Data Fig. 3 Coincidence of the three AMOC phases with global warming slowdown and acceleration.

a, Global mean surface temperature. b, OHC north of 45° N in the Atlantic. c, Salinity north of 45° N in the Atlantic.

Extended Data Fig. 4 Deep Labrador Sea density:

Average density in the 1,000–1,500 m layer of the Labrador Sea, regionally averaged over the ocean area shown in the inset, from the three data sources given. A leading signal for stronger AMOC is the increased deep Labrador Sea salinity (and hence density). The signal propagates southward along the western boundary at depth, changing the cross-basin zonal gradient, and hence the geostrophic southward velocity13. The return flow then strengthens the upper branch of AMOC with a lag of 7–10 years15,16.

Source_Data

Extended Data Fig. 5 SST patterns during different AMOC phases.

a, When AMOC is below climatology. b, When AMOC is above climatology, SST detrended. c, SST linear trend when AMOC is increasing. d, When AMOC is decreasing.

Extended Data Fig. 6 Linear trends, from 1950 to 2017, of temperature, salinity and density.

a–c, Trends in temperature (a), salinity (b) and density (c) as a function of depth. Solid curves indicate where the trend is statistically significant at 95% confidence level.

Source_Data

Extended Data Fig. 7 Temperature–salinity diagram.

The subpolar Atlantic Ocean (45°–65° N) for each depth between 300 m and 1,500 m for the two periods, with the mean of 2000–2016 in red and the mean of 1920–1940 in blue. The dots shown are the five winter month values (NDJFM). At these depths the seasonal cycle is very small38.

Source_Data

Source_Data

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Chen, X., Tung, KK. Global surface warming enhanced by weak Atlantic overturning circulation. Nature 559, 387–391 (2018). https://doi.org/10.1038/s41586-018-0320-y

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