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Indian Ocean warming can strengthen the Atlantic meridional overturning circulation

Abstract

The slowdown of the Atlantic meridional overturning circulation (AMOC)1,2,3 and the accelerated warming of the tropical Indian Ocean (TIO)4,5,6 are two robust features projected for anthropogenic greenhouse warming, affecting both regional and global climates7,8. Here we use coupled climate simulations to investigate a previously overlooked link between the two phenomena. We demonstrate that TIO warming reduces rainfall over the tropical Atlantic by strengthening the Walker circulation and increasing atmospheric vertical stability. The resultant ocean salinity increase intensifies the AMOC as salinity anomalies are advected to northern high latitudes. In addition, TIO warming enhances westerly winds over the subpolar North Atlantic, which helps to maintain the stronger AMOC. A TIO warming of 0.1 °C above the mean warming of tropical oceans intensifies the AMOC by ~1 Sv, leading to a stronger interhemispheric asymmetry and a northward shifted ITCZ. Thus, TIO warming could delay the AMOC weakening under greenhouse warming. Indeed, we find that the AMOC weakens more strongly or completely collapses if we suppress TIO warming under the doubled and quadrupled CO2 scenarios. Simulations replicating the observed tropical ocean warming further confirm this TIO–AMOC link, suggesting that the observed TIO warming might be already playing a role in sustaining the AMOC.

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Fig. 1: TIO warming since the mid-twentieth century.
Fig. 2: The effect of changing TIO SST on the AMOC.
Fig. 3: Global climate response to TIO warming.
Fig. 4: TIO control of the AMOC.

Data availability

The NOAA ERSST v4 data set is available at http://www.esrl.noaa.gov/psd/data. The data that support the findings of this study are available from the corresponding author upon request.

Code availability

We used the NCAR Command Language (NCL)40 for all the analyses and figures in this study, which is available from https://doi.org/10.5065/D6WD3XH5.

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Acknowledgements

A.V.F. was supported by grants from NSF (OCE-1756682, OPP-1741847), the ARCHANGE project of the “Make our planet great again” programme (CNRS, France) and the Guggenheim fellowship. S.H. was supported by the Scripps Institutional Postdoctoral Fellowship Program. We also acknowledge computational support from the NSF/NCAR Yellowstone/Cheyenne Supercomputing Center.

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S.H. conceived the study, conducted the numerical simulations, performed the data analysis and led the writing of the manuscript. S.H. and A.V.F. together interpreted and explained the results and edited the manuscript.

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Correspondence to Shineng Hu.

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The authors declare no competing interests.

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Peer review information: Nature Climate Change thanks Claudia Frauen, Changhyun Yoo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Hu, S., Fedorov, A.V. Indian Ocean warming can strengthen the Atlantic meridional overturning circulation. Nat. Clim. Chang. 9, 747–751 (2019). https://doi.org/10.1038/s41558-019-0566-x

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