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Strong remote control of future equatorial warming by off-equatorial forcing

Abstract

The tropical climate response to GHG forcing is spatially non-uniform1,2,3. Even though enhanced equatorial and eastern Pacific warming is simulated by most climate models, the underlying mechanisms—including the relative roles of atmospheric and oceanic feedbacks—remain debated. Here, we use a climate model with idealized CO2-radiative forcing patterns to show that off-equatorial radiative forcing and corresponding coupled circulation/cloud adjustments are responsible for much of equatorial warming in response to global CO2 forcing. For equatorial forcing, the atmosphere responds by enhancing atmospheric heat export to the extra-tropics, an associated strengthening of the ascending Hadley circulation branch and strong negative equatorial cloud feedbacks. These processes together greatly dampen equatorial surface warming. Intensification of the oceanic subtropical cells and increased cold subsurface water upwelling in the eastern tropical Pacific provide an additional negative feedback for surface temperatures. In contrast, applying off-equatorial forcing, the atmosphere responds by exporting less heat from the tropics, Hadley circulation weakening and weaker negative equatorial cloud feedbacks, while the subtropical cells slow down in the ocean. Our results demonstrate a delicate balance in the coupled climate system between remote circulation adjustments and regional feedbacks that create the patterns of future climate change.

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Fig. 1: Surface temperature and precipitation response to CO2 forcing.
Fig. 2: Ensemble-mean temperature, meridional wind, vertical pressure velocity and cloud radiative effect (CRE) responses to CO2 forcing.
Fig. 3: Ocean response to CO2 forcing.
Fig. 4: Feedback mechanism.

Data availability

The data from the regionally forced model simulations are available on the IBS Center for Climate Physics climate data server (https://climatedata.ibs.re.kr/).

Code availability

The CESM source code can be obtained from http://www.cesm.ucar.edu/models/cesm1.2/ and the code modifications43 that allow prescribing spatially varying CO2 concentrations can be obtained from https://github.com/stuecker/regionalCO2/.

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Acknowledgements

M.F.S., A.T., K.-S.Y., J.-E.C. and E.-S.C. were supported by the Institute for Basic Science (project code IBS-R028-D1). F.-F.J. was supported by NSF grant no. AGS?1813611 and DOE grant no. DE-SC0005110. C.M.B. was supported by NOAA grant no. CPO NA115OAR4310161. C.P. was supported by a JISAO postdoctoral fellowship. K.C.A. was supported by NSF grant no. AGS-1752796. S.M.K. and D.K. were supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (grant no. 2016R1A1A3A04005520). M.H. was supported by JSPS Overseas Research Fellowships (no. 201860671). Computing resources were provided by University of Southern California’s Center for High-Performance Computing. M.F.S. thanks S.-P. Xie and A. N. Babu for valuable discussions.

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M.F.S. designed the study and wrote the initial manuscript draft. M.F.S. and D.K. conducted the model experiments and performed the analysis. All authors contributed to the interpretation of the results and to the improvement of the manuscript.

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Correspondence to Malte F. Stuecker.

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Supplementary Information

Supplementary methods, Tables 1 and 2, Figs. 1–10 and refs. 1 and 2.

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Stuecker, M.F., Timmermann, A., Jin, FF. et al. Strong remote control of future equatorial warming by off-equatorial forcing. Nat. Clim. Chang. 10, 124–129 (2020). https://doi.org/10.1038/s41558-019-0667-6

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