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Strengthening tropical Pacific zonal sea surface temperature gradient consistent with rising greenhouse gases

Abstract

As exemplified by El Niño, the tropical Pacific Ocean strongly influences regional climates and their variability worldwide1,2,3. It also regulates the rate of global temperature rise in response to rising GHGs4. The tropical Pacific Ocean response to rising GHGs impacts all of the world’s population. State-of-the-art climate models predict that rising GHGs reduce the west-to-east warm-to-cool sea surface temperature gradient across the equatorial Pacific5. In nature, however, the gradient has strengthened in recent decades as GHG concentrations have risen sharply5. This stark discrepancy between models and observations has troubled the climate research community for two decades. Here, by returning to the fundamental dynamics and thermodynamics of the tropical ocean–atmosphere system, and avoiding sources of model bias, we show that a parsimonious formulation of tropical Pacific dynamics yields a response that is consistent with observations and attributable to rising GHGs. We use the same dynamics to show that the erroneous warming in state-of-the-art models is a consequence of the cold bias of their equatorial cold tongues. The failure of state-of-the-art models to capture the correct response introduces critical error into their projections of climate change in the many regions sensitive to tropical Pacific sea surface temperatures.

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Data availability

All data used or analysed in this study, or generated by other groups and organizations, are publicly available at the links provided in the Methods. Data from our model simulations are available at http://kage.ldeo.columbia.edu:81/SOURCES/.LDEO/.ClimateGroup/.PROJECTS/.PublicationsData/.Seager_etal_NCC-2019/.

Code availability

The Python code for the atmosphere model is in a Juypter Notebook and is available on request. The ocean model code is built on legacy Fortran 90 and C code, and a TAR file of the source code can be made available on request.

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Acknowledgements

This work was supported by NSF award OCE 1657209 and a grant from World Surf League P.U.R.E. through Columbia University’s Center for Climate and Life. We thank R. Miller, B. Fox-Kemper, T. Shepherd, R. Chadwick, J. Smerdon, P. Williams and I. Simpson for useful discussions.

Author information

R.S. conceived of the study and directed the research. All authors designed the experiments. R.S. and M.C. designed the atmosphere and ocean thermodynamic models. N.H. and M.C. designed the numerical methods for solution, with assistance from R.S. N.H. wrote the new model codes, implemented the ocean model and conducted the modelling. D.-E.L. and N.H. led the analysis of ocean and atmosphere data. D.-E.L. conducted experiments with the ocean model to interpret the results. R.S. wrote the paper, with all authors contributing advice on content and wording.

Correspondence to Richard Seager.

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The authors declare no competing interests.

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

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Fig. 1: SST trends over 1958–2017.
Fig. 2: Atmosphere trends over 1958–2017.
Fig. 3: Results from the coupled atmosphere and ocean model simulations.
Fig. 4: Trends in thermocline depth (20 °C isotherm) over 1958–2017.
Fig. 5: Coupled model trends over 1958–2017, and attribution of erroneous trends in CMIP5 models to model bias.