The equatorial Pacific zonal sea surface temperature (SST) gradient, known to be a pacemaker of global warming, has strengthened since the mid-twentieth century. However, the cause is controversial because a majority of Coupled Model Intercomparison Project Phase 5 (CMIP5) models suggest weakening of the zonal SST gradient from the past to the future. Reconciling this discrepancy is important for the climate change attribution and climate sensitivity assessment. Here we use the CMIP5 ensemble and large ensemble simulations by four climate models to show that the intensifying SST gradient observed during 1951–2010 could arise from internal climate variability. Models and members that simulate historical strengthening of the SST gradient commonly exhibit reversed future trends. Using these models as a constraint, the rate of global-mean temperature rise is amplified by 9–30%, with higher values occurring in low-emission scenarios, because internal variability has a greater impact when the externally forced response is smaller.
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COBE-SST2, ERSSTv5, ICOADS and Kaplan SST data sets are all available from the NOAA/OAR/ESRL PSD website (https://www.esrl.noaa.gov/psd/data/gridded/). Two other SST data sets compiled at the Met Office Hadley Centre are available from https://www.metoffice.gov.uk/hadobs/. The CMIP5 model output analysed in this study is available from the Earth System Grid Federation (ESGF) server (https://esgf-node.llnl.gov/search/cmip5/). The CESM Large Ensemble project simulation output can be obtained from http://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html. The MPI-ESM1.1 large ensemble data are available at the MPI Grand Ensemble project website (https://www.mpimet.mpg.de/en/grand-ensemble/). A part of the IPSL-CM6-LR and MIROC6 large ensemble simulation data can be downloaded from ESGF47,48, but the full data sets are available upon request.
The Fortran codes used for creating main plots in this study are available at https://ccsr.aori.u-tokyo.ac.jp/~hiro/sst_trends/.
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We acknowledge the modelling groups, the PCMDI and the WCRP’s WGCM for their efforts in making the CMIP5 multi-model data set available. We also thank T. Suzuki for processing CMIP5 ocean temperature data. M.W., Y.K. and H.T. were supported by Grant-in-Aid 26247079 and the Integrated Research Program for Advancing Climate Models (JPMXD0717935457) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. J.-L.D. was supported by the European Union Horizon 2020 project #820829. T.M. acknowledges funding from European Research Council grant #770765 and European Union Horizon 2020 project #820829.
The authors declare no competing interests.
Peer review information Nature Climate Change thanks Dong Eun Lee and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Watanabe, M., Dufresne, JL., Kosaka, Y. et al. Enhanced warming constrained by past trends in equatorial Pacific sea surface temperature gradient. Nat. Clim. Chang. (2020). https://doi.org/10.1038/s41558-020-00933-3