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Walker circulation strengthening driven by sea surface temperature changes outside the tropics

Nature Geoscience volume 17, pages 858–865 (2024)Cite this article

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Abstract

The Pacific Walker circulation—the tropical Pacific zonal overturning circulation of the atmosphere—and the associated sea surface temperature distribution in the tropical Pacific significantly impact global climate. However, climate model historical simulations cannot capture the observed Walker circulation enhancement since around 1980. Although a number of mechanisms have been proposed to explain the observed change, quantitative discussion and clues for reconciling the model-observation discrepancy have not yet been settled. Here we show that the Walker circulation strengthening between 1980 and 2020 can be quantitatively explained by the remote influence of subtropical and extratropical sea surface temperature changes. This conclusion is obtained from climate model pacemaker experiments in which sea surface temperature anomalies outside the tropics are restored towards observations. Influence from the southeastern Pacific, which cools the eastern tropical Pacific, is especially crucial for the Walker circulation strengthening. This equatorward influence occurs mostly through the atmosphere and its thermal coupling with the ocean. We further show that current generation climate models have biases in southeastern Pacific surface temperature changes, which may cause the failure in reproducing the Walker circulation trend. Our results suggest that improved representation of air–sea coupling in this region could enable better projections of future climate.

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Fig. 1: Observed and simulated changes in SST, SLP and the PWC indices.
Fig. 2: Atmospheric and air–sea thermal coupling processes of the subtropical influence on the tropical Pacific.
Fig. 3: Evaluation of the pacemaker effects by the North Pacific, subtropical South Pacific and tropical Indo-Atlantic oceans.
Fig. 4: Comparison of the observed and simulated SST trends in selected regions.
Fig. 5: Influence of prescribed SST variations over the tropical Indo-Atlantic oceans.

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Data availability

The ERSSTv5 and COBE-SST2 datasets are available from the NOAA/OAR/ESRL PSD website (https://www.esrl.noaa.gov/psd/data/gridded/). The HadISST1 dataset is available from the Met Office Hadley Centre observations datasets (https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html). The JRA-55 reanalysis data can be downloaded from the Japan Meteorological Agency website at https://jra.kishou.go.jp/JRA-55/index_en.html and ERA5 reanalysis data are available from the Copernicus Climate Service at https://cds.climate.copernicus.eu/. The CMIP6 model outputs analysed in this study are available from the Earth System Grid Federation (ESGF) server (https://esgf-node.llnl.gov/search/cmip6/). The data archiving of the fully coupled experiments, the pacemaker experiments, the slab ocean experiments and the atmospheric model experiments for the main figures is available at https://doi.org/10.5281/zenodo.12749255 (ref. 57). You can also access the archives of CMIP6 and observational data, which have undergone regridding and trend processing for plotting, from the same link. Further data and codes are availabe upon reasonable request to the first author, Masaki Toda (masaki.toda@mpimet.mpg.de). Source data are provided with this paper.

Code availability

The archiving of Python codes used for creating the main figures is available at https://doi.org/10.5281/zenodo.12749255 (ref. 57).

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Acknowledgements

We thank T. Kataoka for the instruction to conduct XTOGA15 with MIROC6. This study is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through the Program for Advanced Studies of Climate Change Projection (SENTAN) Grant number JPMXD0722680395, and by the Japan Society for the Promotion of Science through Grants-in-Aid for Scientific Research JP19H05703, JP23H01241, JP24H02223 and JP23H01250.

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Authors and Affiliations

  1. Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo, Japan

    Masaki Toda & Yu Kosaka

  2. Max-Planck Institute for Meteorology, Hamburg, Germany

    Masaki Toda

  3. Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA

    Ayumu Miyamoto

  4. Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan

    Masahiro Watanabe

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  1. Masaki Toda
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Contributions

M.T. designed the research based on the initial idea by M.W., carried out the experiments, analysed the data, and wrote the paper through discussion with Y.K. and A.M. Y.K. and A.M. prepared an experimental environment and conducted part of the experiments. All authors discussed the results and commented on the paper.

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Correspondence to Masaki Toda.

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Nature Geoscience thanks Michelle L’Heureux and Xiyue Zhang for their contribution to the peer review of this work. Primary Handling Editor: Tom Richardson, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Observed and simulated Pacific Walker circulation index trends for 1980-2014.

Trend of the sea level pressure Pacific Walker circulation index for 1980-2014 in the series of experiments (left) and in the Coupled Model Intercomparison Projects Phase 6 (CMIP6) multi-model ensemble (right). Boxes and whiskers respectively indicate (left) ±0.67σ and ±1.645σ ranges (where σ denotes one s.d.), corresponding to 50% and 90% ranges of a normal distribution, and (right) the interquartile and 5th-95th percentile ranges. The model experiments include 20 members for all scenarios except HIST, AMIP, and POGA, which have 40 members, 10 members, and 10 members, respectively. For the CMIP6 analysis, 242 members are used.

Source data

Extended Data Fig. 2 The restoration region and buffer zones for pacemaker experiments.

The restoration region (pink) and buffer zones (sky-blue) for each pacemaker experiment as indicated: XTOGA15 (a); XTOGA25 (b); XPOGA15 (c); XPOGA25 (d); XNPTOGA15 (e); POGA (f); SPOGA (g). Sea surface temperature (SST) in the pink-colored region is restored toward the model climatology plus the observed anomaly. In pacemaker experiments by CM2.1, the observed SST anomaly is calculated based on ERSSTv5 (except POGA which uses ERSST version 3b extended by ERSSTv5), while in XTOGA15 by MIROC6, SST anomaly is based on COBE-SST2. See Methods for the detailed explanation of the restoration region and buffer zones. The red rectangles in 2ab represent the reference regions of Fig. 4a.

Extended Data Fig. 3 Trends of the 10 m-level zonal wind velocity and vertical velocity anomalies at 500 hPa Pacific Walker circulation indices for 1980-2020.

Trend of the 10 m-level zonal wind velocity (u10) and the vertical velocity anomalies at 500 hPa (ω500) Pacific Walker circulation index for 1980-2020 in the series of experiments (a) and in the CMIP6 multi-model ensemble (b). Boxes and whiskers respectively indicate ±0.65σ and ±1.645σ ranges (where σ denotes one s.d.), corresponding to 50% and 90% ranges of a normal distribution. The model experiments include 20 members for all scenarios except HIST, AMIP, and POGA, which have 40 members, 10 members, and 10 members, respectively.

Source data

Extended Data Fig. 4 Spatial trend patterns of surface zonal wind velocity and mid-tropospheric vertical velocity.

(a,c,e) The horizontal distribution of 10 m-level zonal wind velocity (u10) trend for 1980-2020 in (a) ERA5 and CM2.1 ensemble means of (c) HIST and XTOGA15. (b,d,f) As in (a,c,e), but for vertical velocity anomalies at 500 hPa (ω500) (positive if downward). The green boxes in (a) and (b) are the reference region for the u10 and ω500 Pacific Walker circulation indices.

Extended Data Fig. 5 Annual mean sea surface temperature forcing prescribed to aXTOGA15 and sXTOGA15.

The 1980-2020 sea surface temperature (SST) trend difference between XTOGA15 and HIST scaled to 100 yr changes. In aXTOGA15, this difference, weighted in the same manner as in XTOGA15 restoring, is superposed on HIST climatology. In sXTOGA15, SST is restored toward this difference plus HIST climatology with the same weighting as in XTOGA15.

Extended Data Fig. 6 Surface heat flux and surface atmospheric circulation responses obtained by atmosphere-only experiments.

(a-c) Maps of (a) net (turbulent and radiation) heat flux at surface, (b) latent heat flux (color) and scalar wind speed at the 10 m level (contour), (c) surface short wave radiation flux (color) and low-level cloud cover (contour) obtained as aXTOGA15 minus aCLM. Downward flux is positive. Contours are drawn (b) from -0.3 to 1.5 W m–2 with intervals of 0.3 and (c) from 0 to 24 % with intervals of 8. Hatching indicates that the difference is not significant (p > 0.05) for each surface flux. The gray solid lines and dot lines represent 15°N or S and 10°N or S, respectively. The region between a solid line and a dotted line corresponds to the buffer zone indicated in Extended Data Fig. 2a.

Extended Data Fig. 7 Sea surface temperature and sea level pressure trends for 1980-2020 induced by the tropical Pacific SST changes with and without radiative forcing.

As in Fig. 1e,f but for POGA (a) and POGA minus HIST (b). The contours are drawn from -5.6 to 8.4 hPa at every 1.4 hPa interval.

Extended Data Fig. 8 Observed and simulated sea surface temperature trends in the tropical Indian and Atlantic Oceans.

The sea surface temperature trends in the tropical Indian Ocean (15°S-15°N, 40°-100°E), Atlantic Ocean (5°S-15°N, 50°W-0°E) and their average for 1980-2020. Crosses and stars indicate ensemble mean trends of individual models, and numbered makers represent observational datasets. White lines represent multi-model average of individual model ensemble means, and boxes and whiskers indicate ±0.67σ and ±1.645σ ranges, respectively. Models with 7 or more ensemble members are used for this analysis. The standard deviation was calculated from the values of 11 models.

Source data

Supplementary information

Supplementary Information

Supplementary Tables 1–3.

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Source Data Fig. 1

Source data for Fig. 1.

Source Data Fig. 3

Source data for Fig. 3.

Source Data Fig. 4

Source data for Fig. 4.

Source Data Extended Data Fig. 1

Source data for Extended Data Fig. 1.

Source Data Extended Data Fig. 3

Source data for Extended Data Fig. 3.

Source Data Extended Data Fig. 8

Source data for Extended Data Fig. 8.

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Toda, M., Kosaka, Y., Miyamoto, A. et al. Walker circulation strengthening driven by sea surface temperature changes outside the tropics. Nat. Geosci. 17, 858–865 (2024). https://doi.org/10.1038/s41561-024-01510-5

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