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Collapsed upwelling projected to weaken ENSO under sustained warming beyond the twenty-first century

Abstract

The El Niño–Southern Oscillation (ENSO) in a warming climate has been studied extensively, but the response beyond 2100 has received little attention. Here, using long-term model simulations, we find that while ENSO variability exhibits diverse changes in the short term, there is a robust reduction in ENSO variability by 2300. Continued warming beyond 2100 pushes sea surface temperature above the convective threshold over the eastern Pacific, causing collapsed mean equatorial upwelling with intensified deep convection. We show that the weakened thermocline feedback due to the collapsed upwelling and increased thermal expansion coefficient, along with enhanced thermodynamic damping, are crucial to reducing ENSO amplitude under sustained warming. Our results suggest a threshold behaviour in the tropical Pacific, where a convective atmosphere over the eastern equatorial Pacific causes dramatic shifts in ENSO variability. This threshold is not crossed under low-emission scenarios.

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Fig. 1: ENSO amplitude changes under sustained global warming.
Fig. 2: Mean state changes in the tropical Pacific.
Fig. 3: Eastern equatorial Pacific mean state changes.
Fig. 4: ENSO-related air–sea feedback changes.
Fig. 5: Changes in subsurface temperature anomalies due to the increased thermal expansion coefficient.
Fig. 6: Reduced ENSO variability under sustained global warming.

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Data availability

All data supporting the findings of this study are openly available. The CMIP6 data can be found at https://esgf-data.dkrz.de/search/cmip6-dkrz/. The CMIP5 data can be found at https://esgf-node.llnl.gov/search/cmip5/.

Code availability

The code is publicly available via Zenodo at https://doi.org/10.5281/zenodo.11416550 (ref. 35).

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Acknowledgements

Q.P. and S.-P.X. are supported by the National Science Foundation (AGS 1637450). The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation under Cooperative Agreement No. 1852977. We acknowledge high-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation. We thank M. T. Luongo for his help with grammar corrections and suggestions.

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Q.P. and S.-P.X. designed the study. Q.P., S.-P.X. and C.D. carried out the analysis. Q.P. wrote the first draft. S.-P.X. and C.D. contributed to writing and editing the manuscript.

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Correspondence to Shang-Ping Xie.

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Extended data

Extended Data Fig. 1 The projected ENSO variability and mean state changes.

(a) Histograms of 10,000 realizations of the Bootstrap method for Niño 3.4 SST standard deviation (s.d.) (°C) in the 20th century (1941-1990, blue) and 23rd century (2241-2290, red). The blue and red lines indicate the mean values of the 10,000 realizations for each period. The grey shaded areas correspond to the respective one s.d. of the 10,000 realizations (see Method). (b) The difference in s.d. (°C) of Niño 3.4 SST anomalies between the 21st-century SSP585/RCP8.5 scenario period (2041-2090) and the historical reference period (1941-1990). (c) The difference in equatorial Pacific zonal SST gradient between the 23rd (2241-2290) and 20th (1941-1990) centuries under the SSP585/RCP8.5 scenario. Here, zonal SST gradient change is defined as the SST change difference between the eastern (120°W-170°W, 5°S–5°N) and western (150°E-170°W, 5°S–5°N) boxes, with positive values indicating an El Niño-like warming pattern. (d) Latitude-time Hovmöller diagrams of the eastern Pacific (90°W-170°W) zonal mean monthly climatological wind (vectors; m/s) and the \(-v\frac{\partial u}{\partial y}\) term (color shading; m/s2) during 2241-2290.

Extended Data Fig. 2 The ENSO simulations in the 16 climate models.

ap, The simulated spatial pattern of present-day (1941-1990) SSTA s.d. (°C, color shading) from the 15 climate models used in this study.

Extended Data Fig. 3 The simulated seasonal cycle of ENSO amplitude.

ao, The simulated seasonal cycle of the Niño 3.4 SSTA s.d. from the 15 models with reduced ENSO variability during the historical period (1941-1990) (blue line) and the 23rd-century (2241-2290) (red line) under the SSP585/RCP8.5 scenario.

Extended Data Fig. 4 Time variation of simulated ENSO amplitude from ACCESS-ESM1-5 large ensembles.

The running 50-year ENSO amplitude change (°C) from ACCESS-ESM1-5 large ensembles during the historical period and under the (a) SSP585 and (b) SSP126 emission scenarios. The ACCESS-ESM1-5 ensemble mean is shown as the thick red curve.

Extended Data Fig. 5 ENSO-related air-sea feedback changes for each model.

aj, The strength of ENSO-related air-sea feedback changes (yr-1) (see Methods) in the eastern Pacific (5°S–5°N, 90°W–170°W) from each of the ten models with a positive Niño 3 skewness. TH, EK, ZAF, and TD represent thermocline feedback, Ekman feedback, zonal advective feedback, and thermodynamic damping, respectively. The CNRM-CM5, CCSM4, and GISS-E2-R belong to CMIP5, while the rest are from CMIP6.

Extended Data Fig. 6 Impacts of thermal expansion coefficient changes.

(a) Scatter plots of eastern Pacific (5°S–5°N, 90°W–170°W) averaged thermal expansion coefficient changes (Δα, 10-4°C-1) and Niño 3.4 SST s.d. changes from SSP585/RCP8.5 extended simulations. The running 10-year upper 500 m averaged (b) α (10-4°C-1) from the historical and SSP585/RCP8.5 outputs. (c) Regression of upper 500 m ocean temperature (°C) against steric height (SH, mm) anomalies averaged in the eastern Pacific Ocean from CMIP6 models. (d) The impacts of Δα on the regression of upper 500 m ocean temperature against SH anomalies (see Methods). The MME is shown as the thick red curve and the color shadings indicate one inter-member standard deviation (n = 10) above and below the MME. The regression coefficient is calculated over a 10-year moving window, with a one-year shift forward starting from 1850 to 2300. The results in (b)-(d) are derived from the ten models exhibiting positive Niño 3 skewness (see Methods).

Extended Data Fig. 7 Thermodynamic response changes to Niño 3.4 SST variability.

Projected changes in (a) Qnet (W/m2), (b) latent heat flux (W/m2), (c) shortwave radiation (W/m2), and (d) rainfall (mm/day) response to Niño 3.4 SST anomalies under SSP585 during 2241-2290 relative to the present-day (1941-1990). These responses are estimated using linear regression between variables and Niño 3.4 SST anomalies from the ten climate models with positive Niño 3 skewness. The stippled areas denote signals that are significant at the 95% confidence level from the bootstrap test.

Extended Data Fig. 8 ENSO-related air-sea feedback changes among models with positive and negative Niño 3 skewness.

af, The MME (bars) ENSO-related air-sea feedback (yr-1) in the eastern Pacific (5°S–5°N, 90°W–170°W) during the present-day (1941-1990) (left panels), future (2241-2290) (middle panels), and their difference (right panels) across models with positive (upper panels; n = 10) and negative (lower panels; n = 4) Niño 3 skewness (see methods). The dots indicate individual model results.

Extended Data Fig. 9 ENSO variation changes under SSP126/RCP2.6.

(a) The MME (thick red curve) running 50-year ENSO variation change (°C) from the historical and SSP126/RCP2.6 outputs; the color shadings indicate one inter-member standard deviation above and below the MME (n = 14; Supplementary Table 1). (b) Future (2241-2290) mean state relative SST (°C, color shading), rainfall (contours with an interval of 3 mm/day; positive in green), and wind stress (N/m2, vectors) under SSP126/RCP2.6. (c) The Hovmöller diagram of the equatorial mean upwelling at 60 m (w; 10-5 m/s, color shading; derived from models with direct vertical velocity outputs), and the zonal wind stress (contours with an interval of 0.01 N/m2; positive in black and negative in gray). (d) The MME ENSO-related air-sea feedbacks (yr-1) during 2241-2290 under SSP126/RCP2.6 (red bars) from seven climate models (Supplementary Table 1), along with the differences between these air-sea feedbacks and their present-day counterparts (blue bars); The dots indicate individual model results.

Extended Data Fig. 10 ENSO amplitude changes under SSP245/RCP4.5.

(a) The running 50-year ENSO variation change (°C) from the historical and SSP245/RCP4.5 outputs. (b) Future (2241-2290) annual mean relative SST (°C, color shading), rainfall (contours with an interval of 3 mm/day; positive in green), and wind stress (N/m2, vectors) under SSP245/RCP4.5. The running 50-year EP (c) relative SST (°C) and (d) zonal wind stress (10-2 N/m2) from the historical and SSP245/RCP4.5 outputs. The MME is shown as the thick curve, and the color shadings indicate one inter-member standard deviation above and below the MME (n = 7; Supplementary Table 1).

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Peng, Q., Xie, SP. & Deser, C. Collapsed upwelling projected to weaken ENSO under sustained warming beyond the twenty-first century. Nat. Clim. Chang. (2024). https://doi.org/10.1038/s41558-024-02061-8

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