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Contrasting fast and slow intertropical convergence zone migrations linked to delayed Southern Ocean warming

Nature Climate Change volume 14, pages 732–739 (2024)Cite this article

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Abstract

Migrations of the intertropical convergence zone (ITCZ) have significant impacts on tropical climate and society. Here we examine the ITCZ migration caused by CO2 increase using climate model simulations. During the first one to two decades, we find a northward ITCZ displacement primarily related to an anomalous southward atmospheric cross-equatorial energy transport. Over the next hundreds or thousands of years, the ITCZ moves south. This long-term migration is linked to delayed surface warming and reduced ocean heat uptake in the Southern Ocean, which alters the interhemispheric asymmetry of ocean heat uptake and creates a northward atmospheric cross-equatorial energy transport anomaly. The southward ITCZ shift, however, is reduced by changes in the net energy input to the atmosphere at the equator by about two-fifths. Our findings highlight the importance of Southern Ocean heat uptake to long-term ITCZ evolution by showing that the (quasi-)equilibrium ITCZ response is opposite to the transient ITCZ response.

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Fig. 1: CO2-induced zonal-mean ITCZ changes.
Fig. 2: Maps of CO2-induced precipitation changes.
Fig. 3: CO2-induced changes in interhemispheric asymmetry of energy flux.
Fig. 4: Zonal-mean CO2-induced energy flux changes.
Fig. 5: Maps of CO2-induced energy flux changes.
Fig. 6: Maps of CO2-induced SST changes.

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Data availability

CMIP5 model data are available at https://esgf-node.llnl.gov/projects/cmip5/. CMIP6 model data are available at https://esgf-node.llnl.gov/projects/cmip6/. LongRunMIP data are available at https://www.longrunmip.org. The processed variables to generate Figs. 1–6 are available via Zenodo at https://zenodo.org/records/11075601 (ref. 57) in the form of netcdf files.

Code availability

Figures 1–6 were generated using NCL v.6.5.0 (ref. 56). The codes to generate Figs. 1–6 are available via Zenodo at https://zenodo.org/records/11075601 (ref. 57) in the form of NCL files.

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Acknowledgements

This study was supported by the US National Science Foundation (NSF OCE-2123422, AGS-2053121 and AGS-2237743) awarded to W.L. who was also supported by the UC Regents Faculty Development Award. C.L. was supported by the Clusters of Excellence CLICCS (EXC2037), University of Hamburg, funded by the German Research Foundation (DFG). M.R. was supported by NSF AGS-2233673.

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Authors and Affiliations

  1. Department of Earth Sciences and Planetary Sciences, University of California Riverside, Riverside, CA, USA

    Wei Liu, Shouwei Li & Antony P. Thomas

  2. Max Planck Institute for Meteorology, Hamburg, Germany

    Chao Li

  3. Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

    Maria Rugenstein

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Contributions

W.L. conceived the study and wrote the original draft of the paper. S.L. and A.P.T. performed the analysis. C.L. and M.R. provided the data. All authors contributed to interpreting the results and made substantial improvements to the paper.

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Correspondence to Wei Liu.

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Extended data

Extended Data Fig. 1 Maps of TOA radiation changes due to radiative feedback.

(a-c) Maps of TOA radiation changes (relative to preindustrial times, in units of W/m2) due to the cloud feedback in the CO2 quadrupling simulations for the multimodel means of (a) CMIP5/6 and (b) LongRunMIP models over years 1–20, and (c) LongRunMIP models for the difference between years 981–1000 and years 1–20. (d-f) Same as (a-c) but for the water vapor feedback. (g-i) Same as (a-c) but for the albedo feedback. (j-l) Same as (a-c) but for the temperature feedback. The base map is from NCAR Command Language map outline databases.

Extended Data Fig. 2 Maps of CO2-induced energy flux changes in CMIP5/6 models.

Maps of changes (relative to preindustrial times, in units of W/m2) in the net (a) TOA radiation and (b) surface energy flux in the CO2 quadrupling simulation for the multimodel mean of CMIP5/6 models over years 1–20. The base map is from NCAR Command Language map outline databases.

Extended Data Fig. 3 Maps of CO2-induced surface energy flux changes.

(a,b) Maps of (a) surface (shortwave plus longwave radiation energy flux and (b) surface turbulent (sensible plus latent) heat flux changes (relative to preindustrial times, in units of W/m2) in the CO2 quadrupling simulation for the multimodel mean of CMIP5/6 models over years 1–20. (c,d) Same as (a,b) but for LongRunMIP models. (e,f) Same as (c,d) but for years 981–1000. (g,h) Same as (c,d) but for the differences between years 981–1000 and years 1–20. The base map is from NCAR Command Language map outline databases.

Extended Data Fig. 4 CO2-induced changes in energy flux asymmetry in the LongRunMIP_sub.

Changes (relative to preindustrial times, multimodel mean, MMM, dot; intermodel spread, one standard derivation (1 SD) among models, bars) in the atmospheric cross-equatorial energy transport (purple) and interhemispheric asymmetry (Southern minus Northern Hemisphere, Methods) of the net TOA radiation (red), net surface energy flux (blue), surface turbulent heat flux (sensible plus latent, turquoise blue), and surface radiation energy flux (shortwave plus longwave, brown) in the CO2 quadrupling simulation by LongRunMIP_sub models over years 1–20 and years 3981–4000, and for the difference between the two periods (years 3981–4000 minus years 1–20).

Extended Data Fig. 5 Maps of CO2-induced energy flux changes in the LongRunMIP_sub.

(a,b) Maps of changes (relative to preindustrial times, in units of W/m2) in the net (a) TOA radiation and (b) surface energy flux in the CO2 quadrupling simulation for the multimodel mean of LongRunMIP_sub models over years 1–20. (c,d) Same as (a,b) but for years 3981–4000. (e,f) The differences between the two periods for the net (e) TOA radiation and (f) surface energy flux (years 3981–4000 minus years 1–20). (g,h) Same as (e,f) but for surface (shortwave plus longwave) radiation energy flux and surface turbulent (sensible plus latent) heat flux. The base map is from NCAR Command Language map outline databases.

Extended Data Fig. 6 Maps of CO2-induced SST changes in the LongRunMIP_sub.

SST changes (relative to preindustrial times, in units of K) in the CO2 quadrupling simulation for the multimodel mean of LongRunMIP_sub models over (a) years 1–20 and (b) years 3981–4000, respectively. (c) Same as (a) but for the difference between years 3981–4000 and years 1–20. The base map is from NCAR Command Language map outline databases.

Extended Data Fig. 7 CO2-induced changes in the AMOC and the integrated surface energy flux asymmetry.

(a) Changes (relative to preindustrial times) in AMOC strength (multimodel mean, black; intermodel spread, one standard derivation among models, grey) in the CO2 quadrupling simulation by LongRunMIP models except ECHAM5-MPIOM. The AMOC strength is defined as the maximum in the meridional overturning stream function below 500 m in the North Atlantic. The first 20-year average of AMOC strength is plotted at year 10 in the form of multimodel mean (MMM, dot) ± one standard deviation (1 SD) among models (bars). (b) Same as (a) but for surface energy fluxes integrated over 30°N–65°N (multimodel mean, red; intermodel spread, light red), over the Pacific and land areas (multimodel mean, orange; intermodel spread, yellow) and the Atlantic area (multimodel mean, green; intermodel spread, light green) within 30°N–65°N. Note that the first 20-year average of surface energy fluxes integrated over the Pacific and land areas with 30°N–65°N is plotted at year 9 for a clear visualization. (c) Same as (a) but for changes in surface energy fluxes integrated over 30°N–65°N (multimodel mean, red; intermodel spread, one standard derivation among models, light red), 30°S–65°S (multimodel mean, blue; intermodel spread, light blue) and the difference (30°S–65°S minus 30°N–65°N, multimodel mean, black; intermodel spread, grey).

Extended Data Table 1 List of the LongRunMIP models
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Supplementary Table 1.

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Liu, W., Li, S., Li, C. et al. Contrasting fast and slow intertropical convergence zone migrations linked to delayed Southern Ocean warming. Nat. Clim. Chang. 14, 732–739 (2024). https://doi.org/10.1038/s41558-024-02034-x

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