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Hysteresis of the intertropical convergence zone to CO2 forcing

Nature Climate Change volume 12, pages 47–53 (2022)Cite this article

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Abstract

With the unprecedented rate of global warming in recent decades, whether or not anthropogenic climate change is irreversible is an important question. Based on idealized CO2 ramp-up until 1,468 ppm and symmetric ramp-down model experiments, here we show that the intertropical convergence zone (ITCZ) does not respond linearly to CO2 forcing, but exhibits strong hysteresis behaviour. While the location of the ITCZ changes minimally during the ramp-up period, it moves sharply south as soon as CO2 begins to decrease, and its centre eventually resides in the Southern Hemisphere during the ramp-down period. Such ITCZ hysteresis is associated with delays in global energy exchanges between the tropics and extratropics. The delayed energy exchanges are explained by two distinct hysteresis behaviours of the Atlantic Meridional Overturning Circulation and slower warming/cooling in the Southern Ocean. We also suggest that the ITCZ hysteresis can lead to hysteresis in regional hydrological cycles.

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Fig. 1: Evolution and hysteresis of temperature, precipitation and ITCZ location.
Fig. 2: Changes in the global SAT and hydrological cycle.
Fig. 3: Changes in atmospheric meridional energy exchanges.
Fig. 4: Changes in the land hydrological cycle.

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Data availability

The data used in this study are available from https://doi.org/10.6084/m9.figshare.1666932461, and the CMIP6 archives are freely available from https://esgf-node.llnl.gov/projects/cmip6.

Code availability

The code used in this study is available from the corresponding author on reasonable request.

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Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF-2018R1A5A1024958), the National Supercomputing Center with supercomputing resources including technical support (KSC-2018-CHA-0062) and the Korea Research Environment Open NETwork (KREONET).

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Author notes

  1. These authors contributed equally: Jong-Seong Kug, Ji-Hoon Oh.

Authors and Affiliations

  1. Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea

    Jong-Seong Kug, Ji-Hoon Oh, Soon-Il An, Seung-Ki Min & Jonghun Kam

  2. Institute for Convergence Research and Education in Advanced Technology, Yonsei University, Seoul, South Korea

    Jong-Seong Kug, Seung-Ki Min & Jonghun Kam

  3. Department of Atmospheric Sciences/Irreversible Climate Change Research Center, Yonsei University, Seoul, South Korea

    Soon-Il An & Jongsoo Shin

  4. Department of Marine Sciences and Convergence Technology, Hanyang University, ERICA, Ansan, South Korea

    Sang-Wook Yeh

  5. School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea

    Seok-Woo Son

  6. Department of Oceanography, Chonnam National University, Gwangju, South Korea

    Yoo-Geun Ham

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Contributions

J.-H.O. compiled the data, conducted analyses, prepared the figures and wrote the manuscript. J.-S.K. designed the research and wrote the majority of the manuscript content. All of the authors discussed the study results and reviewed the manuscript.

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Correspondence to Jong-Seong Kug.

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Peer review information Nature Climate Change thanks Antonios Mamalakis, Didier Swingedouw and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Time evolution of the ITCZ width, northern and southern edge of the ITCZ.

Time series of a, ITCZ width, b, northern edge and c, southern edge of ITCZ. ITCZ edge is defined as the latitude that is changed from a zonal-mean upward motion to a downward motion in the mid-troposphere (700hPa). Lines and shadings as in Fig. 1b.

Extended Data Fig. 2 Precipitation anomalies in the extreme El Niño period.

Composite of DJF averaged precipitation anomalies during 82/83, 97/98, and 15/16 El Niño period in the observational data, b, during the cases when the DJF Nino3.4 index is greater than the 2standard deviation of the DJF Nino3.4 index in the model (CESM1). The observational precipitation data are the Climate Prediction Merged Analysis of Precipitation (CMAP), which uses a horizontal resolution of 2.5° and covers the period from 1979 to 2018. Note that the seasonal cycle and linear trend are removed from the data.

Extended Data Fig. 3 Changes in global hydrological cycle in CMIP6.

Same as Fig. 2c, but the data from the individual models in CMIP6 (ACCESS-ESM1-5, CanESM5, CESM2, GFDL-ESM4, MIROC-ES2L, UKESM1-0-LL).

Extended Data Fig. 4 Changes in global SAT and hydrological cycle in CMIP6.

Same as Fig. 2a and d-e, but the data in the multi-model mean from the CMIP6 (ACCESS-ESM1-5, CanESM5, CESM2, GFDL-ESM4, MIROC-ES2L, UKESM1-0-LL). The regions denoted by the cross-shaped dots a, dots b, and colors c, indicate where more than 2/3 of models disagree a, agree b, and agree c with the sign of the multi-model mean, respectively.

Extended Data Fig. 5 Changes in poleward ocean heat transport.

Time-series of the AMOC strength, global poleward ocean heat transport at 30°N and 30°S, b, poleward ocean heat transport anomalies at 30°N in global, Atlantic, and Indo-Pacific oceans relative to the Year 2000. The ocean heat transport variable (N_HEAT; both global and Atlantic components are included) is obtained directly from the model output. Note that the heat transport at 30°S is the absolute value. Lines and shadings as in Fig. 1b.

Extended Data Fig. 6 Land and ocean contribution to global surface warming.

The percentage of the land and ocean SAT anomalies in the NH and SH for global surface temperature anomaly relative to long-term climatological values of the present climate simulation. Dotted lines represent the ratio of each area to the total earth surface (SH ocean: 40.5%, SH land: 9.5%, NH ocean: 30.2%, NH land: 19.8%). Solid line indicates the boundary of SH land and NH ocean, which represent hemispheric warming contrast. For example, less than 50% of the solid line means that the NH is warmer than the SH. b, The deviation of the percentage of each component from the ratio to the total earth surface (deonted in Dotted lines in a). c, Time-series of the top 2000m ocean heat content anomaly integrated over the Southern Ocean (90°S–50°S) relative to the Year 2000. Lines and shadings as in Fig. 1b.

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Kug, JS., Oh, JH., An, SI. et al. Hysteresis of the intertropical convergence zone to CO2 forcing. Nat. Clim. Chang. 12, 47–53 (2022). https://doi.org/10.1038/s41558-021-01211-6

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