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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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).
The authors declare no competing interests.
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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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.
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.
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).
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.
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.
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
Nature Climate Change (2022)
Nature Climate Change (2022)