Enhanced equatorial warming causes deep-tropical contraction and subtropical monsoon shift

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Abstract

Under anthropogenic warming, deep-tropical ascent of the intertropical convergence zone (ITCZ) is projected to contract equatorward1,2,3 while subtropical descent associated with the Hadley cell edge is predicted to expand poleward4. These changes have important implications for regional climate2,5,6,7, but their mechanisms are not well understood. Here we reveal a key role of enhanced equatorial surface warming (EEW) in driving the deep-tropical contraction and modulating the Hadley expansion. By shifting the seasonally warmed sea surface temperature equatorward, EEW reduces the meridional migration of the seasonal ITCZ and causes an annual-mean deep-tropical contraction. This process further contracts the subtropical circulation, as seen during El Niño, and counteracts the Hadley expansion caused by the global-scale warming. The EEW-induced contraction even dominates in the Northern Hemisphere early summer (June–July), when atmospheric circulation responses to the global-scale warming are weak8. Regionally, this alters the East Asian summer monsoon, shifting both the subtropical jet and Meiyu–Baiu rainband equatorward. Among models in Phase 5 of the Coupled Model Intercomparison Project9, the degrees of the equatorward shift in the ITCZ, the early-summer subtropical circulation and the East Asian summer monsoon are correlated with EEW. Our results suggest that a better constraint on EEW is critical for accurate projection of tropical and subtropical climate change.

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Fig. 1: Deep-tropical contraction integrated from equatorward-shifted seasonal ITCZ driven by EEW.
Fig. 2: Denial experiment and intermodel spread of the deep-tropical contraction.
Fig. 3: Seasonal-dependent contraction effect of EEW.
Fig. 4: Equatorward shift of the EASM controlled by EEW.

Data availability

The data supporting the findings of this study are available within the manuscript and its supplementary information. Data associated with the GFDL-AM2.1 simulations are available at https://github.com/wenyuz/EEW. The AMIP and CMIP outputs can be obtained from the CMIP5 archive, accessed through http://www.ipcc-data.org/sim/gcm_monthly/AR5/Reference-Archive.html.

Code availability

The data analysis code is available from the corresponding author on request.

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Acknowledgements

This work is supported by the National Science Foundation (grant no. NSF-1637450) and Laboratory Directed Research and Development funding from Berkeley Lab, provided by the Director, Office of Science, of the US Department of Energy under contract no. DE-AC02-05CH11231. Numerical simulations were conducted using the computing resources provided by the NCAR Cheyenne: HPE/SGI ICE XA System (University Community Computing, https://doi.org/10.5065/D6RX99HX).

Author information

W.Z. designed the research, conducted the simulations and analysed the results. S.-P.X. and D.Y. contributed to improving the analysis and interpretation. W.Z. wrote the first draft, and all authors discussed and edited the paper.

Correspondence to Wenyu Zhou.

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The authors declare no competing interests.

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

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Supplementary Information

Supplementary Figs. 1–11.

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Zhou, W., Xie, S. & Yang, D. Enhanced equatorial warming causes deep-tropical contraction and subtropical monsoon shift. Nat. Clim. Chang. 9, 834–839 (2019) doi:10.1038/s41558-019-0603-9

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