Wei et al. (2025) Quasi-invariance of tropical meridional surface temperature gradient in a wide range of climates

Identification

• Journal: Nature Communications
• Year: 2025
• Date: 2025-12-06
• Authors: Mengyu Wei, Jun Yang, Yongyun Hu, Yohai Kaspi, Ji Nie
• DOI: 10.1038/s41467-025-66811-8

Research Groups

• Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China.
• Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel.

Short Summary

This study reveals that the annual- and zonal-mean tropical (30°S-30°N) meridional surface air temperature gradient (TMSTG) remains remarkably stable across a vast spectrum of climates, from extremely cold to extremely hot. This quasi-invariance is robustly maintained by the small gradient of incoming solar radiation and the tropical dynamics of weak temperature gradient (WTG) and convective moist adiabat (CMA).

Objective

• To investigate the behavior of the tropical meridional surface air temperature gradient (TMSTG) across a wide range of past and future climates, spanning global-mean surface temperatures from 6.2 °C to 36.1 °C.
• To uncover the underlying physical mechanisms responsible for the observed quasi-invariance of the TMSTG, including the roles of solar radiation, atmospheric dynamics, and radiative transfer.

Study Configuration

• Spatial Scale: Global, with a specific focus on the tropical region (30°S-30°N), and comparisons to mid- and high-latitude gradients (up to 90°S/N).
• Temporal Scale: A wide range of climates from the past 540 million years to present-day and future projections, including specific periods like the Last Glacial Maximum (~21,000 years ago), Holocene (~6,000 years ago), Paleocene-Eocene Thermal Maximum (~56 million years ago), and other geological epochs (~70, 50, 30, 5 million years ago). Also covers recent decades (1979-1988 vs. 2013-2022).

Methodology and Data

• Models used:
  • Community Earth System Model (CESM1.2)
  • Hadley Centre Model (HadCM3)
  • 17 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6)
  • iCESM (an extension of CESM)
  • Atmospheric component of CESM coupled to a slab-ocean model for sensitivity experiments.
  • Parallel Offline Radiative Transfer (PORT) tool for radiation transfer calculations.
• Data sources:
  • Observations: National Centers for Environmental Prediction (NCEP) reanalysis data, ECMWF Reanalysis 5th Generation (ERA5).
  • Assimilation data: Three groups of assimilation data from previous studies (refs. 29, 56, 57) for LGM, Holocene, and PETM.
  • Proxy data: Seven groups of proxy data from previous studies (refs. 8, 28, 56) for various geological periods.
  • Modelling data:
    • Slice experiments from 540 Ma to PI using CESM (ref. 31).
    • CESM experiments focusing on PETM (ref. 28).
    • CESM experiments for ~95, 55, and 14 Ma (ref. 33).
    • HadCM3 simulations covering the Phanerozoic (ref. 30).
    • Coupled Model Intercomparison Project Phase 4 (PMIP4) experiments (39 experiments from 14 models).
  • Sensitivity experiments: Slab-ocean CESM experiments varying planetary obliquity, ocean heat transport, cloud radiative effects, and planetary rotation rate.

Main Results

• The annual- and zonal-mean tropical meridional surface air temperature gradient (TMSTG, defined as TSeq-30) exhibits small changes across a global-mean surface temperature range of 6.2 °C to 36.1 °C.
• A linear regression of TSeq-30 against global-mean surface temperature shows a statistical slope of -0.08, indicating that a 50 °C change in global-mean temperature results in only a 4.0 °C change in TSeq-30.
• The changes in TMSTG are significantly smaller than those observed in mid- and high-latitude temperature gradients (e.g., equator-to-60°S/N or equator-to-90°S/N).
• The quasi-invariance of TMSTG is primarily driven by:
  1. The small meridional gradient of incoming solar radiation in the tropics, which is robust to changes in solar constant but sensitive to planetary obliquity.
  2. Tropical dynamical processes: the weak temperature gradient (WTG) approximation and the convective moist adiabat (CMA) in the free troposphere.
• When planetary obliquity is increased from 23.3° to 40°, TSeq-30 decreases significantly (from 10.8 °C to 4.9 °C), and the quasi-invariant region expands poleward.
• Decreasing the planetary rotation rate to 1/10 of the modern value expands the WTG region from ~30°S/N to ~60°S/N, also extending the meridional range of TMSTG quasi-invariance.
• Other factors like surface albedo, cloud feedback, and meridional ocean heat transport have limited effects on TMSTG, with ocean heat transport changes affecting TMSTG by at most ~4.7 °C.
• The clear-sky greenhouse effect and downward longwave radiation at the surface also exhibit a meridionally uniform pattern in the tropics, contributing to the uniform surface temperature response.

Contributions

• This study identifies a previously overlooked fundamental law of Earth's climate system: the quasi-invariance of the tropical meridional surface air temperature gradient across a wide range of climate states.
• It provides a crucial tool for reconstructing past tropical surface air temperatures, especially in data-sparse paleoclimate records, by allowing estimation of the entire tropical zonal-mean surface temperature from a single latitude.
• The finding offers valuable implications for predicting future climate change, suggesting that the tropical meridional surface temperature gradient will remain relatively stable even under substantial anthropogenic greenhouse gas emissions.
• The identified rule can be used to constrain parameters, such as diffusivity, in simplified energy balance climate models.

Funding

• Natural Science Foundation of China (NSFC) Grants 42488201
• National Key Research and Development Program of China Grants 2024YFF0807903

Citation

@article{Wei2025Quasiinvariance,
  author = {Wei, Mengyu and Yang, Jun and Hu, Yongyun and Kaspi, Yohai and Nie, Ji},
  title = {Quasi-invariance of tropical meridional surface temperature gradient in a wide range of climates},
  journal = {Nature Communications},
  year = {2025},
  doi = {10.1038/s41467-025-66811-8},
  url = {https://doi.org/10.1038/s41467-025-66811-8}
}