Quasi-invariance of tropical meridional surface temperature gradient in a wide range of climates

Sep 30, 2026

Compared with polar amplification, tropical meridional surface air temperature gradient (TMSTG) is shown to exhibit only small changes across a wide span of climates from extremely cold to extremely hot. This quasi-invariance holds against variations in CO2, solar constant, land-sea configuration, vegetation coverage, heat transport, and cloud parameterization. It is maintained by weak changes in incoming solar radiation gradients and by tropical dynamics involving horizontal weak temperature gradient (WTG) and convective moist adiabat (CMA). The resulting law supports reconstructions and predictions of Earth’s tropical climate in the past and future.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages10

Reading Time25 min

AccessFree to download

Introduction

Background: polar amplification and its mechanisms

Focus: tropical meridional temperature gradient

Results and evidence

Observations and reanalysis

Model experiments under CO2 forcing

Comparison across datasets and climate states

Mechanisms

Solar radiation gradient

Tropical dynamics: WTG and convective moist adiabat (CMA)

Implications and applications

Reconstructing and predicting tropical climate

Article [https://doi.org/10.1038/s41467-025-66811-8](https://doi.org/10.1038/s41467-025-66811-8) Quasi-invariance of tropical meridional surface temperature gradient in a wide range of climates Received: 22 September 2024 | Accepted: 17 November 2025 | | --- | | | | Check for updates | Mengyu Wei 1, Jun Yang 1 , Yongyun Hu 1, Yohai Kaspi 2 & Ji Nie 1 Compared to the well-known polar ampliﬁcation in a warmer or cooler world, the trend oftropical surface air temperature gradient is frequently overlooked. Through analyzing various observations, assimilation data, proxy data, and modeling, here we show that annual-and zonal-mean tropical (30oS-30oN) meridional surface air temperature gradient (TMSTG) exhibits small changes in a wide range of climates from extremely cold to extremely hot. This phenomenon is robust to CO2 concentration, solar constant, land-sea conﬁguration, vegetation coverage, heat transport, and cloud parameterization. The quasi-invariance oftheTMSTG is maintained by the small gradient of incoming solar radiation and by the dynamics of horizontal weak temperature gradient (WTG) and convective moist adiabat (CMA) in the tropics. When planetary obliquity or rotation period is increased, the quasi-invariant region becomes larger. TMSTG’s quasi-invariance is a fundamental and useful law that can be used to reconstruct or predict Earth’s tropical climate in the past and future. Meridional surface air temperature gradient has important effects on the climate system through affecting the Hadley and Ferrel cells, subtropical and midlatitude jets, eddies and waves, and polar ice and snow coverages1–6. In general, the equator-to-pole surface temperature gradient becomes larger in a cooler climate and smaller in a warmer climate, with the high latitudes being more sensitive than low latitudes, known as “polar ampliﬁcation”2,7–9. Besides the widely known sea ice and snow albedo feedback, poleward energy transport, cloud feedback, water vapor feedback, lapse rate feedback, and Planck feedback also contribute to the polar ampliﬁcation10–13. Contrast to the polar ampliﬁcation, here we show that zonal-mean tropical meridional surface temperature gradient (TMSTG) nearly does not change in a wide range of climates. This ﬁnding has critical implications for reconstructing past climate and predicting future climate change. Tropical surface temperature gradient is much smaller than that in the middle and high latitudes. For example, under the present-day climate, the equator-to-30°S/N zonal-mean surface temperature difference is ~6.6 °C whereas it is ~20.1 °C for 30°S/N to 60°S/N and~30.5 °C for 60°S/N to 90°S/N. More importantly, the changes of TMSTG under external forcings such as increasing CO2 are very small. In observations, the surface temperature changes in the tropics from 30°S to 30°N are nearly uniform during the last few decades14. In the doubling CO2 experiment using the model EC-Earth, the warming magnitude at 30°S/N, ~1.9°C, is very close to that at the equator,~2.0 °C (Fig. 1 in ref. 15). In abrupt 4×CO2 experiments, the multimodelmean warming magnitude at 30°S(N), 5 °C, is nearly the same as that atthe equator (Fig. 2c of ref. 16). Similar phenomenon of the small change in surface temperature gradient between the equator and 30°S/N was found in other studies of global warming10,14,17–22. However, the range of the climate states investigated in their studies is narrow, within ~7 K in global mean compared to present. Due to the narrow range ofthe climates investigated in previous studies, the small change of the TMSTG was usually viewed as a coincidence rather than as a physical rule; as a result, the trend of TMSTG was usually overlooked. An exception is the study by ref. 19, within which the highest CO2 concentration examined is 16 times the pre-industrial (PI) level and the changes in tropical surface temperature are about 10–12 °C. However, ref. 19 focused on the hemispheric asymmetry in polar warmin

FAQs

What does the study claim about the tropical meridional surface temperature gradient (TMSTG) across climates?

TMSTG exhibits small changes, demonstrating quasi-invariance from extremely cold to extremely hot climates, unlike the larger equator-to-pole changes seen in higher latitudes.

Which forcings does the quasi-invariance of TMSTG remain robust against?

The quasi-invariance remains robust to CO2 concentration, solar constant, land-sea configuration, vegetation coverage, heat transport, and cloud parameterization.

What mechanisms are proposed to maintain the quasi-invariance in the tropics?

It is maintained by the small gradient of incoming solar radiation and by tropical dynamics involving horizontal weak temperature gradient (WTG) and convective moist adiabat (CMA).

How is the quasi-invariance useful for studying past and future climates?

The study describes TMSTG quasi-invariance as a fundamental and useful law for reconstructing or predicting Earth’s tropical climate in the past and future.

Sep 30, 2026

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