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

Compared to the well-known polar amplification in a warmer or cooler world, the trend of tropical 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 (30^oS-30^oN) 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 of the TMSTG 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.