Weakening of Tropical Free-Tropospheric Temperature Gradients with Global Warming

The weak temperature gradients in the tropical free troposphere due to the vanishing Coriolis force near the equator lead to a strong dynamical coupling over the entire tropics. Using theory and a suite of targeted model experiments, we show that the weak temperature gradients further weaken under global warming.We show that the temperature gradient is set by the circulation strength, with a weaker circulation being associated with weaker gradients. Thus, the known scaling difference between atmospheric radiative cooling and static stability that leads to a slowdown of the circulation under warming also leads to a weakening of the temperature gradients in the tropical free troposphere. The impact from the weakening circulation on the weakening of temperature gradients is shown to dominate over the impact of masked CO2 forcing and the El Niño-like tropical Pacific warming pattern in model projections. Key to the result is the nonlinear zonal momentum advection term. Using the well-known Matsuno-Gill model with the correct scaling of heating and static stability may give the correct sign of the response in the temperature gradients, but inaccurate scaling, due to the linear momentum damping in that model. The robust scaling of the magnitude of the tropical quasi-stationary structure with temperature opens possibilities for theoretical advances on questions of societal relevance, ranging from changes in tropical cloudiness to heat stress under climate change.