Elucidating the Mechanisms Responsible for Hadley Cell Weakening Under 4 × CO₂ Forcing

The projected weakening of the Northern Hemisphere Hadley cell will have large climatic impacts at low latitudes. Several mechanisms have been proposed to explain this weakening. In order to isolate and assess their relative importance, we here use the abrupt 4 × CO₂ experiment of the Coupled Model Intercomparison Project phase 5, as this forcing separates the different mechanisms which respond on different time scales. We find that the Hadley circulation responds relatively quickly to quadrupling CO₂ concentrations, reaching its steady-state value after less than a decade. This fast response demonstrates that the weakening could not be solely due to the much slower increase in surface temperature. In addition, we show that the Hadley cell's weakening results from a combination of an increase in tropical static stability, partially offset by an increase in the latitudinal gradient of latent heating.