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. Plain Language Summary The Hadley circulation has large effects on the variability of temperature and precipitation at low latitudes. By the end of the 21st century, climate models project a weakening of the circulation, which will have great societal impacts. It is thus critical to elucidate the underlying mechanisms of the projected weakening. Here, we assess different mechanisms for the projected weakening and show that it stems from an increase in the vertical temperature gradient in the tropics, which is partially offset by an increase in latent heating. Key Points Surface warming alone is unable to explain Hadley cell weakening under 4 × CO2 forcing, as the weakening exhibits a much faster response Each of the previously proposed mechanisms analyzed in this study cannot fully explain the weakening of the Hadley cell The weakening stems from a combination of the opposing effects of increases in stratification and in meridional gradient of latent heating