Elucidating the mechanisms responsible for Hadley cell weakening under 4xCO2 forcing

Abstract:

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 analyze the abrupt 4xCO₂ integrations of the Coupled Model Intercomparison Project phase 5 (CMIP5), as this forcing separates the different response times of different mechanisms. We find that the circulation shows a fast response to quadrupling CO₂ concentrations, reaching its steady-state value after less than a decade. This fast response demonstrates that the weakening of the Hadley cell is not a feedback from the relatively slow increase in surface temperature. In addition, we show that previous suggested mechanisms do not accurately follow the evolution of the circulation. Instead, the circulation's response results from a cancellation between a reduction in tropical static stability and an increase in the meridional gradient of latent heating, which act to weaken and strengthen the circulation, respectively.