A mechanism for future changes in Hadley circulation strength in CMIP5 climate change simulations

The Coupled Model Intercomparison Project Phase 5 (CMIP5) 21st century climate change simulations exhibit a robust (slight) weakening of the Hadley cell (HC) during the boreal winter (summer, respectively) season in the future climate. Using 30 different coupled model simulations, we investigate the main mechanisms for both the multimodel ensemble mean changes in the HC strength and its intermodel changes in response to global warming during these seasons. A simple scaling analysis relates the strength of the HC to three factors: the meridional potential temperature gradient, gross static stability, and tropopause height. We found that changes in the meridional potential temperature gradients across the subtropics in a warming climate play a crucial role in the ensemble mean changes and model-to-model variations in the HC strength for both seasons. A larger reduction in the meridional temperature gradient in the Northern Hemisphere in boreal winter leads to the larger reduction of the HC strength in that season.