Recalibrated projections of the Hadley circulation under global warming

Abstract

Climate models project a weakening and expansion of the Hadley circulation (HC) under global warming but with considerable spread in the magnitude of these changes. Here, utilizing models from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6), we illustrate how the variance in projected changes in the HC arises from equilibrium climate sensitivity (ECS) uncertainty across models. Models with higher ECS project a greater extent of static stability increase hence larger HC changes. Using the best estimate of ECS with value of 3 K (∼2.5–4.0 K) to constrain the HC projection, we reveal that the constrained projection yields a 15% (11%) decrease in the weakening (poleward shift) of the HC in the Northern (Southern) Hemisphere compared to the multimodel mean under the SSP5-8.5 scenario. The corresponding projection uncertainty is reduced by about 77.4% and 75.6%, respectively. Our results indicate a smaller-than-expected change in the HC in response to increased CO _2 concentrations.

Keywords

• Hadley circulation
• projection uncertainty
• emergent constraint
• equilibrium climate sensitivity