Hadley cell expansion in CMIP6 models

• Published in: Atmospheric chemistry and physics Vol. 20; no. 9; pp. 5249 - 5268
• Main Authors: Grise, Kevin M, Davis, Sean M
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 06.05.2020; Copernicus Publications
• Subjects: Aerosols; Air pollution; Analysis; Atmospheric circulation; Atmospheric models; Cell culture; Climate; Climate models; Climate sensitivity; Computer simulation; Contraction; Gases; General circulation models; Global climate; Global climate models; Greenhouse effect; Greenhouse gases; Hadley circulation; Intercomparison; Northern Hemisphere; Ocean models; Oceans; Prescription writing; Southern Hemisphere; Studies; Subtropical circulation; Summer circulation; Trends
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-20-5249-2020

In response to increasing greenhouse gases, the subtropical edges of Earth's Hadley circulation shift poleward in global climate models. Recent studies have found that reanalysis trends in the Hadley cell edge over the past 30–40 years are within the range of trends simulated by Coupled Model Intercomparison Project Phase 5 (CMIP5) models and have documented seasonal and hemispheric asymmetries in these trends. In this study, we evaluate whether these conclusions hold for the newest generation of models (CMIP6). Overall, we find similar characteristics of Hadley cell expansion in CMIP5 and CMIP6 models. In both CMIP5 and CMIP6 models, the poleward shift of the Hadley cell edge in response to increasing greenhouse gases is 2–3 times larger in the Southern Hemisphere (SH), except during September–November. The trends from CMIP5 and CMIP6 models agree well with reanalyses, although prescribing observed coupled atmosphere–ocean variability allows the models to better capture reanalysis trends in the Northern Hemisphere (NH). We find two notable differences between CMIP5 and CMIP6 models. First, while both CMIP5 and CMIP6 models contract the NH summertime Hadley circulation equatorward (particularly over the Pacific sector), this contraction is larger in CMIP6 models due to their higher average climate sensitivity. Second, in recent decades, the poleward shift of the NH annual-mean Hadley cell edge is slightly larger in CMIP6 models. Increasing greenhouse gases drive similar trends in CMIP5 and CMIP6 models, so the larger recent NH trends in CMIP6 models point to the role of other forcings, such as aerosols.