Hadley cell expansion in CMIP6 models

Kevin M Grise,Sean M Davis

doi:10.5194/acp-20-5249-2020

Abstract

Abstract. 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.

Highlights

The poleward expansion of the Hadley circulation is one of the most robust aspects of the atmospheric general circulation’s response to a warming climate in global climate models
We examined the trends in the latitudes of the Northern Hemisphere (NH) and Southern Hemisphere (SH) Hadley cell edges over the late 20th century and early 21st century in Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6 models and compared them to trends from five reanalyses
In response to increasing greenhouse gases, global climate models show a robust poleward expansion of the Hadley circulation (Davis et al, 2016; GP16; Watt-Meyer et al, 2019), and numerous lines of observational evidence suggest that the Hadley circulation has already expanded over the last 30– 40 years (Birner et al, 2014; Davis and Rosenlof, 2012; Seidel et al, 2008; Staten et al, 2018)

Summary

Introduction

The poleward expansion of the Hadley circulation is one of the most robust aspects of the atmospheric general circulation’s response to a warming climate in global climate models. A decade ago, a number of studies began estimating rates of Hadley cell expansion using various observational data sets (Fu et al, 2006; Hu and Fu, 2007; Seidel and Randel, 2007; Seidel et al, 2008). These rates varied widely by study, ranging from 0.2 to 3◦ latitude per decade over the period from 1979 until the mid-2000s (Birner et al, 2014; Davis and Rosenlof, 2012; Lucas et al, 2014). Some studies identified an important role for anthropogenic forcing, including increasing greenhouse gases

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