An updated view of Hadley cell expansion from CMIP6 models

            One of the most robust aspects of the atmospheric circulation response to increasing greenhouse gases is the poleward shift in the subsiding branches of the Hadley circulation, potentially pushing subtropical dry zones poleward toward midlatitudes.  Numerous lines of observational evidence suggest that this tropical expansion may have already begun.  Yet, the degree to which the observed tropical widening is anthropogenically forced has remained a topic of great debate, as previous studies have attributed the recent circulation trends to some combination of increasing greenhouse gases, stratospheric ozone depletion, anthropogenic aerosols, and natural variability.  During the past few years, two international working groups have synthesized recent findings about the magnitude and causes of the observed tropical widening, primarily using output from global climate models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5).  In this presentation, we update those findings using the recently released CMIP6 global climate models.

            Over recent decades, the poleward expansion of the Hadley circulation estimated from modern reanalyses is relatively modest (< 0.5 degrees latitude per decade).  The reanalysis trends have similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but both CMIP5 and CMIP6 models suggest that increasing greenhouse gases should drive 2–3 times larger circulation shifts in the SH.  The reanalysis trends fall within the bounds of the models’ simulations of the late 20^th century and early 21^st century, although prescribing observed coupled atmosphere-ocean variability allows the models to better capture the observed trends in the NH.  We find two notable differences between CMIP5 and CMIP6 models.  First, both CMIP5 and CMIP6 models contract the NH summertime Hadley circulation equatorward (particularly over the Pacific sector) in response to increasing greenhouse gases, but 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 the historical runs of CMIP6 models.  Increasing greenhouse gases drive similar trends in CMIP5 and CMIP6 models, so CMIP6 models imply a stronger role for other forcings (such as aerosols) in recent circulation trends than CMIP5 models.