Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.4.2.2 Hadley Circulation and Subtropical Belt ====

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The AR5 found that the Hadley cells are ''likely'' to slow down and expand in response to radiative forcing, but with considerable internal variability. Given the complexities in forcing mechanisms, AR5 assigned ''low confidence'' to near-term changes in the structure of the Hadley circulation. The widening Hadley cells were expected to result in a poleward expansion of subtropical dry zones.

Model simulations since AR5 project a more noticeable and consistent weakening of the Northern Hemisphere (NH) winter Hadley cell than the Southern Hemisphere (SH) winter cell ( [[#Seo--2014|Seo et al., 2014]] ; [[#Zhou--2016|Zhou et al., 2016]] ), related to changes in meridional temperature gradient, static stability, and tropopause height ( [[#Seo--2014|Seo et al., 2014]] ; [[#D’Agostino--2017|D’Agostino et al., 2017]] ). Changes in SST patterns reduces the magnitude of Hadley cell weakening ( [[#Gastineau--2009|Gastineau et al., 2009]] ; [[#Ma--2012|Ma et al., 2012]] ). There is considerable structure in Hadley circulation strength changes with longitude, associated with cloud-circulation interactions ( [[#Su--2014|Su et al., 2014]] ). Subtropical anticyclones are projected to intensify over the north Atlantic and south Pacific but to weaken elsewhere ( [[#He--2017|He et al., 2017]] ).

A consistent poleward expansion of the edges of the Hadley cells is projected ( [[#Nguyen--2015|Nguyen et al., 2015]] ; [[#Grise--2020|Grise and Davis, 2020]] ), particularly in the SH, consistent with observed trends (Figure 8.21 and [[#8.3.2.2|Section 8.3.2.2]] ; Nguyen et al. , 2015) . The main driver of future expansion appears to be greenhouse gas forcing ( [[#Grise--2019|Grise et al., 2019]] ), with uncertainty in magnitude due to internal variability ( [[#Kang--2013|Kang et al., 2013]] ). Proposed mechanisms for poleward expansion include increased dry static stability (Frierson et al. , 2007; Lu et al. , 2007) , increased tropopause height ( [[#Chen--2007|Chen and Held, 2007]] ; Chen et al. , 2008) , stratospheric influences ( [[#Kidston--2015|Kidston et al., 2015]] ) and radiative effects of clouds and water vapour ( [[#Shaw--2016|Shaw and Voigt, 2016]] ; see also [[IPCC:Wg1:Chapter:Chapter-4#4.5.1.5|Section 4.5.1.5]] ). Hadley cell expansion is thought to be associated with the precipitation declines projected in many subtropical regions ( [[#Shaw--2016|Shaw and Voigt, 2016]] ), but more recent work suggests that these reductions are mainly due to the direct radiative effect of CO <sub>2</sub> forcing ( [[#He--2015|He and Soden, 2015]] ), land – sea contrasts in the response to forcing (Shaw and Voigt, 2016; Brogli et al. , 2019) and SST changes ( [[#Sniderman--2019|Sniderman et al., 2019]] ). In semi-arid, winter rainfall-dominated regions (such as the Mediterranean), thermodynamic processes associated with the land – sea thermal contrast and lapse rate changes dominate the projected precipitation decline in summer, whereas circulation changes are of greater importance in winter (Brogli et al. , 2019) . The hydroclimates in these regions are projected to evolve with time due to changing contributions from rapid atmospheric circulation changes and their associated SST responses, as well as slower SST responses to anthropogenic forcing (Zappa et al., 2020) .

In summary, CMIP5 and CMIP6 models project a weakening of the Hadley cells, with ''high confidence'' for the NH in boreal winter and ''low confidence'' for the SH in austral winter. The Hadley cells are projected to expand polewards with global warming, most notably in the SH ( ''high confidence'' ). There is currently ''low confidence'' in the impacts on regional precipitation in subtropical regions.

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