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

==== 8.3.2.2 Hadley Circulation and Subtropical Belt ====

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The AR5 reported ''low confidence'' in trends in the strength of the Hadley circulation (HC) due to uncertainties in reanalyses but ''high confidence'' on the widening of the tropical belt since 1979. In AR6, [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.1|Section 2.3.1.4.1]] ) states that the HC has ''very likely'' widened and strengthened since at least the 1980s, mostly in the NH ( ''medium con'' ''fidence'' ).

The poleward shift of the HC is closely related to migration of the location of tropical cyclone trajectories in bothhemispheres ( [[#Sharmila--2018|Sharmila and Walsh, 2018]] ; [[#Studholme--2018|Studholme and Gulev, 2018]] ), with a ''very likely'' poleward shift over the western North Pacific Oceans since the 1940s ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.2|Section 11.7.1.2]] ). Moreover, the Western North Pacific Subtropical High has extended westward since the 1970s, resulting in a monsoon rain band shift over China, with excessive rainfall along the middle and lower reaches of the Yangtze River valley along about 30°N over eastern China. At the same time, the effect of anthropogenic aerosols dominated the response to GHG increases over East Asia, resulting in a weakening of the East Asian summer monsoon and causing a drying trend in north-eastern China ( [[#Hu--2003|Hu, 2003]] ; [[#Yu--2007|Yu and Zhou, 2007]] ; T. [[#Wang--2013|]] [[#Wang--2013|]] [[#Wang--2013|]] [[#Wang--2013|Wang et al., 2013]] ; Z. [[#Li--2016|Li et al., 2016]] b; [[#Lau--2017|Lau and Kim, 2017]] ) and northern parts of South Asia ( [[#8.3.2.4.2|Section 8.3.2.4.2]] ; [[#Preethi--2017|Preethi et al., 2017]] ). During 1977 – 2007, the precipitation variability over the eastern USA increased due to changes in the intensity and position of the western ridge of the North Atlantic Subtropical High ( [[#Li--2011|Li et al., 2011]] ; [[#Diem--2013|Diem, 2013]] ).

In the Southern Hemisphere (SH), the HC expansion has been associated with both the intensification and poleward shift of the subtropical high pressure belt ( [[#Nguyen--2015|Nguyen et al., 2015]] ), with consequences for precipitation amount over Africa, Australia, South America, and subtropical Pacific islands (Cai et al. , 2012; Grose et al. , 2015; Nguyen et al. , 2015; [[#Sharmila--2018|Sharmila and Walsh, 2018]] ; McGree et al. , 2019). The subtropical ridge in Australia has intensified significantly since 1970, with marked declines observed in April to October rainfall across south-eastern and south-western Australia ( [[#Timbal--2013|Timbal and Drosdowsky, 2013]] ).

The local tropical edges of the meridional overturning cells (as diagnosed from the horizontally divergent wind) are more closely associated with hydroclimate variations than the subtropical ridge ( [[#Staten--2019|Staten et al., 2019]] ). Poleward expansion of the tropical belt strongly contributes to precipitation decline in the poleward edge of the subtropics ( [[#Cai--2012|Cai et al., 2012]] ; [[#Scheff--2012|Scheff and Frierson, 2012]] ; [[#Timbal--2013|Timbal and Drosdowsky, 2013]] ; [[#He--2017|He and Soden, 2017]] ; H. [[#Nguyen--2018|]] [[#Nguyen--2018|Nguyen et al., 2018]] ; [[#Tang--2018|Tang et al., 2018]] ), although recent modelling evidence suggests that subtropical precipitation declines are a response to direct CO <sub>2</sub> radiative forcing mainly over ocean, irrespective of the HC expansion ( [[#He--2017|He and Soden, 2017]] ). Both reanalyses datasets and climate model simulations suggest that the HC expansion is not associated with widespread, zonally symmetric subtropical drying over land ( [[#Schmidt--2017|Schmidt and Grise, 2017]] ).

Since AR5, an improved understanding of the key drivers of the recent HC expansion has been achieved, identifying the role of both internal variability and anthropogenic climate change. Part of the recent expansion (1979 – 2005) of the HC has been driven by a swing from warm to cold phase of the Pacific Decadal Variability (PDV; [[#Meehl--2016|Meehl et al., 2016]] ; [[#Grise--2019|Grise et al., 2019]] ). The presence of large multi-decadal variability in 20th-century reanalyses means there is ''limited evidence'' on the human influence on the recent HC strengthening, yet the southward shift of the southern edge and widening of the SH HC appeared as robust features in all reanalysis datasets, and their trends have accelerated during 1979 – 2010 ( [[#D’Agostino--2017|D’Agostino and Lionello, 2017]] ). As assessed in [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.1|Section 3.3.3.1]] , GHG increases and stratospheric ozone depletion have contributed to the expansion of the zonal mean HC in the SH since around 1980, and the expansion of the NH HC has not exceeded the range of internal variability ( ''medium confidence'' ). Moreover, Antarctic ozone depletion can cause a poleward shift in the SH mid-latitude jet and HC (Sections 3.3.3 and 6.3.3.2). Further assessment of the attribution of recently observed changes in the HC extent and intensity is found in [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.1|Section 3.3.3.1]] .

In summary, it is ''very likely'' that the recent HC expansion was associated with poleward shifts of tropical cyclone tracks over the western North Pacific Ocean since the 1940s, and of extratropical storm tracks in the SH since the 1970s. Changes to the HC in the NH may have contributed to subtropical drying and a poleward expansion of aridity during the boreal summer, but there is ''low confidence'' due to ''limited evidence'' . GHG increases and stratospheric ozone depletion have contributed to expansion of the zonal mean HC in the SH since around 1970, while the expansion of the NH HC has not exceeded the range of internal variability ( ''medium co'' ''nfidence'' ).

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