Editing IPCC:AR6/WGI/Chapter-3 (section)

===== 3.3.3.1.1 Hadley cell extent =====

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[[#Grise--2019|Grise et al. (2019)]] found that a metric based on surface zonal winds, which are well constrained by surface observations, best compares reanalyses with CMIP5 models. With this method and new reanalysis products, the CMIP5 historical simulations exhibit comparable mean states and variability of the subtropical edge latitude of the Hadley cells to those observed ( [[#Grise--2019|Grise et al., 2019]] ).

( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] assesses that there has ''very likely'' been a widening of the Hadley circulation since the 1980s ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.1|Section 2.3.1.4.1]] ). The CMIP5 ( [[#Davis--2017|Davis and Birner, 2017]] ; [[#Grise--2018|Grise et al., 2018]] ) and CMIP6 ( [[#Grise--2020|Grise and Davis, 2020]] ) historical simulation ensembles span the observed trends of the zonal-mean Hadley cell edges since the 1980s (Figure 3.16a–c). Studies based on CMIP5 models find a contribution from human influence to the observed widening trend, especially in the Southern Hemisphere ( [[#Gerber--2014|Gerber and Son, 2014]] ; [[#Staten--2018|Staten et al., 2018]] , 2020; [[#Grise--2019|Grise et al., 2019]] ; [[#Jebri--2020|Jebri et al., 2020]] ), which is confirmed based on CMIP6 (Figure 3.16b,c; [[#Grise--2020|Grise and Davis, 2020]] ).

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[[File:a1a79ce037eeab534e19613ebdd8b703 IPCC_AR6_WGI_Figure_3_16.png]]

Figure 3.16 | '''Model evaluation and attribution of changes in Hadley cell extent and Walker circulation strength. (a–c)''' Trends in subtropical edge latitude of the Hadley cells in '''(a)''' the Northern Hemisphere for 1980–2014 annual means and '''(b, c)''' Southern Hemisphere for '''(b)''' 1980–2014 annual means and '''(c)''' 1980/81–1999/2000 December–January–February means. Positive values indicate northward shifts. '''(d–f)''' Trends in the Pacific Walker circulation strength for '''(d)''' 1901–2010, '''(e)''' 1951–2010 and '''(f)''' 1980–2014. Positive values indicate strengthening. Based on CMIP5 historical (extended with RCP4.5), CMIP6 historical, AMIP, pre-industrial control, and single forcing simulations along with HadSLP2 and reanalyses. Pre-industrial control simulations are divided into non-overlapping segments of the same length as the other simulations. White boxes and whiskers represent means, interquartile ranges and 5th and 95th percentiles, calculated after weighting individual members with the inverse of the ensemble size of the same model, so that individual models are equally weighted ( [[#3.2|Section 3.2]] ). The filled boxes represent the 5–95% confidence interval on the multi-model mean trends of the models with at least three ensemble members, with dots indicating the ensemble means of individual models. The edge latitude of the Hadley cell is where the surface zonal wind velocity changes sign from negative to positive, as described in the Appendix of [[#Grise--2018|Grise et al. (2018)]] . The Pacific Walker circulation strength is evaluated as the annual mean difference of sea level pressure between 5°S–5°N, 160°W–80°W and 5°S–5°N, 80°E–160°E. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

In the annual mean, internal variability, including Pacific Decadal Variability (PDV; Annex IV.2.6), contributed to the observed zonal-mean Hadley cell expansion since 1980 comparably with human influence ( [[#Allen--2014|Allen et al., 2014]] ; [[#Allen--2017|Allen and Kovilakam, 2017]] ; [[#Mantsis--2017|Mantsis et al., 2017]] ; [[#Amaya--2018|Amaya et al., 2018]] ; [[#Grise--2018|Grise et al., 2018]] ). Indeed, the ensemble-mean expansion in historical simulations is significantly weaker than in most of the reanalyses shown in Figure 3.16a–c, while the Atmospheric Model Intercomparison Project (AMIP) simulations forced by observed SSTs (Figure 3.16a–c) show stronger trends than historical coupled simulations on average ( [[#Nguyen--2015|Nguyen et al., 2015]] ; [[#Davis--2017|Davis and Birner, 2017]] ; [[#Grise--2018|Grise et al., 2018]] ). The human-induced change has not yet clearly emerged out of the internal variability range in the Northern Hemisphere ( [[#Quan--2018|Quan et al., 2018]] ; [[#Grise--2019|Grise et al., 2019]] ), whereas the trend in the annual-mean Southern Hemisphere edge is outside the 5th–95th percentile range of internal variability in CMIP6 in three out of the four reanalyses (Figure 3.16b). For the Southern Hemisphere summer when the simulated human influence is strongest, the 1981–2000 trend in three out of the four reanalyses falls outside the 5th–95th percentile range of internal variability (Figure 3.16c; L. [[#Tao--2016|]] [[#Tao--2016|Tao et al., 2016]] ; [[#Grise--2018|Grise et al., 2018]] , 2019).

In CMIP5 simulations, greenhouse gas increases and, in austral summer, stratospheric ozone depletion, contribute to the Southern Hemisphere expansion ( [[#Gerber--2014|Gerber and Son, 2014]] ; [[#Nguyen--2015|Nguyen et al., 2015]] ; L. [[#Tao--2016|]] [[#Tao--2016|Tao et al., 2016]] ; Y.H. [[#Kim--2017|]] [[#Kim--2017|Kim et al., 2017]] ), but the ozone influence is not significant in available CMIP6 simulations (Figure 3.16b–c). Since the 2000s, the stabilization or slight recovery of stratospheric ozone ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.5.2|Section 2.2.5.2]] ) is consistent with the smaller observed trends ( [[#Banerjee--2020|Banerjee et al., 2020]] ). While many CMIP5 models under-represent the magnitude of the PDV, implying potential overconfidence on the detection of human influence on the Hadley cell expansion, this is less the case for the CMIP6 models ( [[#3.7.6|Section 3.7.6]] ). However, the mechanism underlying the Hadley cell expansion remains unclear ( [[#Staten--2018|Staten et al., 2018]] , 2020), precluding a process-based validation of the simulated human influence.

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