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

==== 3.3.3.1 The Hadley and Walker Circulations ====

<div id="h3-9-siblings" class="h3-siblings"></div>

The tropical tropospheric circulation features meridional and zonal overturning circulations, called Hadley and Walker circulations. In the zonal mean, the downwelling branch of the Hadley circulation cell is located in the subtropics and is often used as an indicator of the meridional extent of the tropics. In the equatorial zonal-vertical section, the major rising branch of the Walker circulation is located over the Maritime continent with secondary ascending regions over northern South America and Africa. The zonal component of the surface trade winds over most of the equatorial Pacific and Atlantic is associated with the Walker circulation. This section assesses the zonal-mean Hadley cell extent and the Pacific Walker circulation strength. Regional and water cycle aspects of these circulations are assessed in more detail in Section 8.3.2.

AR5 found ''medium confidence'' that the depletion of stratospheric ozone had contributed to Hadley cell widening in the Southern Hemisphere in austral summer ( [[#Bindoff--2013|Bindoff et al., 2013]] ). It also noted that in contrast to a simulated weakening in response to greenhouse gas forcing, the Walker circulation had actually strengthened since the early 1990s, precluding any detection of human influence.

<div id="3.3.3.1.1" class="h4-container"></div>

<span id="hadley-cell-extent"></span>
===== 3.3.3.1.1 Hadley cell extent =====

<div id="h4-5-siblings" class="h4-siblings"></div>

[[#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]] ).

<div id="_idContainer041" class="•-2-columns"></div>

[[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.

<div id="3.3.3.1.2" class="h4-container"></div>

<span id="walker-circulation-strength"></span>
===== 3.3.3.1.2 Walker circulation strength =====

<div id="h4-6-siblings" class="h4-siblings"></div>

CMIP5 models reproduce the mean state of the Walker circulation with reasonable fidelity, evidenced by the spatial pattern correlations of equatorial zonal mass stream function between models and observations being larger than 0.88 ( [[#Ma--2016|Ma and Zhou, 2016]] ). CMIP5 historical simulations on average simulate a significant weakening of the Pacific Walker circulation over the 20th century ( [[#DiNezio--2013|DiNezio et al., 2013]] ; [[#Sandeep--2014|Sandeep et al., 2014]] ; [[#Kociuba--2015|Kociuba and Power, 2015]] ), which is also seen in CMIP6 (Figure 3.16d). This weakening is accompanied by a reduction of convective activity over the Maritime Continent and an enhancement over the central equatorial Pacific ( [[#DiNezio--2013|DiNezio et al., 2013]] ; [[#Sandeep--2014|Sandeep et al., 2014]] ; [[#Kociuba--2015|Kociuba and Power, 2015]] ). In the CMIP6 simulations, greenhouse gas forcing induces this weakening (Figure 3.16d), which is consistent with theories based on radiative-convective equilibrium ( [[#Vecchi--2006|Vecchi et al., 2006]] ; [[#Vecchi--2007|Vecchi and Soden, 2007]] ) and thermodynamic air-sea coupling ( [[#Xie--2010|Xie et al., 2010]] ), but inconsistent with a theory highlighting the ocean dynamical effect which suggests a strengthening in response to greenhouse gas increases ( [[#Clement--1996|Clement et al., 1996]] ; [[#Seager--2019|Seager et al., 2019]] ; see also Section 7.4.4.2.1). [[#Seager--2019|Seager et al. (2019)]] attributed this inconsistency to equatorial Pacific SST biases in the models ( [[#3.5.1.2.1|Section 3.5.1.2.1]] ). However, observational and reanalysis datasets disagree on the sign of trends in the Walker Circulation strength over the 1901–2010 period (Figure 3.16d), and [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.1|Section 2.3.1.4.1]] assesses ''low confidence'' in observed long-term Walker Circulation trends. The observational uncertainty remains high in the trends since the 1950s ( [[#Tokinaga--2012|Tokinaga et al., 2012]] ; [[#L’Heureux--2013|L’Heureux et al., 2013]] ), though both CMIP5 and CMIP6 historical simulations span trends of all but one observational data set (Figure 3.16e). For this period, external influence simulated in CMIP6 is insignificant due to a partial compensation of forced responses to greenhouse gases and aerosols and large internal decadal variability (Figure 3.16e). It is notable that while AMIP simulations on average show strengthening over both the periods, those simulations are forced by one reconstruction of SST, which itself is subject to uncertainty before the 1970s ( [[#Deser--2010|Deser et al., 2010]] ; [[#Tokinaga--2012|Tokinaga et al., 2012]] ).

Observational SST products indicate that the equatorial zonal SST gradient from the western to the eastern equatorial Pacific has strengthened since 1870 (Section 7.4.4.2.1). While CMIP5 historical simulations on average simulate a weakening, large ensemble simulations span the observed strengthening since the 1950s ( [[#Watanabe--2021|Watanabe et al., 2021]] ) suggesting an important contribution from internal variability. [[#Coats--2017|Coats and Karnauskas (2017)]] also find that the anthropogenic influence on the SST gradient is yet to emerge out of internal variability even on centennial time scales.

Trends since the 1980s in in-situ and satellite observations and reanalyses exhibit strengthening of the Pacific Walker circulation and SST gradient ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.1|Section 2.3.1.4.1]] and Figure 3.16f; L’Heureux et al., 2013; [[#Boisséson--2014|Boisséson et al., 2014]] ; [[#England--2014|England et al., 2014]] ; [[#Kociuba--2015|Kociuba and Power, 2015]] ; [[#Ma--2016|Ma and Zhou, 2016]] ). AMIP simulations reproduce this strengthening (Figure 3.16d; [[#Boisséson--2014|Boisséson et al., 2014]] ; [[#Ma--2016|Ma and Zhou, 2016]] ), indicating a dominant role of SST changes. However, all reanalysis trends lie outside the 5–95% range of simulated CMIP6 historical Walker circulation trends over this period (Figure 3.16f), consistent with CMIP5 results ( [[#England--2014|England et al., 2014]] ; [[#Kociuba--2015|Kociuba and Power, 2015]] ). This may be in part caused by the underestimation of the PDV magnitude especially in CMIP5 models (Section [[#_idTextAnchor002|3.7.6]] ; [[#Kociuba--2015|Kociuba and Power, 2015]] ; [[#Chung--2019|Chung et al., 2019]] ), but also suggests a potential error in simulating the forced changes of the Walker circulation. Specifically, anthropogenic and volcanic aerosol changes over this period may have driven a strengthening ( [[#DiNezio--2013|DiNezio et al., 2013]] ; [[#Takahashi--2016|Takahashi and Watanabe, 2016]] ; [[#Hua--2018|Hua et al., 2018]] ). This aerosol influence may be indirect via Atlantic Multi-decadal Variability (AMV; Annex IV.2.7) through inter-basin teleconnections ( [[#McGregor--2014|McGregor et al., 2014]] ; [[#Chikamoto--2016|Chikamoto et al., 2016]] ; [[#Kucharski--2016|Kucharski et al., 2016]] ; X. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] a; [[#Ruprich-Robert--2017|Ruprich-Robert et al., 2017]] ), which may be underestimated in models due to SST biases in the equatorial Atlantic ( [[#3.5.1.2.2|Section 3.5.1.2.2]] ; [[#McGregor--2018|McGregor et al., 2018]] ). Note also the large uncertainty in aerosol influence on the Walker circulation ( [[#Kuntz--2016|Kuntz and Schrag, 2016]] ; [[#Hua--2018|Hua et al., 2018]] ; [[#Oudar--2018|Oudar et al., 2018]] ), which is also seen in CMIP6 (Figure 3.16f).

Paleoclimate data from the Pliocene epoch suggest that there was a reduction in the zonal SST gradient in the tropical Pacific under a similar CO <sub>2</sub> concentration as today (Section 7.4.4.2.2 and Cross-Chapter Box 2.4). [[#Tierney--2019|Tierney et al. (2019)]] found that this weaker gradient compared to pre-industrial, which suggests a weaker Walker circulation, is captured by climate models under Pliocene CO <sub>2</sub> levels, in agreement with the CMIP6 response to greenhouse gas forcing (Figure 3.16d), though the magnitude of this effect varies strongly between models ( [[#Corvec--2017|Corvec and Fletcher, 2017]] ).

<div id="3.3.3.1.3" class="h4-container"></div>

<span id="summary"></span>
===== 3.3.3.1.3 Summary =====

<div id="h4-7-siblings" class="h4-siblings"></div>

It is ''likely'' that human influence has contributed to the poleward expansion of the zonal mean Hadley cell in the Southern Hemisphere since the 1980s. This assessment is supported by studies since AR5, which consistently find human influence from greenhouse gas increases on the expansion, with additional influence from ozone depletion in austral summer. For the strong ozone depletion period of 1981–2000, human influence is detectable in the summertime poleward expansion in the Southern Hemisphere ( ''medium confidence'' ). By contrast, there is ''medium confidence'' that the expansion of the zonal mean Hadley cell in the Northern Hemisphere is within the range of internal variability, with contributions from PDV and other internal variability. The causes of the observed strengthening of the Pacific Walker circulation over the 1980–2014 period are not well understood, since the observed strengthening trend is outside the range of variability simulated in the coupled models ( ''medium confidence'' ). Large observational uncertainty, lack of understanding of the mechanism underlying the poleward Hadley cell expansion, and contradicting theories on the greenhouse gas influence and uncertainty in the aerosol influence on the Walker circulation strength, limit confidence in these assessments.

<div id="3.3.3.2" class="h3-container"></div>

<span id="global-monsoon"></span>