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

===== 3.3.3.1.2 Walker circulation strength =====

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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]] ).

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