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

==== 8.3.2.3 Walker Circulation ====

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The AR5 concluded that the long-term weakening of the Pacific Walker circulation (WC) from the late 19th century to the 1990s has been largely offset by a recent strengthening ( ''high confidence'' ), though with ''low confidence'' in trends of the WC strength due to reanalysis uncertainties and large natural variability. The observed trends in the WC since 1980 are consistent with a ''very likely'' WC strengthening in the Pacific, similar to a La Niña pattern, with ''medium confidence'' in the magnitude of these changes due to differences between satellite observations and reanalyses.

The causes of the observed strengthening of the WC during 1980 – 2014 are not well understood due to competing influences from individual external forcings and since this strengthening is outside the range of variability simulated in coupled models ( ''medium confidence'' ), as assessed in [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.1|Section 3.3.3.1]] ). Recent strengthening in the WC has been linked with internal variability ( [[#Chung--2019|Chung et al., 2019]] ), although one study argues that it could be a response forced by GHG that models do not capture because of common sea surface temperature (SST) biases in the equatorial Pacific ( [[#Seager--2019a|Seager et al., 2019a]] ). It could be also related to an interbasin thermostat mechanism whereby the human-induced Indian Ocean warming emerged earlier than in the tropical Pacific (L. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ) and induced a transient strengthening of the zonal sea level pressure gradient and easterly trades in the tropical Pacific (L. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ).

The weakening of the PWC observed during most of the 20th century is associated with reductions in land rainfall over the Maritime Continent during 1950 – 1999 ( [[#Tokinaga--2012|Tokinaga et al., 2012]] ; [[#Yoden--2017|Yoden et al., 2017]] ). In contrast, the recent strengthening of the WC has been associated with an intensification of extreme flooding ( [[#Barichivich--2018|Barichivich et al., 2018]] ) and an increased frequency of wet days ( [[#Espinoza--2016|J.C. Espinoza et al., 2016]] , 2018) over the north-western Amazon, increased precipitation in South America ( [[#Yim--2017|Yim et al., 2017]] ), reduced precipitation over eastern Africa ( [[#Williams--2011|Williams and Funk, 2011]] ; [[#Lyon--2012|Lyon and Dewitt, 2012]] ), and increased rainfall in southern Africa ( [[#Maidment--2015|Maidment et al., 2015]] ). Internal variability has been shown to have a dominant role in the recent strengthening of the WC ( [[#Chung--2019|Chung et al., 2019]] ).

In summary, there is ''high confiden'' ce that changes in the WC are associated with changes in the water cycle over regions like the Maritime Continent, South America and Africa. It is ''very likely'' that the WC has strengthened in the Pacific since the 1980s, with ''medium confidence'' that this strengthening is within the range of internal variability.

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