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

==== 8.4.2.3 Walker Circulation ====

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The AR5 determined that the Pacific Walker circulation was ''likely'' to slow down over the 21st century, which would lead to decreased precipitation over the western tropical Pacific and increases over the central and eastern Pacific. Recent studies show consistency with AR5 conclusions but also show an eastward shift over the Pacific, mostly due to a shift towards more ‘El Niño-like’ conditions under global warming ( [[#Bayr--2014|Bayr et al., 2014]] ). Other studies suggest that the weakening of the Walker circulation is related to the response of the western North Pacific monsoon and to changing land–sea temperature contrasts, while a positive ocean–atmosphere feedback amplifies the weakening of both east–west SST gradient and trade winds in the tropical Pacific (Zhang and Li, 2017).

Since AR5, the paradox between the projected weakening and the observed strengthening of the Walker circulation since the 1990s ( [[#8.3.2.2|Section 8.3.2.2]] ) has triggered debate about the drivers of these changes (England et al. , 2014; McGregor et al. , 2014; [[#Kociuba--2015|Kociuba and Power, 2015]] ; Vilasa et al. , 2017; Chung et al. , 2019) . Projected changes in equatorial SST gradients are not entirely consistent with observed trends (Coats and Karnauskas, 2017; [[#Seager--2019a|Seager et al., 2019a]] ), and one CMIP5 model that projects a future strengthening of the Walker circulation is more consistent with observations than other models (Kohyama et al., 2017). Other studies suggest that these differences arise from the dominant influence of internal climate variability to the observed trends ( [[#Chung--2019|Chung et al., 2019]] ), or as a consequence of a systematic cold bias of most CMIP5 models in their Equatorial Pacific cold tongues ( [[#Seager--2019a|Seager et al., 2019a]] ). However, the latter hypothesis is based on a simplified model of tropical Pacific dynamics and is not consistent with the current physical understanding of the tropical circulation response to increasing CO <sub>2</sub> levels ( [[#8.2.2.2|Section 8.2.2.2]] ) or with independent paleoclimate evidence suggesting a weaker Walker circulation under warmer climates ( [[#Tierney--2019|Tierney et al., 2019]] ; [[#McClymont--2020|McClymont et al., 2020]] ). Different time scales of the tropical Pacific responses to global warming have been highlighted by numerical experiments with both comprehensive and simplified models. Results suggest a transient strengthening of the Walker circulation related to Indian Ocean warming (L. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ), followed by a slower weakening linked to a strengthened eastern Pacific cold tongue warming emerging after 50 – 100 years ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.2.1|Section 7.4.4.2.1]] ; [[#Heede--2020|Heede et al., 2020]] ).

CMIP6 projections provide further evidence of a significant long-term weakening of the Walker circulation (Figure 8.21). For instance, a pronounced weakening of the upper-level tropical easterly jet is projected both over the Indian Ocean and tropical eastern Pacific, where declines are projected to exceed 70% by 2100 in the high-emissions SSP5-8.5 scenario (S. [[#Huang--2020|Huang et al., 2020]] ). CMIP6 models agree on a future decrease of the equatorial zonal temperature gradient ( [[#Fredriksen--2020|Fredriksen et al., 2020]] ), which can lead to weaker trade winds over the tropical Pacific. However, CMIP6 models show a diversity of SST warming patterns in the tropical Pacific ( [[#Freund--2020|Freund et al., 2020]] ), which contributes to uncertainties in the response of both Walker circulation and ENSO to continued warming.

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[[File:1db82c46f3cd13224b03acaa27e2a877 IPCC_AR6_WGI_Figure_8_21.png]]

'''Figure 8.21 |''' '''Schematic depicting large-scale circulation changes and impacts on the regional water cycle.''' The central figures show precipitation minus evaporation (P–E) changes at 3°C or global warming relative to an 1850–1900 base period (mean of 23 CMIP6 SSP5-8.5 simulations). Annual mean changes (large map) include contours (ocean only) depicting control climate P–E = 0 mm day – 1 lines with the solid contour enclosing the tropical rain belt region and dashed lines representing the edges of subtropical regions. Confidence levels assess understanding of how large-scale circulation change affect the regional water.

In summary, there is ''high confidence'' that the Pacific Walker circulation will weaken by the end of the 21st century, and will be associated with decreased precipitation over the western tropical Pacific and increases farther east. Discrepancies between observed and simulated changes in SSTs in the tropics indicate that a temporary strengthening of the Walker Circulation can arise from a transient response to GHG radiative forcing ( ''low confidence'' ) and from internal variability ( ''medium co'' ''nfidence'' ).

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