Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.1.4.1 The Hadley and Walker circulations =====

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The AR5 reported ''low confidence'' in trends in the strength of the Hadley circulation (HC) and the Walker circulation (WC) due to uncertainties in available reanalysis datasets and the large interannual-to-decadal variability of associated circulation patterns. However, AR5 indicated a ''likely'' widening of the tropical belt since the 1970s, albeit with large uncertainty in the magnitude of this change. There was ''high confidence'' that the post-1990s strengthening of the Pacific WC reversed its weakening observed from the mid-19th century to the 1990s.

Paleo reconstructions of rainfall and trade winds extending over the last 100 kyr show an intensification of the NH HC concurrently with a weakening of the SH HC and a southward shift of the inter tropical convergence zone (ITCZ) during Heinrich stadials ( [[#Deplazes--2013|Deplazes et al., 2013]] ; [[#McGee--2018|McGee et al., 2018]] ; [[#Stríkis--2018|Stríkis et al., 2018]] ; [[#Wendt--2019|Wendt et al., 2019]] ). An intensification of the HC associated with conditions similar to La Niña (northward migrations of both the ITCZ and the SH westerlies) was found in reconstructions for the MH ( [[#McGee--2014|McGee et al., 2014]] ; [[#Mollier-Vogel--2019|Mollier-Vogel et al., 2019]] ). Changes in insolation from the mid to late Holocene favoured a southward migration in the position of the ITCZ and the descending branch of the HC in the NH, approaching its current width and position ( [[#Wirth--2013|Wirth et al., 2013]] ; [[#Thatcher--2020|Thatcher et al., 2020]] ). Tree ring chronologies from the NH mid-latitudes over the last 800 years show that the northern edge of the HC tended to migrate southward during positive phases of ENSO and PDV, with northward shifts during negative phases ( [[#Alfaro-Sánchez--2018|Alfaro-Sánchez et al., 2018]] ). Between 1400 and 1850 CE the HC over both hemispheres and the ITCZ were displaced southward, consistent with occurrence of drought conditions in several NH regions ( [[#Wirth--2013|Wirth et al., 2013]] ; [[#Burn--2014|Burn and Palmer, 2014]] ; [[#Lechleitner--2017|Lechleitner et al., 2017]] ; [[#Alfaro-Sánchez--2018|Alfaro-Sánchez et al., 2018]] ; [[#Flores-Aqueveque--2020|Flores-Aqueveque et al., 2020]] ). Moreover, several proxy records showed not only inter-hemispheric shifts in the ITCZ but a contraction of the tropical belt during 1400–1850 CE, which followed an expansion during 950–1250 CE ( [[#Denniston--2016|Denniston et al., 2016]] ; [[#Griffiths--2016|Griffiths et al., 2016]] ).

From centennial-scale reanalyses, [[#Liu--2012|Liu et al. (2012)]] and [[#D’Agostino--2017|D’Agostino and Lionello (2017)]] found divergent results on HC extent over the last 150 years, although with unanimity upon an intensification of the SH HC. A substantial discrepancy between HC characteristics in centennial-scale reanalyses and in ERA-Interim ( [[#D’Agostino--2017|D’Agostino and Lionello, 2017]] ) since 1979 yields significant questions regarding their ability to capture changes in HC behaviour. Taken together with the existence of apparent non-climatic artefacts in the datasets ( [[#Nguyen--2015|Nguyen et al., 2015]] ), this implies ''low confidence'' in changes in the extent and intensity of HC derived from centennial-scale reanalyses. However, using multiple observational datasets and centennial-scale reanalyses, [[#Bronnimann--2015|Bronnimann et al. (2015)]] identified a southward shift in the NH HC edge from 1945 to 1980 of about 0.25° latitude per decade, consistent with observed changes in global land monsoon precipitation ( [[#2.3.1.4.2|Section 2.3.1.4.2]] ).

Since AR5 several studies based upon a range of metrics and different reanalyses products have suggested that the annual mean HC extent has shifted poleward at an approximate rate of 0.1°–0.5° latitude per decade over the last about 40 years ( [[#Allen--2017|Allen and Kovilakam, 2017]] ; [[#Davis--2017|Davis and Birner, 2017]] ; [[#Grise--2018|Grise et al., 2018]] ; [[#Staten--2018|Staten et al., 2018]] , 2020; [[#Studholme--2018|Studholme and Gulev, 2018]] ; [[#Grise--2020|Grise and Davis, 2020]] ). The observed widening of the annual mean HC, revealed by a variety of metrics, is primarily due to poleward shift of the Northern Hemisphere HC. There have been stronger upward trends in the NH extent of HC after 1992 (Figure 2.17a). The estimated magnitude of the recent changes based on modern-era reanalyses is not as large as that in AR5, due to apparent biases in older-generation reanalyses ( [[#Grise--2019|Grise et al., 2019]] ). Moreover, large interannual variability leads to uncertainties in estimates of long-term changes ( [[#Nguyen--2013|Nguyen et al., 2013]] ; [[#Garfinkel--2015b|Garfinkel et al., 2015b]] ; [[#Seviour--2018|Seviour et al., 2018]] ; [[#Staten--2018|Staten et al., 2018]] ), particularly for the NH given its zonal asymmetries ( [[#Staten--2020|Staten et al., 2020]] ; [[#Wang--2020|Wang et al., 2020]] ). These large-scale features of the HC based on reanalyses agree with estimates revealed from the Integrated Global Radiosonde Archive (IGRA) during 1979–2012 ( [[#Lucas--2015|Lucas and Nguyen, 2015]] ; [[#Mathew--2016|Mathew et al., 2016]] ). Recent trends based on reanalyses indicate a larger seasonal widening in the HC for summer and autumn in each hemisphere, although the magnitude of changes in HC extent is strongly dependent on dataset and metrics used ( [[#Grise--2018|Grise et al., 2018]] ; Y. [[#Hu--2018|]] [[#Hu--2018|Hu et al., 2018]] ; [[#Staten--2018|Staten et al., 2018]] ). The shifts in the HC position were accompanied by a narrowing ITCZ over the Atlantic and Pacific basins, with no significant change in its location and increases in the precipitation intensity ( [[#Byrne--2018|Byrne et al., 2018]] ).

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[[File:edccb51f7c800aa7409e4500c5eec32e IPCC_AR6_WGI_Figure_2_17.png]]

'''Figure''' '''2.17 |''' '''Time series of the annual mean Northern Hemisphere (NH, top curves) and Southern Hemisphere (SH, bottom curves) Hadley cell extent (a) and Hadley cell intensity (b) since 1979.''' Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Trends in the HC intensity since 1979 differ between reanalyses, although there is a tendency toward HC intensification (Figure 2.17b; [[#Nguyen--2013|Nguyen et al., 2013]] ; [[#Chen--2014|Chen et al., 2014]] ; [[#D’Agostino--2017|D’Agostino and Lionello, 2017]] ; R. [[#Huang--2019|Huang et al., 2019]] ), which is more marked in the NH than the SH ( [[#Studholme--2018|Studholme and Gulev, 2018]] ). However, the ability of reanalyses to represent the HC strength has been questioned due to inaccurate representation of latent heating distribution, which is directly related to tropical convection and influences the HC dynamics ( [[#Chemke--2019|Chemke and Polvani, 2019]] ; [[#Mathew--2019|Mathew and Kumar, 2019]] ).

Paleo evidence during the LGM indicates a weaker WC over the Indian Ocean ( [[#DiNezio--2018|DiNezio et al., 2018]] ; [[#Windler--2019|Windler et al., 2019]] ) with a stronger Pacific WC ( [[#DiNezio--2013|DiNezio and Tierney, 2013]] ). During the Holocene, a transition from a strong WC located more westward during the Early-to-Mid Holocene towards a weak and eastward shifted WC during the late Holocene was inferred from proxy records from the Pacific Warm Pool and South East Asia ( [[#Barr--2019|Barr et al., 2019]] ; [[#Dang--2020|Dang et al., 2020]] ; [[#Griffiths--2020|Griffiths et al., 2020]] ), in concurrence with changes in ENSO activity ( [[#2.4.2|Section 2.4.2]] ). Reconstructions for the CE showed weakened WC during 1000–1250 and since 1850, with an intensified circulation during 1500–1850 CE ( [[#Xu--2016|Xu et al., 2016]] ; [[#Deng--2017|Deng et al., 2017]] ).

Considering instrumental records, there is considerable interdecadal variability in the strength of the WC, resulting in time-period dependent magnitude and even sign of trends ( [[#Carilli--2015|Carilli et al., 2015]] ; [[#Bordbar--2017|Bordbar et al., 2017]] ; [[#Hou--2018|Hou et al., 2018]] ), with some studies reporting weakening over the 20th century (e.g., [[#Power--2011|Power and Kociuba, 2011]] ; [[#Liu--2019|Liu et al., 2019]] ), while others reported strengthening (Z. [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ), particularly over the last 30–40 years (e.g., [[#Hu--2013|Hu et al., 2013]] ; [[#L’Heureux--2013|L’Heureux et al., 2013]] ; [[#Yim--2017|Yim et al., 2017]] ). Based on estimation of changes in mid-tropospheric velocity from changes in observed cloud cover, [[#Bellomo--2015|Bellomo and Clement (2015)]] suggest a weakening and eastward shift of the WC over 1920–2010, however the robustness of this signal is questionable due to high uncertainty in the ship-reported cloud data used before 1954. Using centennial-scale 20CR reanalysis [[#Tseng--2019|Tseng et al. (2019)]] showed that the vertical westerly wind shear over the western Pacific does not indicate any long-term change during 1900–1980, but shows a marked increase since the 1980s that is not present in ERA-Interim and JRA-55, again calling into question the ability of centennial-scale reanalyses to capture tropical circulation changes. Recent strengthening together with a westward shift of the WC ( [[#Bayr--2014|Bayr et al., 2014]] ; [[#Ma--2016|Ma and Zhou, 2016]] ) was identified across several reanalysis products and observational datasets, and using different metrics for quantifying WC. Nevertheless, satellite observations of precipitation and analyses of upper tropospheric humidity suggest substantially weaker strengthening of the WC than implied by reanalyses ( [[#Chung--2019|Chung et al., 2019]] ). This recent strengthening in the WC is associated with enhanced precipitation in the tropical western Pacific, anomalous westerlies in the upper troposphere, strengthened downwelling in the central and eastern tropical Pacific, and anomalous surface easterlies in the western and central tropical Pacific ( [[#Dong--2013|Dong and Lu, 2013]] ; [[#McGregor--2014|McGregor et al., 2014]] ; [[#Choi--2016|Choi et al., 2016]] ). Positive trends in sea level pressure over the eastern Pacific and concurrent negative trends over the Indonesian region result in a pattern implying a shift towards a La Niña-like WC regime, with strengthening of the Pacific Trade Winds mainly over 1979–2012 ( [[#L’Heureux--2013|L’Heureux et al., 2013]] ; [[#England--2014|England et al., 2014]] ; [[#Sohn--2016|Sohn et al., 2016]] ; [[#Zhao--2019|Zhao and Allen, 2019]] ). Seasonal assessment of the WC showed significant changes in the vertical westerly wind shear over the Pacific during the austral summer and autumn implying a strengthening ( [[#Clem--2017|Clem et al., 2017]] ).

In summary, there has been a ''likely'' widening of the Hadley circulation since the 1980s, mostly due to its extension in the NH, although there is only ''medium confidence'' in the extent of the changes. This has been accompanied by a strengthening of the Hadley circulation, particularly in the NH ( ''medium confidence'' ). There is ''low confidence'' in the estimation of long-term trends in the strength of the Walker circulation, which are time period dependent and subject to dataset uncertainties. Trends since 1980 are better characterized and consistent with a ''very likely'' strengthening that resembles a La Niña-like Walker circulation and a westward shift of the Walker circulation, although with ''medium confidence'' in the magnitude of the changes, arising from the differences between satellite observations and reanalysis products.

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