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

==== 8.3.2.1 Inter-tropical Convergence Zone and Tropical Rain Belts ====

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The AR5 concluded it is ''likely'' that the tropical belt, as delimited by the Hadley circulation, has widened since the 1970s. Observations in the satellite era indicate precipitation increases in the core of the Pacific Inter-tropical Convergence Zone (ITCZ) and decreases on the ITCZ margins ( [[#Gu--2016|Gu et al., 2016]] ; [[#Su--2017|Su et al., 2017]] ). As the satellite period has lengthened, observations have increasingly been used to assess trends in the ITCZ and tropical rain belt. Since AR5, significant narrowing and strengthening of the Pacific ITCZ after 1979 have been identified in atmospheric reanalyses ( [[#Wodzicki--2016|Wodzicki and Rapp, 2016]] ), but no change in the ITCZ location ( [[#Byrne--2018|Byrne et al., 2018]] ). Atmospheric model simulations suggest that with a narrower ITCZ, the subtropical jet becomes baroclinically unstable at a lower latitude and allows mid-latitude eddies to propagate farther equatorward ( [[#Watt-Meyer--2019|Watt-Meyer and Frierson, 2019]] ). Observational analyses also show that the ITCZ narrowing ( [[#Zhou--2020|Zhou et al., 2020]] ) is associated with increased precipitation in the ITCZ core region that is strongly coupled to increasing Outgoing Longwave Radiation (OLR) in the expanding dry zones, particularly over land regions in the subtropics and mid-latitudes ( [[#Lau--2020|Lau and Tao, 2020]] ). In addition, an eastward movement of the South Pacific Convergence Zone (SPCZ) between 1977 and 1999 has been reported, with associated significant precipitation trends in the South Pacific regions ( [[#Salinger--2014|Salinger et al., 2014]] ).

ITCZ trends seen in satellites, precipitation measurements and reanalysis data are further supported by ocean surface-salinity observations. Long-term salinity observations show a freshening in the cores of the Atlantic and Pacific ITCZs and increased salinity on the ITCZ margins ( [[#Durack--2010|Durack and Wijffels, 2010]] ; [[#Durack--2012|Durack et al., 2012]] ; [[#Terray--2012|Terray et al., 2012]] ; [[#Skliris--2014|Skliris et al., 2014]] ). By investigating simultaneous changes in precipitation, temperature and continental aridity in CMIP5 historical simulations, [[#Bonfils--2020|Bonfils et al. (2020)]] found a secondary signal (Figure 8.9, right column) characterized by a robust inter-hemispheric temperature contrast ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1.1|Section 3.3.1.1]] ), a latitudinal shift in the ITCZ (in accordance with the theory of cross-equatorial energy transport; [[#8.2.2.2|Section 8.2.2.2]] ), and changes in aridity in the Sahel ( [[#8.3.1.6|Section 8.3.1.6]] ). These forced changes are statistically detectable in reanalyses datasets over the 1950 – 2014 period at the 95% confidence level.

Reconstructions in the Sahel ( [[#Carré--2019|Carré et al., 2019]] ) and Belize ( [[#Ridley--2015|Ridley et al., 2015]] ) support the southward displacement of the tropical rain belt since 1850 and the narrowing trend of the tropical rainbelt detected in observations ( [[#Rotstayn--2002|Rotstayn et al., 2002]] ; [[#Hwang--2013|Hwang et al., 2013]] ). Decreasing precipitation trends in the NH during the 1950s to 1980s have been attributed to anthropogenic aerosol emissions from North America and Europe, which peaked during the late1970s and declined thereafter following improved air quality regulations, causing dimming (brightening) through reduced (increased) surface solar radiation (Box 8.1 Figure 1), in agreement with model simulations ( [[#Chiang--2013|Chiang et al., 2013]] ; [[#Hwang--2013|Hwang et al., 2013]] ). This is consistent with energetic constraints where tropical precipitation shifts are anti-correlated with cross-equatorial energy transport (Section 6.3.3, Box 8.1). It also provides a physical mechanism for the severe drought in the Sahel that peaked in the mid-1980s (Sections 8.3.2.4.3 and 10.4.2.1) and the southward shift of the NH tropical edge from the 1950s to the 1980s ( [[#Allen--2014|Allen et al., 2014]] ; [[#Brönnimann--2015|Brönnimann et al., 2015]] ). However, CMIP5 and CMIP6 models still exhibit strong biases in representing the ITCZ, such as the simulation of a double ITCZ ( [[#Oueslati--2015|Oueslati and Bellon, 2015]] ; [[#Adam--2018|Adam et al., 2018]] ; [[#Tian--2020|Tian and Dong, 2020]] ). The impacts of aerosols and volcanic activity on the position of the ITCZ have been investigated but changes are difficult to characterize from observations (Section 6.3.3.2; Friedman et al. , 2013; J.M. Haywood et al. , 2013; [[#Iles--2014|Iles and Hegerl, 2014]] ; Colose et al. , 2016; [[#Chung--2017|Chung and Soden, 2017]] ). Such systematic shifts of the ITCZ can have important regional impacts like changes in precipitation (Figure 8.9).

In summary, there is ''medium confidence'' that the tropical rain belts over the oceans have been narrowing and strengthening in recent decades, leading to increased precipitation in the ITCZ core region ( [[#8.2.2.2|Section 8.2.2.2]] ). Decreasing precipitation trends in the NH during the 1950s – 1980s have been attributed to anthropogenic aerosol emissions from North America and Europe ( ''high co'' ''nfidence'' ).

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