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

==== 8.4.2.1 ITCZ and Tropical Rain Belts ====

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CMIP5 projections show no consistent shift in the zonal mean position of the ITCZ ( Donohoe et al. , 2013; [[#Donohoe--2017|Donohoe and Voigt, 2017]] ; Byrne et al. , 2018 ). The ITCZ position is strongly connected to cross-equatorial energy transport ( Kang et al. , 2008; [[#Bischoff--2014|Bischoff and Schneider, 2014]] ), which also shows no consistent change in future projections ( [[#Donohoe--2013|Donohoe et al., 2013]] ). Since AR5 it has been reported that most CMIP5 models project a narrowing of the ITCZ in response to surface warming together with intensified ascent in the core region and weakened ascent on the ITCZ edges ( [[#Lau--2015|Lau and Kim, 2015]] ; [[#Byrne--2018|Byrne et al., 2018]] ), implying a narrowing of precipitation regions influenced by the ITCZ. Modelled changes in the width and intensity of the zonal mean ITCZ are strongly anti-correlated, for example, narrowing is associated with increased intensity while broadening with decreased intensity. Such changes are associated with changes in tropical high cloud fraction and outgoing longwave radiation ( [[#Su--2017|Su et al., 2017]] ; [[#Byrne--2018|Byrne et al., 2018]] ).

Regional shifts in tropical convergence zones are much larger than their zonal mean, and associated regional changes in precipitation ( [[#Chadwick--2013|Chadwick et al., 2013]] ; [[#Mamalakis--2021|Mamalakis et al., 2021]] ) are characterized by considerable uncertainties across models ( [[#Kent--2015|Kent et al., 2015]] ; [[#Oueslati--2016|Oueslati et al., 2016]] ). Over the tropical oceans, shifts in rain bands are strongly coupled with changes in SSTs (Xie et al. , 2010; Huang et al. , 2013) . Over tropical land, factors including remote SST increases ( [[#Giannini--2010|Giannini, 2010]] ), the direct CO <sub>2</sub> effect ( [[#Biasutti--2013|Biasutti, 2013]] ) and land–atmosphere interactions ( [[#Chadwick--2017|Chadwick et al., 2017]] ; [[#Kooperman--2018|Kooperman et al., 2018]] ) influence projections. CMIP6 models project a clear northward ITCZ shift over eastern Africa and the Indian Ocean as well as a southward shift over the eastern Pacific and Atlantic oceans, as a result of regionally-contrasting inter-hemispheric energy flows ( [[#Mamalakis--2021|Mamalakis et al., 2021]] ). The northward movement of the ITCZ over Africa has been linked to an intensification of the Saharan heat low associated with greenhouse gas (GHG) warming ( [[#Dong--2015|Dong and Sutton, 2015]] ), causing the tropical rain belt to seasonally migrate farther northward and reside there longer ( [[#Cook--2012|Cook and Vizy, 2012]] ; [[#Dunning--2018|Dunning et al., 2018]] ). In southern Africa, the projected delay in the wet season onset (Dunning et al. , 2018) is also associated with a circulation-based northward shift in the tropical rain band (Lazenby et al., 2018) .

In summary, consistent with the AR5, the overall weakening of the tropical circulation is projected in CMIP5 and CMIP6 simulations with ''high confidence'' . It is ''likely'' that the zonal mean of the ITCZ will narrow and strengthen in the core region with projected surface warming ( ''high confidence'' ). Distinct regional shifts in the ITCZ will be associated with regional changes in precipitation amount and seasonality ( ''medium co'' ''nfidence'' ).

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