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

==== 8.3.2.7 Atmospheric Blocking ====

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Atmospheric blocking refers to persistent, semi-stationary weather patterns characterized by a high-pressure (anticyclonic) anomaly that interrupts the westerly flow in the mid-latitudes of both hemispheres. By redirecting the pathways of mid-latitude cyclones, blocking can affect the water cycle and lead to negative precipitation anomalies in the region of the blocking anticyclone and positive anomalies in the surrounding areas ( [[#Sousa--2017|Sousa et al., 2017]] ). In this way, blocking can also be associated with extreme events such as heavy precipitation ( [[#Lenggenhager--2019|Lenggenhager et al., 2019]] ), drought ( [[#Schubert--2014|Schubert et al., 2014]] ) and heatwaves ( [[#Miralles--2014a|Miralles et al., 2014a]] ). The AR5 reported ''low confidence'' in global-scale changes in blocking, due to methodological differences between studies.

Currently no consensus exists on observed trends in blocking during 1979 – 2013. ( [[#Horton--2015|Horton et al., 2015]] ) identified increasing trends in anticyclonic circulation regimes based on geopotential height fields in the mid-troposphere, which may be partly related to the tropospheric warming itself and thus not represent real changes in the statistics of weather ( [[#Horton--2015|Horton et al., 2015]] ; [[#Woollings--2018|Woollings et al., 2018]] ). [[#Hanna--2018|Hanna et al. (2018)]] and ( [[#Davini--2020|Davini and D’Andrea, 2020]] ) reported a significant increase in the frequency of summer blocking over Greenland. A weakening of the zonal wind, eddy kinetic energy and amplitude of Rossby waves in summer in the NH ( [[#Coumou--2015|Coumou et al., 2015]] , [[#Kornhuber--2019|Kornhuber et al., 2019]] ) and an increased ‘waviness’ of the jet stream associated with Arctic warming ( [[#Francis--2015|Francis and Vavrus, 2015]] ; [[#Pfahl--2015|Pfahl et al., 2015]] ; [[#Luo--2019|Luo et al., 2019]] ) have also been identified, which may be linked to increased blocking.

In contrast, it has been shown that observed trends in blocking are sensitive the choice of the blocking index, and that there is a large internal variability that complicates the detection of forced trends ( [[#Barnes--2014|Barnes et al., 2014]] ; [[#Cattiaux--2016|Cattiaux et al., 2016]] ; [[#Woollings--2018|Woollings et al., 2018]] ), compromising the attribution of any observed changes in blocking. Many climate models still underestimate the occurrence of blocking, at least in winter over north-eastern Atlantic and Europe ( [[#Dunn-Sigouin--2013|Dunn-Sigouin and Son, 2013]] ), which leads to caution in the interpretation of their results for these regions. However, over the Pacific Ocean there have been large improvements in the simulation of blocking for the last 20 years ( [[#Davini--2016|Davini and D’Andrea, 2016]] ; Patterson et al. , 2019) . In the SH, increases in blocking frequency have occurred in the South Atlantic in austral summer ( [[#Dennison--2016|Dennison et al., 2016]] ) and in the southern Indian Ocean in austral spring ( [[#Schemm--2018|Schemm, 2018]] ). A reduced blocking frequency has been found over the south-western Pacific in austral spring (Sections 2.3.1.4.3 and 3.4.1.3.3; [[#Schemm--2018|Schemm, 2018]] ).

In summary, no robust trend in atmospheric blocking has been detected in modern reanalyses and in CMIP6 historical simulations ( ''medium confidence'' ). The lack of trend is explained by strong internal variability and/or the competing effects of low-level Arctic amplification and upper-level tropical amplification of the equator-to-pole temperature gradient ( ''medium co'' ''nfidence'' ).

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