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

==== 8.4.2.7 Atmospheric Blocking ====

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In AR5, the increased ability of models to simulate blocking and higher agreement on projections led to an assessment with ''medium confidence'' that the frequency of NH and SH blocking will not increase, but future changes in blocking intensity and persistence were deemed uncertain (AR5 Chapter 14, ES and Box 14.2). Blocking influences precipitation (e.g., [[#Trigo--2004|Trigo et al., 2004]] ), flooding (e.g., Yamada et al. , 2016) , drought (e.g., Dong et al. , 2018b) , snow (e.g., [[#García-Herrera--2006|García-Herrera and Barriopedro, 2006]] ) , and glacier melt (e.g., [[#Hanna--2013|Hanna et al., 2013]] ), and so is of broad importance to the water cycle in areas of blocking activity.

Blocking projections are assessed in this Report in [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.1.6|Section 4.5.1.6]] ), and model performance in simulating blocking is also discussed in [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.3|Section 3.3.3.3]] ). CMIP5 projections suggest a complex response in blocking frequencies with an eastward shift in NH winter blocking, mid-latitude decreases in boreal summer except in eastern Europe – western Russia, and SH decreases in the Pacific sector during austral spring and summer. CMIP6 projections (Figure. 4.28) show a notable decrease in blocking activity over Greenland and the North Pacific for the SSP3-7.0 and SSP5-8.5 scenarios. However, the continued large differences among current models as well as the sensitivity to blocking detection methods limits confidence in projected regional changes in blocking (see also [[IPCC:Wg1:Chapter:Chapter-10#10.3.3.3.1|Section 10.3.3.3.1]] ). The influence of blocking on multiple elements of the water cycle means that the uncertainty in blocking projections adds a corresponding layer of uncertainty to water cycle projections.

In summary, and despite recent improvements in the simulation of blocking, there is ''limited evidence'' in model projections of future changes, except for boreal winter over Greenland and the North Pacific where there is high confidence that blocking events are not expected to increase in the SSP3-7.0 and SSP5-8.5 scenarios. As with stationary waves, this adds uncertainty to mid-latitude water cycle projections at the regional scale.

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