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

==== 8.4.2.6 Stationary Waves ====

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The AR5 did not provide an assessment of stationary wave projections as distinct from other related aspects of circulation, such as blocking, modes of variability, and storm tracks. Here we provide a brief assessment of stationary wave projections from the water cycle perspective, with the related circulation aspects considered separately in the following sections.

Several studies based on CMIP5 projections show changes in NH winter stationary waves that increase precipitation over the west coast of North America and decrease it over the eastern Mediterranean and parts of south-western North America ( Neelin et al. , 2013; Seager et al. , 2014a, b, 2019b; Simpson et al. , 2016; Wills et al. , 2019 ), although the underlying dynamics are not yet fully understood ( [[#Seager--2019b|Seager et al., 2019b]] ; [[#Wills--2019|Wills et al., 2019]] ). For the NH winter global teleconnection pattern, the majority of the models analyzed in ( [[#Sandler--2020|Sandler and Harnik, 2020]] ) project the development of a preferred longitudinal phasing for the pattern, but with strong disagreement among models over the details of the phasing and therefore the associated regional hydrologic impacts.

While the potential role of increasing hydrologic extremes with quasi-resonant stationary waves during NH summer has received considerable attention (see [[#8.3.2.6|Section 8.3.2.6]] ), as yet there is no clear evidence in model projections that this variability will increase ( [[#Teng--2019|Teng and Branstator, 2019]] ). The influence of the Arctic on mid-latitude circulation is assessed in Cross-Chapter Box 10.1, which reports that there is ''low confidence'' in the dominant contribution of Arctic warming compared to other drivers in future projections. Potential changes to the stratospheric polar vortex in CMIP5 models have a substantial influence on tropospheric stationary waves and associated hydrologic impacts in both the NH ( [[#Zappa--2017|Zappa and Shepherd, 2017]] ) and SH ( [[#Mindlin--2020|Mindlin et al., 2020]] ). CMIP5 models have some important limitations in their representation of stationary waves ( [[#Lee--2013|Lee and Black, 2013]] ; [[#Simpson--2016|Simpson et al., 2016]] ; [[#Garfinkel--2020|Garfinkel et al., 2020]] ) and this aspect of CMIP6 models has not yet been comprehensively evaluated.

In summary, future changes in stationary waves may have an important influence on both the mean state and variability of the water cycle. Limitations in model representation, dynamical understanding, and the number of targeted studies on the topic currently constrain the assessment of future changes in stationary waves. Based on current knowledge, there is ''low confidence'' that projected changes in stationary wave activity will contribute to decreases of cold season precipitation over the eastern Mediterranean and increases over the west coast of North America.

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