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

==== 8.3.2.6 Stationary Waves ====

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Stationary waves are planetary-scale waves that are approximately stable (stationary) in terms of geographic position, as opposed to propogating planetary waves, and are important both as part of the climatological general circulation and seasonal and shorter-term anomalies. They are related to surface features including land – ocean contrasts and major mountain ranges, as well as atmospheric features including the jet stream, storm tracks, and blocking, which are considered separately in the following sections. While zonal mean changes in P–E (precipitation minus evaporation) are dominated by thermodynamic effects ( [[#8.2.2.1|Section 8.2.2.1]] ), changes in stationary waves are of key importance in understanding zonal asymmetries in the water cycle response to global warming ( [[#Wills--2015|Wills and Schneider, 2015]] ; [[#Wills--2019|Wills et al., 2019]] ). The AR5 did not explicitly assess stationary waves, but noted changes in related circulation features such as a ''likely'' poleward shift of the Northern Hemisphere (NH) storm tracks and an increase in frequency and eastward shift in North Atlantic blocking anticyclones, although there was ''low confidence'' in the global assessment of blocking.

Since AR5, several studies have demonstrated a link between stationary wave amplitude and wet and dry extremes in several different regions of the NH ( [[#Liu--2012|Liu et al., 2012]] ; [[#Coumou--2014|Coumou et al., 2014]] ; [[#Screen--2014|Screen and Simmonds, 2014]] ; [[#Yuan--2015|Yuan et al., 2015]] ) with changes in moisture transport playing an important role ( [[#Yuan--2015|Yuan et al., 2015]] ). A ‘resonance mechanism’ has been proposed for an increasing amplitude of stationary waves ( [[#Petoukhov--2013|Petoukhov et al., 2013]] , 2016; [[#Coumou--2014|Coumou et al., 2014]] ; [[#Kornhuber--2017|Kornhuber et al., 2017]] ) and several studies have linked increasing amplitude of stationary waves to Arctic warming ( [[#Francis--2012|Francis and Vavrus, 2012]] , 2015; Liu et al. , 2012; Tang et al. , 2014) as well as to global warming (Mann et al. , 2017). However, other studies have not identified an increase in stationary wave amplitude ( [[#Barnes--2013|Barnes, 2013]] ; Screen and Simmonds, 2013a, b).

There has been considerable work on linkages (teleconnections) between Arctic warming and the mid-latitude circulation (see also Cross-Chapter Box 10.1). The limited amount of research on Southern Hemisphere (SH) stationary waves suggests changes in high-latitude, mid-tropospheric stationary waves which influence Antarctic precipitation ( [[#Turner--2017|Turner et al., 2017]] ) and changes in stratospheric stationary waves that are associated with ozone depletion rather than increases in GHGs (L. [[#Wang--2013|]] [[#Wang--2013|]] [[#Wang--2013|]] [[#Wang--2013|Wang et al., 2013]] ). The observed climatology of NH winter stationary waves is well-represented in the CMIP5 multi-model mean ( [[#Wills--2019|Wills et al., 2019]] ) but individual models have important deficiencies in reproducing stationary wave variability ( [[#Lee--2013|Lee and Black, 2013]] ). In the SH, the observed climatology of stationary waves in CMIP5 models has considerable bias in both phase and amplitude ( [[#Garfinkel--2020|Garfinkel et al., 2020]] ). A comprehensive assessment is not yet available for CMIP6 models.

In summary, there is ''low confidence'' in strengthened winter stationary wave activity over the North Atlantic, associated with increased poleward moisture fluxes east of North America There is ''medium confidence'' in a recent amplification of the NH stationary waves in summer, but no formal attribution to anthropogenic climate change.

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