Editing IPCC:AR6/WGI/Chapter-4 (section)

===== 4.5.1.6.2 Zonal wind and westerly jets =====

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Storm tracks and mid-latitude westerly jets are dynamically related aspects of mid-latitude circulation. The AR5 assessed that a poleward shift of the SH westerlies and storm track is ''likely'' by the end of the 21st century under RCP8.5 ( ''medium confidence'' ). In contrast, ''low confidence'' was assessed for the storm-track response in the NH.

Under both SSP1-2.6 and SSP3-7.0 there is a strengthening and lifting of the subtropical jets in both hemispheres (Figure 4.26), consistent with the response to large-scale tropospheric warming found in earlier generations of climate models ( [[#Collins--2013|Collins et al., 2013]] ). In the SH, GHG emissions tend to force a poleward shift of the jet, but this is opposed, particularly in austral summer, by the stratospheric ozone hole recovery ( [[#Barnes--2013|Barnes and Polvani, 2013]] ; [[#Barnes--2014|Barnes et al., 2014]] ; [[#Bracegirdle--2020b|Bracegirdle et al., 2020b]] ). Consistent with sea level pressure changes, CMIP6 models project a strengthening and poleward shift of the SH jet in austral summer and winter under SSP3-7.0, but smaller and non-robust changes in SH mid-latitude zonal winds under SSP1-2.6 (Figure 4.26; see also [[#4.5.3.1|Section 4.5.3.1]] ). CMIP6 models show an improved simulation of the SH jet stream latitude ( [[#Bracegirdle--2020a|Bracegirdle et al., 2020a]] ; [[#Curtis--2020|Curtis et al., 2020]] ). This has been linked to a reduction in the projected poleward shift of the SH jet in austral summer compared to the CMIP5 models ( [[#Curtis--2020|Curtis et al., 2020]] ; [[#Goyal--2021|Goyal et al., 2021]] ), although differences in the pattern of SST response may also play a role ( [[#Wood--2020|Wood et al., 2020]] ). In the NH extratropics, the changes in lower-tropospheric zonal-mean zonal winds by the end of the century are generally smaller than in the SH. In boreal winter, there is a weak poleward shift of the NH zonal-mean westerly jet maximum in SSP3-7.0.

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[[File:f54d6787f8e183a379169ed8be4933e4 IPCC_AR6_WGI_Figure_4_26.png]]

'''Figure 4.26 |''' '''Long-term change of zonal-mean, zonal wind.''' Displayed are multi-model mean changes in '''(left)''' boreal winter(December–January–February, DJF) and '''(right)''' austral winter (June–July–August, JJA) zonal mean, zonal wind (m s <sup>–1</sup> ) in 2081–2100 for (top) SSP1-2.6 and (bottom) SSP3-7.0 relative to 1995–2014. The 1995–2014 climatology is shown in contours with spacing 10 m s <sup>–1</sup> . Diagonal lines indicate regions where less than 80% of the models agree on the sign of the change and no overlay where at least 80% of the models agree on the sign of the change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

CMIP5 and CMIP6 models show a strong seasonal and regional dependence in the response to climate change of NH westerlies ( [[#Barnes--2013|Barnes and Polvani, 2013]] ; [[#Grise--2014b|Grise and Polvani, 2014b]] ; [[#Simpson--2014|Simpson et al., 2014]] ; [[#Zappa--2015|Zappa et al., 2015]] ; [[#Harvey--2020|Harvey et al., 2020]] ; [[#Oudar--2020|Oudar et al., 2020]] ). CMIP5 projections indicate a poleward shift of the westerlies in the North Atlantic in boreal summer, while the North Pacific jet weakens in this season ( [[#Simpson--2014|Simpson et al., 2014]] ; [[#Davini--2020|Davini and D’Andrea, 2020]] ; [[#Harvey--2020|Harvey et al., 2020]] ). There is a poleward shift in the westerlies in both the North Pacific and North Atlantic in Autumn ( [[#Barnes--2013|Barnes and Polvani, 2013]] ; [[#Simpson--2014|Simpson et al., 2014]] ). However, the shift of the westerlies is more uncertain in the other seasons, particularly in the North Atlantic in winter ( [[#Simpson--2014|Simpson et al., 2014]] ; [[#Zappa--2017|Zappa and Shepherd, 2017]] ). Here, the circulation response is not well described as a simple shift, since the North Atlantic jet tends to be squeezed on both its equatorward and poleward flanks, together with an eastward extension into Europe ( [[#Li--2018|Li et al., 2018]] ; [[#Peings--2018|Peings et al., 2018]] ; [[#Simpson--2019a|Simpson et al., 2019a]] ; [[#Harvey--2020|Harvey et al., 2020]] ; [[#Oudar--2020|Oudar et al., 2020]] ). Simulations indicate that most of the changes in winter storminess over the Euro-Atlantic region will occur only after exceeding the 1.5°C warming level ( [[#Barcikowska--2018|Barcikowska et al., 2018]] ).

Progress since AR5 has improved understanding of the climate change aspects that can drive these different, and potentially opposite, responses in the mid-latitude jets and storm tracks. A poleward shift of the jets and storm tracks is expected in response to an increase in the atmospheric stratification and in the upper-tropospheric equator-to-pole meridional temperature gradient, while it is opposed by the decrease in the meridional temperature gradient in the lower troposphere associated with the polar amplification of global warming ( [[#Harvey--2014|Harvey et al., 2014]] ; [[#Shaw--2016|Shaw et al., 2016]] ). Recent analyses have identified additional climate aspects that can drive mid-latitude jet changes, including patterns in sea surface warming ( [[#Mizuta--2014|Mizuta et al., 2014]] ; [[#Langenbrunner--2015|Langenbrunner et al., 2015]] ; [[#Ceppi--2018|Ceppi et al., 2018]] ; [[#Wood--2020|Wood et al., 2020]] ), land–sea warming contrast ( [[#Shaw--2015|Shaw and Voigt, 2015]] ), loss of sea ice ( [[#Deser--2015|Deser et al., 2015]] ; [[#Harvey--2015|Harvey et al., 2015]] ; [[#Screen--2018b|Screen et al., 2018b]] ; [[#Zappa--2018|Zappa et al., 2018]] ), and changes in the strength of the stratospheric polar vortex ( [[#Manzini--2014|Manzini et al., 2014]] ; [[#Grise--2017|Grise and Polvani, 2017]] ; [[#Simpson--2018|Simpson et al., 2018]] ; [[#Ceppi--2019|Ceppi and]] [[#Shepherd--2019|Shepherd, 2019]] ). From an energetics perspective,the uncertainty in the response of the jet streams depends on the response of clouds, their non-spatially uniform radiative feedbacks shaping the meridional profile of warming ( [[#Ceppi--2014|Ceppi et al., 2014]] ; [[#Voigt--2015|Voigt and Shaw, 2015]] , 2016; [[#Ceppi--2016|Ceppi and Hartmann, 2016]] ; [[#Ceppi--2017|Ceppi and Shepherd, 2017]] ; [[#Lipat--2018|Lipat et al., 2018]] ; [[#Albern--2019|Albern et al., 2019]] ; [[#Voigt--2019|Voigt et al., 2019]] ). Climate models seem to underestimate the forced component of the year-to-year variability in the atmospheric circulation, particularly in the North Atlantic sector ( [[#Scaife--2018|Scaife and Smith, 2018]] ), which suggests some relevant dynamical processes may not be well represented. Whether and how this may affect long-term projections is unknown. In conclusion, due to the influence from competing dynamical drivers and the absence of observational evidence, there is ''medium confidence'' in a projected poleward shift of the NH zonal-mean low-level westerlies in autumn and summer and ''low confidence'' in the other seasons. There is also overall ''low confidence'' in projected regional changes in the NH low-level westerlies, particularly for the North Atlantic basin in boreal winter.

The anthropogenic forced signal in extratropical atmospheric circulation may well be small compared to internal variability ( [[#Deser--2012b|Deser et al., 2012b]] , 2014) and, as assessed in AR5, there is generally '''low agreement''' across models in many aspects of regional atmospheric circulation change particularly in the NH ( [[#Shepherd--2014|Shepherd, 2014]] ). The latter means that, in some regions, a multi-model average perspective of atmospheric circulation change represents a small residual after averaging over large intermodel spread. This is in strong contrast to thermodynamic aspects of climate change, such as surface temperature change, for which model results are generally highly consistent (see, e.g., Figure 4.19). Furthermore, models share systematic biases in some aspects of extratropical atmospheric circulation such as mid-latitude jets, which can have complex implications for understanding forced changes ( [[#Simpson--2016|Simpson and Polvani, 2016]] ). Given these issues, an emerging field of research since AR5 has focused on the development of ‘storylines’ for regional atmospheric circulation change ( [[#Shepherd--2019|Shepherd, 2019]] ). The storyline approach is grounded in the identification of a set of physical predictors of atmospheric circulation change, such as those described above ( [[#Harvey--2014|Harvey et al., 2014]] ; [[#Manzini--2014|Manzini et al., 2014]] ; [[#Shepherd--2018|Shepherd et al., 2018]] ), which act together to determine a specific outcome in the projected atmospheric circulation change. The consequences of multi-model spread in the physical predictors of atmospheric circulation change can be investigated, conditioned on a specified level of global warming (see also [[IPCC:Wg1:Chapter:Chapter-1#1.4.4.2|Section 1.4.4.2]] and Box 10.2; [[#Zappa--2017|Zappa and Shepherd, 2017]] ; [[#Zappa--2019|Zappa, 2019]] ; [[#Mindlin--2020|Mindlin et al., 2020]] ).

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