Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.1.4.3 Extratropical jets, storm tracks, and blocking =====

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The AR5 reported a ''likely'' poleward shift of storm tracks and jet streams since the 1970s from different datasets, variables and approaches. These trends were consistent with the HC widening and the poleward shifting of the circulation features since the 1970s. There was ''low confidence'' in any large-scale change in blocking.

Proxy records consistent with modelling results imply a southward shift of the storm tracks over the North Atlantic during the LGM ( [[#Raible--2021|Raible et al., 2021]] ). A variety of proxies are available for the changes in the position of the extratropical jets/westerlies during the Holocene. Recent syntheses of moisture-sensitive proxy records indicate drier-than-present conditions over mid-latitudes of western North America ( [[#Hermann--2018|Hermann et al., 2018]] ; [[#Liefert--2020|Liefert and Shuman, 2020]] ) during the MH, which together with a weakened Aleutian Low ( [[#Bailey--2018|Bailey et al., 2018]] ) implies that the winter North Pacific jetstream was shifted northward. A synthesis of lines of evidence from the SH indicates that the westerly winds were stronger over 14–5 ka, followed by regional asymmetry after 5 ka ( [[#Fletcher--2012|Fletcher and Moreno, 2012]] ). There is no consensus on the shifts of the SH westerlies with some studies implying poleward migrations ( [[#Lamy--2010|Lamy et al., 2010]] ; [[#Voigt--2015|Voigt et al., 2015]] ; [[#Turney--2017|Turney et al., 2017]] ; [[#Anderson--2018|Anderson et al., 2018]] ) and others suggesting an equatorward shift ( [[#Kaplan--2016|Kaplan et al., 2016]] ) in the MH.

During 950–1400 CE, hydroclimate indicators suggest a northward shift of Pacific storm tracks over North America ( [[#McCabe-Glynn--2013|McCabe-Glynn et al., 2013]] ; [[#Steinman--2014|Steinman et al., 2014]] ) which was comparable in magnitude to that over 1979–2015 (J. [[#Wang--2017a|]] [[#Wang--2017|Wang et al., 2017]] a ). Storm tracks over the North Atlantic-European sector shifted northward as indicated by multi-proxy indicators over the North Atlantic ( [[#Wirth--2013|Wirth et al., 2013]] ; [[#Orme--2017|Orme et al., 2017]] ) and Mediterranean ( [[#Roberts--2012|Roberts et al., 2012]] ). Reconstructed westerly winds in the SH suggest a poleward shift ( [[#Lamy--2010|Lamy et al., 2010]] ; [[#Schimpf--2011|Schimpf et al., 2011]] ; [[#Goodwin--2014|Goodwin et al., 2014]] ; [[#Koffman--2014|Koffman et al., 2014]] ; [[#Moreno--2018|Moreno et al., 2018]] ), with latitudinal change comparable to that during recent decades ( [[#Swart--2012|Swart and Fyfe, 2012]] ; [[#Manney--2018|Manney and Hegglin, 2018]] ).

Multiple reanalyses show that since 1979 the subtropical jet wind speeds have generally increased in winter and decreased in summer in both hemispheres, but the trends are regionally dependent ( [[#Pena-Ortiz--2013|Pena-Ortiz et al., 2013]] ; [[#Manney--2018|Manney and Hegglin, 2018]] ; S.H. [[#Lee--2019|]] [[#Lee--2019|Lee et al., 2019]] ). Over NH mid-latitudes, the summer zonal wind speeds have weakened in the mid-troposphere ( [[#Francis--2012|Francis and Vavrus, 2012]] ; [[#Coumou--2014|Coumou et al., 2014]] , 2015; [[#Haimberger--2017|Haimberger and Mayer, 2017]] ). Meanwhile there are indications of enhanced jetstream meandering in boreal autumn at the hemispheric scale ( [[#Francis--2015|Francis and Vavrus, 2015]] ; [[#Di%20Capua--2016|Di Capua and Coumou, 2016]] ), whereas the regional arrangement of meandering depends on the background atmospheric state ( [[#Cohen--2020|Cohen et al., 2020]] ). These meandering trends, however, are sensitive to the metrics used ( [[#Screen--2013|Screen and Simmonds, 2013]] ; [[#Hassanzadeh--2014|Hassanzadeh et al., 2014]] ; [[#Cattiaux--2016|Cattiaux et al., 2016]] ; [[#Vavrus--2018|Vavrus, 2018]] ). Hypothesized links to Arctic warming are assessed in Cross-Chapter Box 10.1.

Multiple reanalyses and radiosonde observations show an increasing number of extratropical cyclones over the NH since the 1950s ( [[#Chang--2016|Chang and Yau, 2016]] ; X.L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ). The positive trends are generally consistent among reanalyses since 1979, though with considerable spread ( [[#Tilinina--2013|Tilinina et al., 2013]] ; X.L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ). In recent decades the number of deep extratropical cyclones has increased over the SH (Section 8.3.2.8.1 and Figure 8.12; [[#Reboita--2015|Reboita et al., 2015]] ; X.L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ), while the number of deep cyclones has decreased in the NH in both winter and summer ( [[#Neu--2013|Neu et al., 2013]] ; [[#Coumou--2015|Coumou et al., 2015]] ; [[#Chang--2016|Chang et al., 2016]] ; J. [[#Wang--2017a|]] [[#Wang--2017|Wang et al., 2017]] a ; [[#Gertler--2019|Gertler and O’Gorman, 2019]] ). The regional changes for different intensity extratropical cyclones are assessed in Section 8.3.2.8.1. The assessment of trends is complicated by strong interannual to decadal variability, sensitivity to dataset choice and resolution ( [[#Tilinina--2013|Tilinina et al., 2013]] ; [[#Lucas--2014|Lucas et al., 2014]] ; X.L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ; [[#Pepler--2018|Pepler et al., 2018]] ; [[#Rohrer--2018|Rohrer et al., 2018]] ) and cyclone identification/tracking methods ( [[#Neu--2013|Neu et al., 2013]] ; [[#Grieger--2018|Grieger et al., 2018]] ). Thus there is overall ''low'' ''confidence'' for recent changes in global extratropical storm tracks.

A consistent poleward shift of the tropospheric extratropical jets since 1979 is reported by multiple reanalyses (Figure 2.18; [[#Davis--2012|Davis and Rosenlof, 2012]] ; [[#Davis--2013|Davis and Birner, 2013]] ; [[#Pena-Ortiz--2013|Pena-Ortiz et al., 2013]] ; [[#Manney--2018|Manney and Hegglin, 2018]] ), and radiosonde winds ( [[#Allen--2012|Allen et al., 2012]] ). This is generally consistent with the previously reported shifts retrieved from satellite temperature observations ( [[#Fu--2011|Fu and Lin, 2011]] ; [[#Davis--2012|Davis and Rosenlof, 2012]] ). After the 1960s the magnitude of meridional shifts in extratropical jets over both the North Atlantic and North Pacific in August is enhanced compared to multi-century variability ( [[#Trouet--2018|Trouet et al., 2018]] ). Despite some regional differences ( [[#Woollings--2014|Woollings et al., 2014]] ; [[#Norris--2016|Norris et al., 2016]] ; J. [[#Wang--2017a|]] [[#Wang--2017|Wang et al., 2017]] a ; [[#Xue--2017|Xue and Zhang, 2017]] ; [[#Ma--2018|Ma and Zhang, 2018]] ; [[#Melamed-Turkish--2018|Melamed-Turkish et al., 2018]] ), overall poleward deflection of storm tracks in boreal winter over both the North Atlantic and the North Pacific was identified during 1979–2010 ( [[#Tilinina--2013|Tilinina et al., 2013]] ). Over the SH extra-tropics there is a similarly robust poleward shift in the polar jet since 1979 ( [[#Pena-Ortiz--2013|Pena-Ortiz et al., 2013]] ; [[#Manney--2018|Manney and Hegglin, 2018]] ; [[#WMO--2018|WMO, 2018]] ), although after 2000 the December–January–February (DJF) tendency to poleward shift of the SH jet stream position ceased ( [[#Banerjee--2020|Banerjee et al., 2020]] ). The general poleward movement in midlatitude jet streams ( [[#Lucas--2014|Lucas et al., 2014]] ) is consistent with the expansion of the tropical circulation ( [[#2.3.1.4.1|Section 2.3.1.4.1]] ). The changes of extratropical jets and westerlies are also related to the annular modes of variability ( [[#2.4|Section 2.4]] and Annex IV).

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[[File:fd4b5ce5996815b4bafa896df1e3d7fb IPCC_AR6_WGI_Figure_2_18.png]]

'''Figure 2.1''' '''8 |''' '''Trends in ERA5 zonal-mean zonal wind speed.''' Shown are '''(a)''' DJF (December–January–February); '''(b)''' MAM (March–April–May); '''(c)''' JJA (June–July–August); and '''(d)''' SON (September–October–November). Climatological zonal winds during the data period are shown in solid contour lines for westerly winds and in dashed lines for easterly. Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] (‘×’ marks denote non-significant trends). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Robust trends in blocking have only been found in certain regions and specific seasons during recent decades. Increases in blocking frequency have occurred over low-latitude regions in the North Atlantic in boreal winter ( [[#Davini--2012|Davini et al., 2012]] ), the South Atlantic in austral summer ( [[#Dennison--2016|Dennison et al., 2016]] ) and the southern Indian Ocean in austral spring ( [[#Schemm--2018|Schemm, 2018]] ). Over the subpolar North Atlantic sustained periods of positive Greenland blocking were identified during 1870–1900 and from the late 1990s to 2015 ( [[#Hanna--2015|Hanna et al., 2015]] ). Further analysis of association of Greenland blocking with the NAM is provided in [[#2.4.1.1|Section 2.4.1.1]] . Meanwhile, a reduced blocking frequency has been found over winter in Siberia ( [[#Davini--2012|Davini et al., 2012]] ) and the south-western Pacific in austral spring ( [[#Schemm--2018|Schemm, 2018]] ). Over eastern European Russia and western Siberia (40°E–100°E) a tendency towards longer blocking events was reported by [[#Luo--2016|Luo et al. (2016)]] for 2000–2013 and by [[#Tyrlis--2020|Tyrlis et al. (2020)]] for 1979–2017. Inter-annual variance in the number of blocking events over the SH ( [[#Oliveira--2014|Oliveira et al., 2014]] ) and North Atlantic ( [[#Kim--2015|Kim and Ha, 2015]] ) has enhanced. Blocking events and their trends are sensitive to choice of datasets, calculation periods and methods ( [[#Cheung--2013|Cheung et al., 2013]] ; [[#Barnes--2014|Barnes et al., 2014]] ; [[#Pepler--2018|Pepler et al., 2018]] ; [[#Rohrer--2018|Rohrer et al., 2018]] ; [[#Woollings--2018b|Woollings et al., 2018b]] ; [[#Kononova--2020|Kononova and Lupo, 2020]] ). As a result, hemispheric and global trends in blocking frequency have overall ''low'' ''confidence.''

In summary, the total number of extratropical cyclones has ''likely'' increased since the 1980s in the NH ( ''low confidence'' ), but with fewer deep cyclones particularly in summer. The number of strong extratropical cyclones has ''likely'' increased in the SH ( ''medium confidence'' ). The extratropical jets and cyclone tracks have ''likely'' been shifting poleward in both hemispheres since the 1980s with marked seasonality in trends ( ''medium confidence'' ). There is ''low confidence'' in shifting of extratropical jets in the NH during the mid-Holocene and over 950–1400 CE to latitudes that ''likely'' were similar to those since 1979. There is ''low confidence'' in observed global-scale changes in the occurrence of blocking events.

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