Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.7.2 Extratropical Storms ===

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This section focuses on extratropical cyclones (ETCs) that are either classified as strong or extreme by using some measure of their intensity, or by being associated with the occurrence of extremes in variables such as precipitation or near-surface wind speed ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ). Since AR5, the high relevance of ETCs for extreme precipitation events has been well established ( [[#Pfahl--2012|Pfahl and Wernli, 2012]] ; [[#Catto--2013|Catto and Pfahl, 2013]] ; [[#Utsumi--2017|Utsumi et al., 2017]] ), with 80% or more of hourly and daily precipitation extremes being associated with either ETCs or fronts over oceanic mid-latitude regions, and somewhat smaller, but still very large, proportions of events over mid-latitude land regions ( [[#Utsumi--2017|Utsumi et al., 2017]] ). The emphasis in this section is on individual ETCs that have been identified using some detection and tracking algorithms. Mid-latitude atmospheric rivers are assessed in [[IPCC:Wg1:Chapter:Chapter-8#8.3.2.8|Section 8.3.2.8]] .

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==== 11.7.2.1 Observed Trends ====

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[[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.3|Section 2.3.1.4.3]] concluded that there is overall ''low confidence'' in recent changes in the total number of ETCs over both hemispheres, and that there is ''medium confidence'' in a poleward shift of the storm tracks over both hemispheres since the 1980s. Overall, there is also ''low confidence'' in past-century trends in the number and intensity of the strongest ETCs due to the large interannual and decadal variability ( [[#Feser--2015|Feser et al., 2015]] ; [[#Reboita--2015|Reboita et al., 2015]] ; [[#Wang--2016|Wang et al., 2016]] ; [[#Varino--2019|Varino et al., 2019]] ) and due to temporal and spatial heterogeneities in the number and type of assimilated data in reanalyses, particularly before the satellite era ( [[#Krueger--2013|Krueger et al., 2013]] ; [[#Tilinina--2013|Tilinina et al., 2013]] ; [[#Befort--2016|Befort et al., 2016]] ; [[#Chang--2016|Chang and Yau, 2016]] ; [[#Wang--2016|Wang et al., 2016]] ). There is ''medium confidence'' that the agreement among reanalyses and detection and tracking algorithms is higher when considering stronger cyclones ( [[#Neu--2013|Neu et al., 2013]] ; [[#Pepler--2015|Pepler et al., 2015]] ; [[#Wang--2016|Wang et al., 2016]] ). Over the Southern Hemisphere, there is ''high confidence'' that the total number of ETCs with low central pressures (&lt;980 hPa) has increased between 1979 and 2009, with all eight reanalyses considered by [[#Wang--2016|Wang et al. (2016)]] showing positive trends, and five of them showing statistically significant trends. Similar results were found by [[#Reboita--2015|Reboita et al. (2015)]] using a different detection and tracking algorithm and a single reanalysis product. Over the Northern Hemisphere, there is ''high agreement'' among reanalyses that the number of cyclones with low central pressures (&lt;970 hPa) has decreased in summer and winter during the period 1979–2010 ( [[#Tilinina--2013|Tilinina et al., 2013]] ; [[#Chang--2016|Chang et al., 2016]] ). However, changes exhibit substantial decadal variability and do not show monotonic trends since the 1980s. For example, over the Arctic and North Atlantic, [[#Tilinina--2013|Tilinina et al. (2013)]] showed that the number of cyclones with very low central pressure (&lt;960 hPa) increased from 1979 to 1990 and then declined until 2010 in all five reanalyses considered. Over the North Pacific, the number of cyclones with very low central pressure reached a peak around 2000 and then decreased until 2010 in the five reanalyses considered ( [[#Tilinina--2013|Tilinina et al., 2013]] ). Overall, however, it should be noted that characterising trends in the dynamical intensity of ETCs (e.g., wind speeds) using the absolute central pressure is problematic because the central pressure depends on the background mean sea level pressure, which varies seasonally and regionally (e.g., [[#Befort--2016|Befort et al., 2016]] ).

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==== 11.7.2.2 Model Evaluation ====

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There is ''high confidence'' that coarse-resolution climate models (e.g., CMIP5 and CMIP6) underestimate the dynamical intensity of ETCs, including the strongest ETCs, as measured using a variety of metrics, including mean pressure gradient, mean vorticity and near-surface wind speeds, over most regions ( [[#Colle--2013|Colle et al., 2013]] ; [[#Zappa--2013a|Zappa et al., 2013a]] ; [[#Govekar--2014|Govekar et al., 2014]] ; [[#Di%20Luca--2016|Di Luca et al., 2016]] ; [[#Trzeciak--2016|Trzeciak et al., 2016]] ; [[#Seiler--2018|Seiler et al., 2018]] ; [[#Priestley--2020|Priestley et al., 2020]] ). There is also ''high confidence'' that most current climate models underestimate the number of explosive systems (i.e., systems showing a decrease in mean sea level pressure of at least 24 hPa in 24 hours) over both hemispheres ( [[#Seiler--2016a|Seiler and Zwiers, 2016a]] ; [[#Gao--2020|Gao et al., 2020]] ; [[#Priestley--2020|Priestley et al., 2020]] ). There is ''high confidence'' that the underestimation of the intensity of ETCs is associated with the coarse horizontal resolution of climate models, with higher horizontal resolution models, including HighResMIP and CORDEX, usually showing better performance ( [[#Colle--2013|Colle et al., 2013]] ; [[#Zappa--2013a|Zappa et al., 2013a]] ; [[#Di%20Luca--2016|Di Luca et al., 2016]] ; [[#Trzeciak--2016|Trzeciak et al., 2016]] ; [[#Seiler--2018|Seiler et al., 2018]] ; [[#Gao--2020|Gao et al., 2020]] ; [[#Priestley--2020|Priestley et al., 2020]] ). The improvement by higher-resolution models is found, even when comparing models and reanalyses after post-processing data to a common resolution ( [[#Zappa--2013a|Zappa et al., 2013a]] ; [[#Di%20Luca--2016|Di Luca et al., 2016]] ; [[#Priestley--2020|Priestley et al., 2020]] ). The systematic bias in the intensity of ETCs has also been linked to the inability of current climate models to resolve diabatic processes, particularly those related to the release of latent heat ( [[#Willison--2013|Willison et al., 2013]] ; [[#Trzeciak--2016|Trzeciak et al., 2016]] ) and the formation of clouds ( [[#Govekar--2014|Govekar et al., 2014]] ). There is ''medium confidence'' that climate models simulate well the spatial distribution of precipitation associated with ETCs over the Northern Hemisphere, together with some of the main features of the ETC life cycle, including the maximum in precipitation occurring just before the peak in dynamical intensity (e.g., vorticity) as observed in a reanalysis and observations ( [[#Hawcroft--2018|Hawcroft et al., 2018]] ). There is, however, large observational uncertainty in ETC-associated precipitation ( [[#Hawcroft--2018|Hawcroft et al., 2018]] ) and limitations in the simulation of frontal precipitation, including overly low rainfall intensity over mid-latitude oceanic areas in both hemispheres ( [[#Catto--2015|Catto et al., 2015]] ).

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==== 11.7.2.3 Detection and Attribution, Event Attribution ====

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( [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.3%20|Section 3.3.3.3]] concluded that there is ''low confidence'' in the attribution of observed changes in the number of ETCs in the Northern Hemisphere and ''high confidence'' that the poleward shift of storm tracks in the Southern Hemisphere is linked to human activity, mostly due to emissions of ozone-depleting substances. Specific studies attributing changes in the most extreme ETCs are not available. The human influence on individual extreme ETC events has been considered only a few times and there is overall ''low confidence'' in the attribution of these changes ( [[#NASEM--2016|NASEM, 2016]] ; [[#Vautard--2019|Vautard et al., 2019]] ).

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==== 11.7.2.4 Projections ====

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The frequency of ETCs is expected to change, primarily following a poleward shift of the storm tracks as discussed in [[IPCC:Wg1:Chapter:Chapter-4#4.5.1.6|Section 4.5.1.6]] (see also Figure 4.31) and [[IPCC:Wg1:Chapter:Chapter-8#8.4.2.8|Section 8.4.2.8]] . There is ''medium confidence'' that changes in the dynamical intensity (e.g., wind speeds) of ETCs will be small, although changes in the location of storm tracks can lead to substantial changes in local extreme wind speeds ( [[#Zappa--2013b|Zappa et al., 2013b]] ; [[#Chang--2014|Chang, 2014]] ; [[#Li--2014|Li et al., 2014]] ; [[#Seiler--2016b|Seiler and Zwiers, 2016b]] ; [[#Yettella--2017|Yettella and Kay, 2017]] ; [[#Barcikowska--2018|Barcikowska et al., 2018]] ; [[#Kar-Man%20Chang--2018|Kar-Man Chang, 2018]] ). [[#Yettella--2017|Yettella and Kay (2017)]] detected and tracked ETCs over both hemispheres in an ensemble of 30 Community Earth System Model Large Ensemble simulations, differing only in their initial conditions, and found that changes in mean wind speeds around ETC centres are often negligible between present (1986–2005) and future (2081–2100) periods. Using 19 CMIP5 models, [[#Zappa--2013b|Zappa et al. (2013b)]] found an overall reduction in the number of cyclones associated with low-troposphere (850-hPa) wind speeds larger than 25 m s <sup>–1</sup> over the North Atlantic and Europe with the number of the 10% strongest cyclones decreasing by about 8% and 6% in December–January–February and June–July–August according to the RCP4.5 scenario (2070–2099 vs. 1976–2005). Over the North Pacific, [[#Chang--2014|Chang (2014)]] showed that CMIP5 models project a decrease in the frequency of ETCs, with the largest central pressure perturbation (i.e., the depth, strongly related with low-level wind speeds) by the end of the century according to simulations using the RCP8.5 scenario. Using projections from CMIP5 GCMs under the RCP8.5 scenario (1981–2000 to 2081–2100), [[#Seiler--2016b|Seiler and Zwiers (2016b)]] projected a northward shift in the number of explosive ETCs in the northern Pacific, with fewer and weaker events south, and more frequent and stronger events north of 45°N. Using 19 CMIP5 GCMs under the RCP8.5 scenario, [[#Kar-Man%20Chang--2018|Kar-Man Chang (2018)]] found a significant decrease in the number of ETCs associated with extreme wind speeds (2081–2100 vs. 1980–99) over the Northern Hemisphere (average decrease of 17%) and over some smaller regions, including the Pacific and Atlantic regions.

Over the Southern Hemisphere, future changes (RCP8.5 scenario; 1980–1999 to 2081–2100) in extreme ETCs were studied by [[#Chang--2017|Chang (2017)]] using 26 CMIP5 models, and a variety of intensity metrics (850-hPa vorticity, 850-hPa wind speed, mean sea level pressure and near-surface wind speed). They found that the number of extreme cyclones is projected to increase by at least 20% and as much as 50%, depending on the specific metric used to define extreme ETCs. Increases in the number of strong cyclones appear to be robust across models and for most seasons, although they show strong regional variations, with increases occurring mostly over the southern flank of the storm track, consistent with a shift and intensification of the storm track. Overall, there is ''medium confidence'' that projected changes in the dynamical intensity of ETCs depend on the resolution and formulation (e.g., explicit or implicit representation of convection) of climate models ( [[#Booth--2013|Booth et al., 2013]] ; [[#Michaelis--2017|Michaelis et al., 2017]] ; [[#Zhang--2017|Zhang and Colle, 2017]] ).

As reported in AR5 and in [[IPCC:Wg1:Chapter:Chapter-8#8.4.2.8|Section 8.4.2.8]] , despite small changes in the dynamical intensity of ETCs, there is ''high confidence'' that the precipitation associated with ETCs will increase in the future ( [[#Zappa--2013b|Zappa et al., 2013b]] ; [[#Marciano--2015|Marciano et al., 2015]] ; [[#Pepler--2016|Pepler et al., 2016]] ; [[#Michaelis--2017|Michaelis et al., 2017]] ; [[#Yettella--2017|Yettella and Kay, 2017]] ; [[#Zhang--2017|Zhang and Colle, 2017]] ; [[#Barcikowska--2018|Barcikowska et al., 2018]] ; [[#Hawcroft--2018|Hawcroft et al., 2018]] ; [[#Zarzycki--2018|Zarzycki, 2018]] ; [[#Kodama--2019|Kodama et al., 2019]] ; [[#Bevacqua--2020a|Bevacqua et al., 2020a]] ; [[#Reboita--2021|Reboita et al., 2021]] ). There is ''high confidence'' that increases in precipitation will follow increases in low-level water vapour (i.e., about 7% per 1°C of surface warming; see Box 11.1) and will be larger for higher warming levels ( [[#Zhang--2017|Zhang and Colle, 2017]] ). There is ''medium confidence'' that precipitation changes will show regional and seasonal differences due to distinct changes in atmospheric humidity and dynamical conditions ( [[#Zappa--2015|Zappa et al., 2015]] ; [[#Hawcroft--2018|Hawcroft et al., 2018]] ), with decreases in some specific regions such as the Mediterranean ( [[#Zappa--2015|Zappa et al., 2015]] ; [[#Barcikowska--2018|Barcikowska et al., 2018]] ). There is ''high confidence'' that snowfall associated with winter ETCs will decrease in the future, because increases in tropospheric temperatures lead to a lower proportion of precipitation falling as snow ( [[#O’Gorman--2014|O’Gorman, 2014]] ; [[#Rhoades--2018|Rhoades et al., 2018]] ; [[#Zarzycki--2018|Zarzycki, 2018]] ). However, there is ''medium confidence'' that extreme snowfall events associated with winter ETCs will change little in regions where snowfall will be supported in the future ( [[#O’Gorman--2014|O’Gorman, 2014]] ; [[#Zarzycki--2018|Zarzycki, 2018]] ).

In summary, there is ''low confidence'' in past changes in the dynamical intensity (e.g., maximum wind speeds) of ETCs and ''medium confidence'' that, in the future, these changes will be small, although changes in the location of storm tracks could lead to substantial changes in local extreme wind speeds. There is ''high confidence'' that average and maximum ETC precipitation-rates will increase with warming, with the magnitude of the increases associated with increases in atmospheric water vapour. There is ''medium confidence'' that projected changes in the intensity of ETCs, including wind speeds and precipitation, depend on the resolution and formulation of climate models.

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