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

==== 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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