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

===== 4.5.1.6.3 Storm tracks =====

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As stated in AR5, the number of extratropical cyclones (ETC) composing the storm tracks is projected to weakly decline in future projections, but by no more than a few percent change. The reduction is mostly located on the equatorward flank of the storm tracks, which is associated with the Hadley cell expansion and a poleward shift in the mean genesis latitude of ETCs ( [[#Tamarin-Brodsky--2017|Tamarin-Brodsky and Kaspi, 2017]] ). Furthermore, the poleward propagation of individual ETCs is expected to increase with warming ( [[#Graff--2014|Graff and LaCasce, 2014]] ; [[#Tamarin-Brodsky--2017|Tamarin-Brodsky and Kaspi, 2017]] ), thus contributing to a poleward shift in the mid-latitude transient-eddy kinetic energy. The increased poleward propagation results from the strengthening of the upper tropospheric jet and increased cyclone-associated precipitation ( [[#Tamarin-Brodsky--2017|Tamarin-Brodsky and Kaspi, 2017]] ), which are robust aspects of climate change.

In the NH boreal winter, CMIP6 models show a northward shift of the ETC density in the North Pacific, a tripolar pattern in the North Atlantic, and a weakening of the Mediterranean storm track (Figure 4.27a). CMIP6 models show overall ''low agreement'' on changes in ETC density in the North Atlantic in boreal winter (Figure 4.27a). A poleward shift of the storm track is evident in the SH (Figure 4.27b), particularly in the Indian and Pacific Ocean sectors. CMIP6 models still feature long-standing biases in the representation of storm tracks; for example, the winter storm track into Europe is too zonal, though different measures of storm track activity indicate some improvements compared to the previous generations of models ( [[#Harvey--2020|Harvey et al., 2020]] ; [[#Priestley--2020|Priestley et al., 2020]] ).

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[[File:efca96cede6e9a3da0fcdf9a22d101a5 IPCC_AR6_WGI_Figure_4_27.png]]

'''Figure''' '''4.27 |''' '''Changes in extratropical storm track density.''' Displayed are projected spatial pattern of multi-model mean change of extratropical storm track density in winter (Northern Hemisphere December –January–Februrary, NH DJF, and Southern Hemisphere June–July–August, SH JJA) in 2080–2100 for SSP5-8.5 relative to 1979–2014 based on 13 CMIP6 models. Diagonal lines indicate regions where fewer 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 change. Units are number density per 5° spherical cap per month. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

Regarding the dynamical intensity of the storm tracks (Section 11.7.2), the number of ETCs associated with intense surface wind speeds and undergoing explosive pressure deepening are projected to strongly decrease in the NH winter ( [[#Seiler--2016|Seiler and Zwiers, 2016]] ; [[#Chang--2018|Chang, 2018]] ). The weakening of surface winds of ETCs in the NH is attributed to the reduced low-level baroclinicity from SST and sea ice changes ( [[#Harvey--2014|Harvey et al., 2014]] ; [[#Seiler--2016|Seiler and Zwiers, 2016]] ; J. [[#Wang--2017a|]] [[#Wang--2017|Wang et al., 2017]] a ). There are, however, regional exceptions such as in the northern North Pacific, where explosive and intense ETCs are projected to increase in association with the poleward shift of the jet and increased upper-level baroclinicity ( [[#Seiler--2016|Seiler and Zwiers, 2016]] ). Eddy kinetic energy and intense cyclone activity are also projected to decrease in the NH summer in association with a weakening of the jet ( [[#Lehmann--2014|Lehmann et al., 2014]] ; [[#Chang--2016|Chang et al., 2016]] ). However, explosive cyclones tend to be too weak in climate models ( [[#Seiler--2016|Seiler and Zwiers, 2016]] ; [[#Priestley--2020|Priestley et al., 2020]] ), though this bias seems to be reduced in high-resolution simulations ( [[#Jiaxiang--2020|Jiaxiang et al., 2020]] ). Furthermore, models may not fully capture the contribution of the future increase in mesoscale latent heating to cyclone intensification ( [[#Li--2014|Li et al., 2014]] ; [[#Pfahl--2015|Pfahl et al., 2015]] ; [[#Willison--2015|Willison et al., 2015]] ; [[#Michaelis--2017|Michaelis et al., 2017]] ). In conclusion, there is only ''medium confidence'' in the projected decrease in the frequency of intense NH ETCs.

In contrast to the Northern Hemisphere, the Southern Hemisphere shows an increase in the frequency of intense ETCs in CMIP5 models ( [[#Chang--2017|Chang, 2017]] ), and there is ''high confidence'' that wind speeds associated with ETCs are expected to intensify in the SH storm track for high emissions scenarios. These changes in intensity are accompanied by an overall southward shift of the SH winter storm track (Figure 4.27b) due to the poleward shift in the upper-level jet and the increase in the meridional SST gradient linked to the slower warming of the Southern Ocean ( [[#Grieger--2014|Grieger et al., 2014]] ).

Regardless of dynamical intensity changes, there is ''high confidence'' that the number of ETCs associated with extreme precipitation is projected to increase with warming, due to the increased moisture-loading capacity of the atmosphere (Section 8.4.2; [[#Yettella--2017|Yettella and Kay, 2017]] ; [[#Hawcroft--2018|Hawcroft et al., 2018]] ).

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