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

==== 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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