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

==== 11.7.1.2 Observed Trends ====

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Identifying past trends in TC metrics remains a challenge due to the heterogeneous character of the historical instrumental data, which are known as ‘best-track’ data ( [[#Schreck--2014|Schreck et al., 2014]] ). There is ''low confidence'' in most reported long-term (multi-decadal to centennial) trends in TC frequency- or intensity-based metrics due to changes in the technology used to collect the best-track data. This should not be interpreted as implying that no physical (real) trends exist, but rather as indicating that either the quality or the temporal length of the data is not adequate to provide robust trend detection statements, particularly in the presence of multi-decadal variability.

There are previous and ongoing efforts to homogenize the best-track data ( [[#Elsner--2008|Elsner et al., 2008]] ; [[#Kossin--2013|Kossin et al., 2013]] , 2020; [[#Choy--2015|Choy et al., 2015]] ; [[#Landsea--2015|Landsea, 2015]] ; [[#Emanuel--2018|Emanuel et al., 2018]] ) and there is substantial literature that finds positive trends in intensity-related metrics in the best-track during the ‘satellite period’, which is generally limited to around the past 40 years ( [[#Kang--2012|Kang and Elsner, 2012]] ; [[#Kishtawal--2012|Kishtawal et al., 2012]] ; [[#Kossin--2013|Kossin et al., 2013]] , 2020; [[#Mei--2016|Mei and Xie, 2016]] ; [[#Zhao--2018|Zhao et al., 2018]] ; [[#Tauvale--2019|Tauvale and Tsuboki, 2019]] ). When best-track trends are tested using homogenized data, the intensity trends generally remain positive, but are smaller in amplitude ( [[#Kossin--2013|Kossin et al., 2013]] ; [[#Holland--2014|Holland and Bruyère, 2014]] ). [[#Kossin--2020|Kossin et al. (2020)]] extended the homogenized TC intensity record to the period 1979–2017 and identified significant global increases in major TC exceedance probability of about 6% per decade. In addition to trends in TC intensity, there is evidence that TC intensification rates and the frequency of rapid intensification events have increased within the satellite era ( [[#Kishtawal--2012|Kishtawal et al., 2012]] ; [[#Balaguru--2018|Balaguru et al., 2018]] ; [[#Bhatia--2018|Bhatia et al., 2018]] ). The increase in intensification rates is found in the best-track and the homogenized intensity data.

A subset of the best-track data corresponding to hurricanes that have directly impacted the USA since 1900 is considered to be reliable, and shows no trend in the frequency of USA landfall events ( [[#Knutson--2019|Knutson et al., 2019]] ). However, an increasing trend in normalized USA hurricane damage, which accounts for temporal changes in exposed wealth ( [[#Grinsted--2019|Grinsted et al., 2019]] ), and a decreasing trend in TC translation speed over the USA (Kossin, 2019) have also been identified in this period. A similarly reliable subset of the data representing TC landfall frequency over Australia shows a decreasing trend in Eastern Australia since the 1800s ( [[#Callaghan--2011|Callaghan and Power, 2011]] ), as well as in other parts of Australia since 1982 ( [[#Chand--2019|Chand et al., 2019]] ; [[#Knutson--2019|Knutson et al., 2019]] ). A paleoclimate proxy reconstruction shows that recent levels of TC interactions along parts of the Australian coastline are the lowest in the past 550–1500 years ( [[#Haig--2014|Haig et al., 2014]] ). Existing TC datasets show substantial inter-decadal variations in basin-wide TC frequency and intensity in the western North Pacific, but a statistically significant north-westward shift in the western North Pacific TC tracks since the 1980s ( [[#Lee--2020|]] [[#Lee--2020|T.-C. Lee et al., 2020]] ). Inthe case of the North Indian Ocean, analyses of trends are highly dependent on the details of each analysis (e.g., pre- and/or post-monsoon season period, or Bay of Bengal and/or Arabian Sea region). The most consistent trends are an increase in the occurrence of the most intense TCs, and a decrease in the overall TC frequency, in particular in the Bay of Bengal ( [[#Sahoo--2016|Sahoo and Bhaskaran, 2016]] ; [[#Balaji--2018|Balaji et al., 2018]] ; [[#Singh--2019|Singh et al., 2019]] ; [[#Baburaj--2020|Baburaj et al., 2020]] ). In the South Indian Ocean (SIO), an increase in the occurrence of the most intense TCs has been noted; however, there are well-known data quality issues there ( [[#Kuleshov--2010|Kuleshov et al., 2010]] ; [[#Fitchett--2018|Fitchett, 2018]] ). When the SIO data are homogenized, a significant increase is found in the fractional proportion of global Category 3–5 TC instances (6-hourly intensity estimates during the lifetime of each TC) to all Category 1–5 instances ( [[#Kossin--2020|Kossin et al., 2020]] ).

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[[File:0adffd6e83a0f7f63ea9c6dad9ad8006 IPCC_AR6_WGI_Figure_11_20.png]]

'''Figure 11.20 |''' '''Summary schematic of past and projected changes in tropical cyclone (TC), extratropical cyclone (ETC), atmospheric river (AR), and severe convective storm (SCS) behaviour.''' Global changes (blue shading) from top to bottom: ''(i)'' Increased mean and maximum rain rates in TCs, ETCs, and ARs [past ( ''low confidence'' due to lack of reliable data) and projected ( ''high confidence'' )]; ''(ii)'' Increased proportion of stronger TCs [past ( ''medium confidence'' ) and projected ( ''high confidence'' )]; ''(iii)'' Decrease or no change in global frequency of TC genesis [past ( ''low confidence'' due to lack of reliable data) and projected ( ''medium confidence'' )]; and (iv) Increased and decreased ETC wind speed, depending on the region, as storm tracks change [past ( ''low confidence'' due to lack of reliable data) and projected ( ''medium confidence'' )]. Regional changes, from left to right: ''(i)'' Poleward TC migration in the western North Pacific and subsequent changes in TC exposure [past ( ''medium confidence'' ) and projected ( ''medium'' ''confidence'' )]; ''(ii)'' Slowdown of TC forward translation speed over the contiguous USA and subsequent increase in TC rainfall [past ( ''medium confidence'' ) and projected ( ''low'' ''confidence'' due to lack of directed studies)]; and ''(iii)'' Increase in mean and maximum SCS rain rate and increase in spring SCS frequency and season length over the contiguous USA [past ( ''low confidence'' due to lack of reliable data) and projected ( ''medium confidence'' )].

As with all confined regional analyses of TC frequency, it is generally unclear whether any identified changes are due to a basin-wide change in TC frequency, or to systematic track shifts (or both). From an impacts perspective, however, these changes over land are highly relevant and emphasize that large-scale modifications in TC behaviour can have a broad spectrum of impacts on a regional scale.

Subsequent to AR5, two metrics have been analysed that are argued to be comparatively less sensitive to data issues than frequency- and intensity-based metrics. Trends in these metrics have been identified over the past 70 years or more ( [[#Knutson--2019|Knutson et al., 2019]] ). The first metric – the mean latitude where TCs reach their peak intensity – exhibits a global and regional poleward migration during the satellite period ( [[#Kossin--2014|Kossin et al., 2014]] ). The poleward migration can influence TC hazard exposure and risk ( [[#Kossin--2016a|Kossin et al., 2016a]] ) and is consistent with the independently observed expansion of the tropics ( [[#Lucas--2014|Lucas et al., 2014]] ). The migration has been linked to changes in the Hadley circulation ( [[#Altman--2018|Altman et al., 2018]] ; [[#Sharmila--2018|Sharmila and Walsh, 2018]] ; [[#Studholme--2018|Studholme and Gulev, 2018]] ). The migration is also apparent in the mean locations where TCs exhibit eyes ( [[#Knapp--2018|Knapp et al., 2018]] ), which is when TCs are most intense. Part of the Northern Hemisphere poleward migration is due to basin-wide changes in TC frequency ( [[#Kossin--2014|Kossin et al., 2014]] , 2016b; [[#Moon--2015|Moon et al., 2015]] , 2016) and the trends, as expected, can be sensitive to the time period chosen ( [[#Tennille--2017|Tennille and Ellis, 2017]] ; [[#Kossin--2018|Kossin, 2018]] ; [[#Song--2018|Song and Klotzbach, 2018]] ) and to subsetting of the data by intensity ( [[#Zhan--2017|Zhan and Wang, 2017]] ). The poleward migration is particularly pronounced and well-documented in the western North Pacific basin ( [[#Kossin--2016a|Kossin et al., 2016a]] ; [[#Oey--2016|Oey and Chou, 2016]] ; [[#Liang--2017|Liang et al., 2017]] ; [[#Nakamura--2017|Nakamura et al., 2017]] ; [[#Altman--2018|Altman et al., 2018]] ; [[#Daloz--2018|Daloz and Camargo, 2018]] ; J. [[#Sun--2019|]] [[#Sun--2019|]] [[#Sun--2019|]] [[#Sun--2019|Sun et al., 2019]] ; [[#Lee--2020|]] [[#Lee--2020|T.-C. Lee et al., 2020]] ; [[#Yamaguchi--2020a|Yamaguchi and Maeda, 2020a]] ; [[#Kubota--2021|Kubota et al., 2021]] ).

A second metric that is argued to be comparatively less sensitive to data issues than frequency- and intensity-based metrics is TC translation speed ( [[#Kossin--2018|Kossin, 2018]] ), which exhibits a global slowdown in the best-track data over the period 1949–2016. TC translation speed is a measure of the speed at which TCs move across the Earth’s surface, and is very closely related to local rainfall amounts (i.e., a slower translation speed causes greater local rainfall). TC translation speed also affects structural wind damage and coastal storm surge by changing the hazard event duration. The slowdown is observed in the best-track data from all basins except the Northern Indian Ocean, and is also found in a number of regions where TCs interact directly with land. The slowing trends identified in the best-track data by [[#Kossin--2018|Kossin (2018)]] have been argued to be largely due to data heterogeneity. [[#Moon--2019|Moon et al. (2019)]] and [[#Lanzante--2019|Lanzante (2019)]] provide evidence that meridional TC track shifts project onto the slowing trends, and argue that these shifts are due to the introduction of satellite data. Kossin (2019) provides evidence that the slowing trend is real by focusing on Atlantic TC track data over the contiguous USA in the 118-year period 1900–2017, which are generally considered reliable. In this period, mean TC translation speed has decreased by 17%. The slowing TC translation speed is expected to increase local rainfall amounts, which would increase coastal and inland flooding. In combination with slowing translation speed, abrupt TC track direction changes – that can be associated with track ‘meanders’ or ‘stalls’ – have become increasingly common along the North American coast since the mid-20th century, leading to more rainfall in the region ( [[#Hall--2019|Hall and Kossin, 2019]] ).

In summary, there is mounting evidence that a variety of TC characteristics have changed over various time periods. It is ''likely'' that the global proportion of Category 3–5 tropical cyclone instances and the frequency of rapid intensification events have increased globally over the past 40 years. It is ''very likely'' that the average location where TCs reach their peak wind intensity has migrated poleward in the western North Pacific Ocean since the 1940s. It is ''likely'' that TC translation speed has slowed over the USA since 1900.

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