Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.3.2.5 Tropical Cyclones ====

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The AR5 assessed ''low confidence'' in centennial changes in tropical cyclone (TC) activity globally, and in the attribution of observed changes in TCs to anthropogenic forcing. Since AR5, there has been considerable progress in understanding the observed changes of TCs and an overall improved knowledge of the sensitivity of TCs to both GHG and aerosol forcing ( [[#Knutson--2019|Knutson et al., 2019]] ; [[#Sobel--2019|Sobel et al., 2019]] ).

Although observational data limitations ( [[#Lau--2012|Lau and Zhou, 2012]] ) tend to limit detection of anthropogenic forced increases in TC precipitation ( [[#Knutson--2019|Knutson et al., 2019]] ), there is ''medium confidence'' that anthropogenic forcing has contributed to observed heavy rainfall events over the USA associated with TCs ( [[#Kunkel--2012|Kunkel et al., 2012]] ) and other regions with sufficient data coverage ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.2|Section 11.7.1.2]] ; [[#Bindoff--2013|Bindoff et al., 2013]] ). There has been increased frequency of TC heavy rainfall events over several areas in the USA since the late 19th century that is greater than what would be expected solely from changes in US landfall frequency, suggesting the increasing role of TCs have in causing heavy rainfall events ( [[#Kunkel--2010|Kunkel et al., 2010]] ). For example, there is evidence for an anthropogenic contribution to the extreme rainfall of Hurricane Harvey in 2017 ( [[#Emanuel--2017|Emanuel, 2017]] ; [[#Risser--2017|Risser and Wehner, 2017]] ; [[#van%20Oldenborgh--2017|van Oldenborgh et al., 2017]] ; [[#Trenberth--2018|Trenberth et al., 2018]] ; S.-Y.S. [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ).

While TCs cause extreme local rainfall and flooding, they can be also an important contributor to annual precipitation and regional fresh water resources ( [[#Hristova-Veleva--2020|Hristova-Veleva et al., 2020]] ). Transport of moisture by TCs is an important contributor for precipitation over the coastal areas of East Asia mostly from July through October, with the TC rainfall accounting for nearly 10% to 30% of the total rainfall in the region (L. [[#Guo--2017|]] [[#Guo--2017|Guo et al., 2017]] ). Local TC rainfall totals depend on rain-rate and translation speed (the speed of TC movement along the storm track) with slow TCs such as Hurricane Harvey (2017), providing a clear example of the effect of slow translation speed on local rainfall accumulation, with urbanization exacerbating the storm total rainfall and flooding ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] ; W. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ).

In addition to evidence that rain-rates have increased, there is evidence that TC translation speed has slowed globally ( [[#Kossin--2018|Kossin, 2018]] ) thus amplifying thermodynamic intensification of rainfall and may be linked to anthropogenic forcing ( [[#Gutmann--2018|Gutmann et al., 2018]] ). This is ''limited evidence'' however, so there is ''medium'' ''confidence'' of a detectable change in TC translation speed over the US. Since the 1900s, and there is ''low'' ''confidence'' for a global signal because of ''limited agreement'' among models and due to data heterogeneity. However, the slowdown is consistent with theoretical and modelling studies that indicate a general weakening of the tropical circulation with warming that reduces the speed of the TC system ( [[#Chauvin--2017|Chauvin et al., 2017]] ), though there is ''limited'' observational ''evidence'' (Sections 8.2.3.5 and 11.7.1).

In summary, there is ''medium confidence'' of an observed increase in TC precipitation intensity in regions with sufficient data coverage Robust physical understanding ( [[#8.2.3.2|Section 8.2.3.2]] ) and detailed singular event attribution studies provide evidence that tropical cyclone rainfall has increased with a warming climate ( ''high confidence'' , [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.4|Section 11.7.1.4]] ).

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