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

===== 8.3.2.8.2 Atmospheric rivers =====

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Atmospheric rivers (ARs) are long, narrow (up to a few hundred kilometres wide), shallow (up to few kilometres deep) and transient corridors of strong horizontal water vapour transport that are typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone ( [[#Ralph--2018|Ralph et al., 2018]] ). Atmospheric rivers were not assessed in AR5. ARs are associated with atmospheric moisture transport from the tropics to the mid- and high latitudes ( [[#Zhu--1998|Zhu and Newell, 1998]] ), although the drivers of moisture transport relative to the different airstreams within extratropical cyclones remains a subject of current study ( [[#Dacre--2019|Dacre et al., 2019]] ). While much previous research has focused on the west coast of North America, ARs occur throughout extratropical and polar regions (e.g., [[#Guan--2015|Guan and Waliser, 2015]] ) and are often associated with locally-heavy precipitation, including a substantial fraction of all mid-latitude extreme precipitation events (e.g., [[#Waliser--2017|Waliser and Guan, 2017]] ). ARs also affect East Asia strongly during the period from late spring to summer ( [[#Kamae--2017|Kamae et al., 2017]] ). ARs can be related to warming/melt events trough the intrusions of warm and moist air in Antarctica, Greenland and New Zealand ( [[#Bozkurt--2018|Bozkurt et al., 2018]] ; [[#Mattingly--2018|Mattingly et al., 2018]] ; [[#Little--2019|Little et al., 2019]] ), contributing about 45 – 60% of total annual precipitation in subtropical South America ( [[#Viale--2018|Viale et al., 2018]] ). They also '''''transport moisture from South America to the western and central South Atlantic, feeding the ARs that reach the west coast of South Africa''''' ( [[#Ramos--2019|Ramos et al., 2019]] ). However, the estimation of precipitation rate from ARs can have large uncertainties, especially as ARs hit topographically complex coastal regions ( [[#Behrangi--2016|Behrangi et al., 2016]] ), which can cause complexities in quantifying AR-related precipitation.

Analysis of observed trends in the characteristics of ARs has been limited. [[#Gershunov--2017|Gershunov et al. (2017)]] and Sharma and Déry (2019) have shown a rising trend in land-falling AR activity over the west coast of North American since 1948. ( [[#Gonzales--2019|Gonzales et al., 2019]] ) have also documented a seasonally-asymmetric warming of ARs affecting the West Coast of the USA since 1980, which has hydrological implications for the timing and magnitude of regional runoff. Longer-term paleoclimate analysis of ARs is even more limited, although Lora et al. (2017) reported that in the last glacial maximum, AR landfalls over the North American west coast were shifted southward compared to the present conditions.

In summary, it is ''likely'' that there was an increasing trend in the AR activity in the eastern North Pacific since the mid-20th century. However, there is ''low confidence'' in the magnitude of this trend and no formal attribution, although such an increase in activity is consistent with the expected and observed increase in precipitable water associated with human-induced global warming.

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