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

===== 8.4.2.8.2 Atmospheric rivers =====

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Atmospheric rivers were not assessed in AR5 but are important in the water cycle as they are linked to extreme rainfall, flooding, and changes in terrestrial water storage including melt and ablation of glaciers and snowpack (Sections 8.2.3). In a warming world, there is ''high confidence'' that thermodynamical increases in atmospheric water vapour ensure that atmospheric rivers will become wetter, hence stronger, and longer-lasting ( [[#Payne--2020|Payne et al., 2020]] ). This is clearly observed in several regional ( [[#Ralph--2011|Ralph and Dettinger, 2011]] ; [[#Lavers--2013|Lavers et al., 2013]] ; [[#Gao--2015|Gao et al., 2015]] ; [[#Payne--2015|Payne and Magnusdottir, 2015]] ; [[#Warner--2015|Warner et al., 2015]] ; [[#Hagos--2016|Hagos et al., 2016]] ; [[#Gershunov--2019|Gershunov et al., 2019]] ) and in one global study (V. [[#Espinoza--2018|]] [[#Espinoza--2018|Espinoza et al., 2018]] ) of atmospheric river activity in CMIP5 model projections. [[#Lavers--2015|Lavers et al. (2015)]] indicate that integrated vapour transport under RCP 8.5 and 4.5 could increase, and consequently this thermodynamic response ( [[#O’Gorman--2015|O’Gorman, 2015]] ) could affect mid-latitude regions where orographic precipitation is important ( [[#Gershunov--2019|Gershunov et al., 2019]] ).

Under continued global warming, more intense moisture transport within atmospheric river events is projected to increase the magnitude of heavy precipitation events on the west coast of the USA ( [[#Ralph--2011|Ralph and Dettinger, 2011]] ; [[#Lavers--2015|Lavers et al., 2015]] ; [[#Warner--2017|Warner and Mass, 2017]] ), in Western Europe ( [[#Lavers--2015|Lavers et al., 2015]] ; [[#Ralph--2016|Ralph et al., 2016]] ; [[#Ramos--2016|Ramos et al., 2016]] ), and in East Asia ( ''very likely'' ) ( [[#Kamae--2019|Kamae et al., 2019]] ). All CMIP5 models analysed agreed under a range of scenarios, except over the Iberian Peninsula ( [[#Ramos--2016|Ramos et al., 2016]] ) where there is only ''low confidence'' in projected changes. [[#Kamae--2019|Kamae et al. (2019)]] reported a 1% increase per °C warming in the frequency of atmospheric rivers affecting East Asia, but this is strongly affected by SST changes. Emerging evidence of possible regional changes due to dynamical factors are uncertain ( [[#Lavers--2013|Lavers et al., 2013]] ; [[#Gao--2015|Gao et al., 2015]] ; [[#Payne--2015|Payne and Magnusdottir, 2015]] ). The frequency, magnitude and duration of atmospheric rivers making landfall along the North American west coast are projected to increase ( [[#Gershunov--2019|Gershunov et al., 2019]] ). In contrast, V. [[#Espinoza--2018|]] [[#Espinoza--2018|Espinoza et al. (2018)]] suggest that the number of atmospheric river events is projected to slightly decrease globally.

In semi-arid regions where atmospheric rivers have historically been important and precipitation is mainly confined to the cold season, the contribution of atmospheric rivers to annual total precipitation may be expected to grow disproportionately. For example, in California decreases in precipitation frequency are projected as a result of fewer non-atmospheric river storms, while the projected increase in heavy and extreme precipitation events are almost entirely a result of increased atmospheric river activity ( [[#Gershunov--2019|Gershunov et al., 2019]] ). Interannual variability in precipitation amounts is projected to increase because of the overall decrease in the frequency of storms but a stronger dependence on extremes ( [[#Polade--2014|Polade et al., 2014]] ), particularly due to atmospheric rivers ( [[#Gershunov--2019|Gershunov et al., 2019]] ), especially where interaction with topography are important ( Polade et al. , 2014; Gershunov et al., 2019 ).

In summary, there is ''high confidence'' that the magnitude and duration of atmospheric rivers are projected to increase in future, leading to increased precipitation. This is projected to increase the intensity of heavy precipitation events on the west coast of the USA and in western Europe ( ''high co'' ''nfidence'' ).

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