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A uniform, global approach is used to quantify how atmospheric rivers (ARs) change between Coupled Model Intercomparison Project Phase 5 historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios. The projections indicate that while there will be similar to 10% fewer ARs in the future, the ARs will be similar to 25% longer, similar to 25% wider, and exhibit stronger integrated water vapor transports (IVTs) under RCP8.5. These changes result in pronounced increases in the frequency (IVT strength) of AR conditions under RCP8.5: similar to 50% (25%) globally, similar to 50% (20%) in the northern midlatitudes, and similar to 60% (20%) in the southern midlatitudes. The models exhibit systematic low biases across the midlatitudes in replicating historical AR frequency (similar to 10%), zonal IVT (similar to 15%), and meridional IVT (similar to 25%), with sizable intermodel differences. A more detailed examination of six regions strongly impacted by ARs suggests that the western United States, northwestern Europe, and southwestern South America exhibit considerable intermodel differences in projected changes in ARs.

Plain Language Summary Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes. These "rivers in the sky" have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and West Europe) due to orographic lifting. ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall, which can mitigate droughts, they can also lead to flooding and extreme winds. This study takes a uniform, global approach that is used to quantify how ARs change between Coupled Model Intercomparison Project Phase 5 historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios globally. The projections indicate that while there will be similar to 10% fewer ARs in the future, the ARs will be similar to 25% longer, similar to 25% wider, and exhibit stronger integrated water vapor transports under RCP8.5. These changes result in pronounced increases in the frequency (integrated water vapor transport strength) of AR conditions under RCP8.5: similar to 50% (25%) globally, similar to 50% (20%) in the northern midlatitudes, and similar to 60% (20%) in the southern midlatitudes.