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Atmospheric rivers (ARs) are one of the major causes of extreme precipitation and flooding in many regions around the world and have been found to contribute substantially to global poleward moisture transport. However, the evaporative origin of the moisture in ARs remains unclear, at least on climatological timescales. Here we use the water tracer and water isotope-enabled Community Atmosphere Model version 5 (CAM5) model to examine the moisture sources of ARs that impact the West Coast of the United States. The climatological distribution of moisture sources is determined for both the modern era and for 2100 under a Representative Concentration Pathway (RCP) 8.5 scenario. It is found that 33 to 53% of the precipitable water over the West Coast of the United States originates from the Northeast Pacific, in particular the midlatitudes and subtropics, although in June-July-August more moisture is recycled from continental regions. It is also found that although ARs are at least 70% Northeast Pacific moisture, a significant amount of the moisture is derived from tropical latitudes (>15% from south of 20 degrees N). It is found that in the warmer 2100 climate there is a 39% increase in the magnitude of ARs. In this future epoch, moisture is transported from more remote regions for all seasons and for both ARs and the average climatology. To provide future observational evidence that this model result is robust, it is shown that water isotopes provide an observational constraint on the moisture transport pathways, and thus the possibility to observe changes in moisture source.

Plain Language Summary Atmospheric rivers (ARs) are long, narrow filaments of elevated water vapor and water vapor transport. Although they have a major impact on precipitation and other water cycle processes, where the water is actually sourced from on average is still being debated. Here we use a global climate model that can track water in the atmosphere to determine where the water in ARs that strike the West Coast of the United States originates. It was found that most (70%) of the water comes from the local Northeast Pacific, but that a large fraction (>15%) also comes from the tropics. It was also found that the more intense the AR, the more tropical water it contained. A future climate simulation was also analyzed, and it was found that as the climate gets warmer, the intensity of the water vapor transport in ARs increases, as well as the amount of water from more remote locations. Finally, a relationship between water source and the isotopic composition of the water in the AR was found in the model, providing the possibility for the modeled water source results to be evaluated using observations.