Numerical Evaluation of the Modern and Future Origins of Atmospheric River Moisture Over the West Coast of the United States

Abstract

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°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.

Climate, Landforms, and Geology Affect Baseflow Sources in a Mountain Catchment

“…The interpretations about differences in water sources between average and drought conditions were very clear in our case given that δ 18 O and δ 2 H were higher in 2015 precipitation compared to 2016 precipitation. Precipitation was more enriched in 2015 because of smaller fractionation rates due to the higher temperatures in this year, consistent with model simulations (Liu et al, ; Nusbaumer et al, ; Nusbaumer & Noone, ). This strong contrast in precipitation isotopes between years enhanced our ability to demonstrate that summer baseflow during the drought did not originate from the recent precipitation.…”

Recent Warming of Landfalling Atmospheric Rivers Along the West Coast of the United States

“…One relevant aspect of ARs for understanding their temperature evolution is that they source moisture both locally and remotely. The majority of winter (DJF) moisture comes from the Northeast Pacific (Nusbaumer & Noone, ), yet a number of case studies have demonstrated examples of both large‐scale advection and local moisture convergence as the dominant mechanism (e.g., Dacre et al, ; Sodemann & Stohl, ). Our results (Figure ) likewise suggest that AR temperature evolution varies across the AR's lifespan, depending on the season and region of eventual landfall.…”

The Connected Isotopic Water Cycle in the Community Earth System Model Version 1

“…Water tagging has been implemented in the atmospheric component of CESM with new infrastructure within CAM and CLM to facilitate a variety of applications. For instance, the scheme has already been used to examine global (Singh et al, ) and regional (Dyer et al, ) variations in moisture source, as well as to determine the average moisture sources for specific weather phenomena (Nusbaumer & Noone, ). This water tagging can be applied to water isotopologues as well, allowing one to determine the impact of moisture source and pathway changes on the isotope ratios for a particular region (Tabor et al, ; Zhu et al, ).…”

“…Efforts to simulate water isotope ratios in climate models span at least half a decade. Earlier isotope-enabled simulations typically included only the atmosphere and land surface (Field et al, 2014;Hoffmann et al, 1998;Joussaume et al, 1984;Jouzel et al, 1987;Jouzel et al, 1991;Mathieu et al, 2002;Noone & Simmonds, 2002;Noone & Sturm, 2010;Werner et al, 2011), and have been found to be reliable in reproducing the extensive database of precipitation observations from the Global Network for Isotopes in Precipitation (GNIP) (IAEA/ WMO, 2016). There are fewer more recent atmospheric models with isotopic tracers that have been compared to isotope ratios of tropospheric water vapor from satellite or in situ observations (notably, Schmidt et al, 2005Nusbaumer et al, 2017).…”