The climatological moisture sources and sinks for Atmospheric Rivers impacting the West Coast of the United States in modern and doubled CO2 climates.

Abstract:

Atmospheric rivers are known to be major drivers of precipitation in many parts of the globe, including the West Coast of the United States. Atmospheric rivers are usually identified as long, filamentary structures of elevated vertically-integrated water vapor transport, and can extend over thousands of km, from the tropics all the way to the midlatitudes. However, it is generally unclear where exactly the moisture in atmospheric rivers comes from, and if that moisture is transported over long distances across the entire river, or if it is continuously precipitated out and replenished by local moisture. By better constraining the sources and sinks of moisture in atmospheric rivers, one can learn which geographic regions or meteorological processes are important for forecasting the rivers’ hydrological impacts. Also, by determining how these moisture sources and sinks will change with the global climate, one can gain a better understanding of future atmospheric river conditions, and thus better future hydroclimate projections.

We show the sources and sinks of water vapor and cloud condensate in atmospheric rivers as simulated by the water tracer-enabled NCAR Community Atmosphere Model Version 5 (CAM5). Specifically, numerous regions in the North Pacific and Western North America are tagged with water tracers, and their relative contributions to atmospheric river moisture and precipitation are calculated for rivers that impact the West Coast of the United States. Two simulations using the model are performed, one with modern-day boundary conditions, and another with doubled-CO₂ conditions. It is found that the majority of moisture in atmospheric rivers comes from the subtropical (20 to 30° N) Pacific, although a substantial portion of moisture for some atmospheric rivers also comes from the tropics (<20° N), indicating an occasional direct tropical connection. It is also found that the proportional amount of water tracer moisture decreases the farther away the river is from the source, indicating a continuous loss by precipitation, and a replacement by locally evaporated moisture. However, there can still be non-trivial amounts of all the moisture sources at the termination end of the river, indicating that long-distance moisture transport can occur.