Enhanced Hydrological Extremes in the Western United States Under Global Warming Through the Lens of Water Vapor Wave Activity
The large-scale nature of the atmospheric water cycle poses a great challenge for predicting how regional hydrology will respond to future climate change. Led by scientists from the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL), researchers developed a new analysis method—called local finite amplitude atmospheric wave activity, or LWA—to examine how hydrological extremes at distinct locations around the globe will change in the future. They found that the rate at which water vapor cycles through the atmosphere will slow down, allowing more water vapor to reside in the atmosphere, ready to be converted to extreme precipitation under the right conditions.
Tapping moisture from the tropics, atmospheric rivers making landfall over the U.S. Pacific Coast will grow longer and more frequent in a warmer climate. The ability to predict these types of Earth system changes are important for society, both at the regional and global scale.
Globally, the atmosphere is a “reservoir” of moisture, with precipitation acting as a moisture sink and evaporation acting as a moisture source. In a changing climate, the amount of precipitation that reaches Earth’s surface could fluctuate greatly by region compared to historical trends. Researchers developed the novel LWA diagnostic method for water vapor to represent local hydrological cycles. Notably, a strong linear relationship emerged between LWA and its “sink”—precipitation. Using this relationship to explore the response of hydrological extremes to a climate warming scenario, researchers applied the method to simulations by the Climate Model Intercomparison Project Phase 5 (CMIP5) for both the past and future. The projections showed intensified winter precipitation-evaporation extremes over the West Coast of the United States. This was due to the lengthening of the atmospheric rivers carrying more water vapor from the tropics and making landfall at the Pacific Coast. These trends occurred both for the eastward extension of the westerly jet in the eastern Pacific, and the eastward shift of the teleconnection pattern of the El Niño-Southern Oscillation. These projections indicate that the unusually wet winter experienced on the U.S. West Coast in 2016/2017 might be a harbinger of more frequent wet extremes in a warmer climate.