Cool season precipitation projections for California and the Western United States in NA-CORDEX models

Mahoney, K., Scott, J. D., Alexander, M., McCrary, R., Hughes, M., Swales, D., & Bukovsky, M. (2021). Cool season precipitation projections for California and the Western United States in NA-CORDEX models. Climate Dynamics, 56(9–10), 3081–3102. https://doi.org/10.1007/s00382-021-05632-z

Summary and relevance

Mahoney et al. examined the projected cool-season (October-March) precipitation changes for the Western U.S. for the late 21st century (2070-2099) from the dynamically downscaled NA-CORDEX dataset, in which 6 regional climate models (RCMs) run at 25-km and/or 50-km resolution were driven by one or more projections from CMIP5 global climate models, most under RCP8.5. Dynamical downscaling with RCMs can provide more insights about physical processes operating at local and regional scales to produce climate changes than statistical downscaling.

Mahoney et al. found, as expected, that the RCMs run at 25 km were better able to reproduce historical precipitation magnitudes across the West because of more realistic simulation of the topography and its influence on precipitation. They found that the NA-CORDEX projected future changes in cool-season precipitation for the Colorado River Basin were relatively consistent in sign across the GCM-RCM combinations; the ensemble average changes under RCP8.5 were mainly increases (0 to +15%) across the Upper Basin, and mainly no change or decreases (0 to -20%) across the Lower Basin. Changes were more pronounced for the January-March sub-season. The models run at 25 km showed slightly larger precipitation changes (both increases and decreases) than the same models run at lower resolution.

Mahoney et al.'s other findings from the NA-CORDEX projections for 2070-2099:

• Extreme daily precipitation (>99th percentile, i.e., the 1-in-0.3-year event) during the cool season increased in magnitude across all parts of the Upper Basin (+5 to +20%) and most of the Lower Basin (0 to +15%).
• The ratio of annual snowfall to annual precipitation declined by 10-30% across the mountains of the Upper and Lower basins, indicating a shift away from snow and towards rain.
• Seasonal peak snow-water equivalent (SWE) decreased markedly across the Upper and Lower basins.

Overall, Mahoney et al.'s findings show considerable agreement with previous studies of Western U.S. cool-season precipitation change in which statistical downscaling was applied to CMIP5 projections (e.g., LOCA, MACA, BCSD). For the Colorado River Basin, the range or uncertainty seen across the NA-CORDEX model ensemble is likewise similar to the uncertainty across the statistically downscaled CMIP5 ensemble. While a tendency towards wetter cool seasons for the Upper Basin, but towards drier for the Lower Basin, is seen in most of the NA-CORDEX models, the opposite outcomes are seen in some models.