The Importance of Anthropogenic Aerosols to Recent and Future Precipitation Trends in the Southwest United States

Abstract:

Observations indicate that the Southwest United States has experienced significant multi-decadal precipitation variability, with an increase from 1950 to ~1980, followed by a decrease since. Using the Coupled Model Intercomparison Project version 5 (CMIP5) archive, we show that the model simulations also yield similar changes, with anthropogenic aerosols accounting for nearly all of the simulated response and about 1/3 of the observed trends. Consistent with these precipitation changes, both observations and models forced with anthropogenic aerosols yield a reversal in the trend of the leading pattern of sea surface temperature variability in the North Pacific, the Pacific Decadal Oscillation. Future CMIP5 RCP4.5 ensemble mean also shows a significant negative trend in SW US precipitation. Additionally, NCAR Community Atmosphere Model version 5 (CAM5) RCP4.5 anthropogenic aerosol forcing (CAM5-AA) experiment shows a significant drying trend throughout 21 century. The CAM5-AA experiment also shows a significant negative PDO trend throughout 21 century. By altering the position of the jet stream, our results suggest that anthropogenic aerosols have perturbed Southwest United States precipitation and have significantly contributed to the recent—and future—drying trend.