Summary/Abstract
The western United States (WUS) experienced a record-breaking drought from September 2020 to August 2021. Here, the authors examine how anthropogenic climate change impacted its severity by comparing its present-day meteorological and hydrological conditions with an identical drought had it played out in a preindustrial climate. These events are simulated using the Weather Research and Forecasting (WRF) Model as well as an offline calibrated land surface model. To simulate preindustrial hydrologic conditions, the authors modify the forcing data used to drive our offline land surface model using two different techniques: (i) implementing the pseudo–global warming method with global climate model (GCM) deltas (13 model mean) to produce dynamically downscaled changes and (ii) directly applying GCM deltas. The authors’ dynamically downscaled simulation exhibits significant anthropogenic drought-period precipitation increases leading to enhanced streamflow in all three major WUS riverine basins. This is distinct from the hydrologic changes derived directly from GCM deltas and found in previous literature, especially across the Colorado River basin where GCMs show precipitation decreases in response to warming. These disparities may be the result of several interconnected factors, such as (i) large uncertainty in GCM precipitation changes, (ii) variation in GCM connective parameterizations, (iii) inaccurate portrayal of the North American monsoon in GCMs and WRF, and (iv) WRF grid boundary placement. The authors do not make any conclusive statements about the drought’s climate response, but instead, use this work as a laboratory to explore methodological and physical complexities in WUS drought attribution to support other modelers undertaking future studies.