The Synoptic-Scale Circulation during the Western U.S. Drought of 2021 and 2022

Abstract While large-scale climate drivers such as El Niño–Southern Oscillation (ENSO) have been widely studied in relation to western U.S. droughts, the influence of synoptic-scale atmospheric patterns remains less well understood. This study examines the 2021–22 drought in the western United States by analyzing synoptic-scale conditions using the fifth major global reanalysis produced by ECMWF (ERA5) and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis data, comparing them to the 1991–2020 climatology and the nondrought year of 2019. Results show that both winters during the drought period were dominated by anomalously strong upper-level ridging and expansive surface high pressure systems over the Pacific, producing persistent blocking patterns that limited precipitation across much of the West. In summer, while surface pressure anomalies were near climatological norms, a persistent upper-level ridge continued to reinforce regional dryness. However, the North American monsoon brought substantial moisture to the interior Southwest in both 2021 and 2022, partially alleviating drought conditions in that region. These findings underscore the critical role of synoptic-scale circulation features—particularly atmospheric blocking—in determining seasonal drought severity. The study highlights the importance of incorporating synoptic-scale analysis into drought forecasting and water resource management to improve preparedness and long-term resilience. Significance Statement This study highlights how persistent high pressure blocking systems during the wet winter season can prolong drought conditions into the dry summer months in the western United States, exacerbating water scarcity and impacting ecosystems and agriculture. However, the North American monsoon plays a crucial role in delivering seasonal moisture to the Southwest, partially alleviating drought in that region. Understanding these synoptic patterns is essential for improving drought forecasting and water resource management, helping policymakers and stakeholders develop more effective mitigation strategies in an increasingly variable climate.