In summer 2021, 90% of the western United States (WUS) experienced drought, with over half of the region facing extreme or exceptional conditions, leading to water scarcity, crop loss, ecological degradation, and significant socio-economic consequences. Beyond the established influence of oceanic forcing and internal atmospheric variability, this study highlights the importance of land-surface conditions in the development of the 2020-21 WUS drought, using observational data analysis and novel numerical simulations.

Our results demonstrate that the soil moisture state preceding a meteorological drought, due to its intrinsic memory, is a critical factor in the development of soil droughts. Specifically, wet soil conditions can delay the transition from meteorological to soil droughts by several months or even nullify the effects of La Niña-driven meteorological droughts, while drier conditions can exacerbate these impacts, leading to more severe soil droughts. For the same reason, soil droughts can persist well beyond the end of meteorological droughts. Our numerical experiments suggest a relatively weak soil moisture-precipitation coupling during this drought period, corroborating the primary contributions of the ocean and atmosphere to this meteorological drought. Additionally, drought-induced vegetation losses can mitigate soil droughts by reducing evapotranspiration and slowing the depletion of soil moisture.

This study highlights the importance of accurate representations of land-surface processes in seasonal-to-interannual drought predictions. Findings from this study may have implications for regions like the WUS, which are experiencing anthropogenically-driven soil aridification and vegetation greening, suggesting that future soil droughts in these areas might develop more rapidly, become more severe, and persist longer.

Our research highlights the important role of vegetation and soil moisture in influencing drought evolution in the WUS. Understanding these land-surface conditions is crucial for characterizing hydroclimatic changes at both regional and global scales under anthropogenically-driven climate change.