What caused the summer 2020 to spring 2021 drought in southwestern North America?

At the end of summer 2021 the U.S. Drought Monitor reported, "90% of the West region (including Colorado and Wyoming) is characterized as ’in drought’ with 54% in Extreme Drought or Exceptional Drought". Reservoir levels were low across almost the whole West including on the Colorado River’s Lakes Powell and Mead at just 31% and 35% of capacity, respectively. Multiple states rated most of their rangelands and pasture in poor to very poor conditions and in October a statewide drought emergency was declared for California. Drought extended from northern Mexico into western Canada and from the Pacific Ocean to the Plains. Notably, the drought was intense in the southwest despite large areas of above-normal precipitation in the summer of 2021. While not as severe as in 2021, drought conditions are still present over 90% of the Southwest in September 2022.

In a new Journal of Climate article, authors Richard Seager, Mingfang Ting, Patrick Alexander, Jennifer Nakamura, Haibo Liu, Cuihua Li, and Isla R. Simpson use reanalyses and sea surface temperature-forced climate models to examine what large-scale atmosphere-ocean conditions were responsible for the onset and intensification of this latest widespread and severe drought in southwestern North America.