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What Caused The Summer 2020 to Spring 2021 Drought in Southwestern North America?



Caption: View of Lake Oroville showing extremely low water during a drought. On this date, the storage was 915,957 reservoir acre-feet (AF), which is 26 percent of the total capacity. Photo taken July 26, 2021 by Kelly M. Grow/ California Department of Water Resources.
  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.  The study reveals the drought onset over summer 2020 to spring 2021 was caused by four consecutive seasons of below-normal precipitation. The driest summer on record occurred in 2020 and started the drought. This was dominated by extreme dry conditions in August. From winter 2020 through spring 2021 the worsening drought conditions were guided by the development of a La Niña in the tropical Pacific Ocean. Decadal variability in the Pacific Ocean aided drought in the southern part of the region by driving the cool-season to be drier during the last two decades. In addition, there is also evidence that the southern part of the region is drying in spring due to human-driven climate change. In conclusion, the drought onset was driven by a combination of internal atmospheric variability and interannual climate variability, while also aided by natural decadal variability and human-driven climate change. This work motivates work to improve our observational records of decadal variability and its impacts on remote hydroclimate as well as the ability of climate models to capture those variability modes and their impacts. Funding for this project was provided in part by the NOAA Climate Program Office, MAPP program and National Integrated Drought Information System.

Read the full study here.

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