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For details of ERA-20C are available at https://www.ecmwf.int/search/siteen/forecasts/datasets/search?search_api_fulltext=era20cera-20c
ECMWF also released an "AMIP style" (Atmospheric Model Intercomparison Project) version of the 20th Century reanalysis, known as ERA-20CM, which includes no data assimilation. It's an ensemble of ten atmospheric model integrations for the years 1899–2010.
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