The Character and Changing Frequency of Extreme California Fire Weather

Five of California's 10 largest wildfires occurred in 2020, with the largest complex exceeding the previous largest by more than 100%. The year follows a decade containing extraordinary fire activity. Previous trend investigations focused on changes in human activities and atmospheric thermodynamics, while the impacts of changing atmospheric dynamics are largely unknown. Here, we identify weather types (WTs) associated with historically large daily burned areas in eight Californian regions. These WTs characterize dominant fire weather regimes varying in fire behavior types (plume‐driven vs. wind‐driven fires) and seasonality. Most of the strongly large‐scale forced WTs such as Santa Ana and Diablo events increased in frequency during the 20th century particularly in the San Diego and Bay Area regions. These changes are likely not anthropogenically caused and the frequency of such events is projected to decrease under continuing climate change. However, significant future increases are found for WTs associated with thermal‐low‐pressure systems along the California coast and in the Sierra west region. These increases in southern California are mainly due to increasing greenhouse‐gas forcing and arise from the larger ocean‐land temperature gradient while aerosol forcing changes are driving most of the increased frequency in central and northern California due to a reduction of relative humidity over land and a strengthening of low‐pressure anomalies over the coast. These WT frequency changes could permit more weather favorable for large fire growth in summer and less in fall, further enhancing the risk of catastrophic fires due to hotter and drier summers in future climates. Key Points Fire weather types (WTs) are shown to be related to the largest burned area days in recent decades in eight Californian regions Increases in Diablo and Santa Ana WT events during the 20th century are primarily caused by natural climate variability Changing greenhouse gas and aerosol forcing significantly increase thermal‐low WTs in coastal regions and the western Sierra