Multi-decadal variability of fire weather and fire extent in northern California and Oregon

We used the Haines Index (HI) and the Energy Release Component (ERC) as fire weather indices to investigate how interannual fire weather variability in Oregon and northern California is related to atmospheric circulation and to fire extent. HI (1961-2000) and ERC (1973-2005) time series showed strong spatial correlation and regional averages were used. Tree-ring based reconstructions were used to extend the HI and ERC time series back to 1922, to allow comparison with the complete annual area burned record. High fire risk years corresponded to years with a high annual area burned, and vice versa. The association between fire extent and HI is highest for plume-driven fires in Oregon. We found a consistent association between reconstructed HI and ERC variation and fire extent variation, indicating the reliability of our reconstruction and confirming the strong associations in the instrumental period. High fire risk years correspond to positive PNA conditions, characterized by enhanced regional mid-tropospheric ridging and low atmospheric moisture, and vice versa. HI combines atmospheric moisture with atmospheric instability and provides a better measure of wildfire extent than indices based on moisture alone.