The relationship between the polar jet stream and extreme wildfire events in North America

Abstract

Northern Hemisphere midlatitude weather is strongly influenced by the polar jet stream (PJS), which dictates the position of storm tracks; this influence also extends to weather patterns conducive to the ignition and growth of large wildfires. We examined the role of the PJS on extreme wildfire events in North America (NA) between 408 and 708N latitude, using fire spread events (FSEs) for 2002-16. Climatologies of the 300-hPa wind components and derived quantities show that the PJS weakens and moves northward in the boreal summer coincident with the fire season. We use spatiotemporal compositing of 300-hPa winds and 500-hPa geopotential height anomalies to show that FSEs are associated with an upper-level ridge and high centered over events, except eastern Canada where patterns are displaced westward. Ridge patterns persist longer for FSEs in western NA compared with eastern NA, as well as for May-August compared with April, September, and October. These tropospheric patterns also occur concomitantly with surface weather drivers of fire spread including positive daily mean temperature and vapor pressure deficit anomalies and negative precipitation anomalies. Distributions of maximum and minimum latitudinal jet stream peak, and ridge and trough positions, relative to FSEs, confirm that events occur predominantly southward of the jet stream core and near a ridge for low split flow configurations but not necessarily for high split flow configurations. These findings have wide-reaching implications for better understanding NA fire regimes and potential fire management strategies (e.g., resource prepositioning and tactical suppression) through improved forecasting of fire weather.