Seminar Abstract

Ze'ev Gedalof - October 8, 2002

Atmospheric and Climatic Controls on Severe Wildfire Years in the Northwestern United States

Ze'ev Gedalof, David L. Peterson and Nate Mantua

Wildland fire is an important ecological agent in forested ecosystems of the American Northwest. Historical fire suppression efforts have contributed to an accumulation of fuels in many northwestern forests, and may result in more frequent and / or more severe wildfire events. Current management approaches focus on fuels treatments as the primary method of avoiding severe wildfire years. Here we investigate the extent to which atmospheric variability may contribute to dramatic increases in area burned on 20 national forests in Washington, Oregon and Idaho. Empirical Orthogonal Function (EOF) analysis was used to identify patterns in severe wildfire years in the Pacific Northwest. Anomaly fields of 500 hPa height were regressed onto the resulting principal component time series to determine the degree to which atmospheric circulation anomalies force severe wildfire years. Additionally, cross correlation functions were calculated for the Palmer drought severity index (PDSI) over the year preceding the wildfire season. A second analysis was undertaken using the same fire and climatic data sets, but based on superposed epoch analysis rather than linear regressions. This analysis focused only on the extreme fire years (both large and small), and does not assume that the association between fire and climate is linear in nature. Four significant "modes" of burning were identified; each associated with distinct climatic processes:

1. Region-wide (with the exception of coastal temperate rainforest) increases in wildfire extent are associated with the formation of a high-pressure blocking ridge over western North America throughout the fire season. This pattern is associated with drought in the spring and summer preceding the fire season maximum, but exhibits no association with either winter precipitation or snow accumulation.
2. Increases in area burned in eastern Washington and Idaho, coupled with reductions in area burned in southern and western Oregon are associated with reduced 500 hPa heights in the region of the winter-time Aleutian Low, and increased heights over central and western North America. This pattern resembles the summer PNA pattern, and is unrelated to antecedent drought.
3. Increases in area burned in the temperate rainforests of the Pacific Northwest occur when intense drought occurs, beginning in the preceding summer and persisting throughout the fire season, coupled with particularly intense blocking.
4. Increased wildfire in the forests of southern Oregon is associated with summer cyclone activity. At these particularly dry locations these storms can deliver lightning and strong winds, but very little rain - thereby providing an ignition source and a mechanism for rapid spread.

The results of these analyses support the hypothesis that climate variability acts as an important control on the occurrence of severe wildfire years. The extent of this control is modulated by the underlying ecology of the region, with coastal temperate rainforests responding to lower frequency variability than the drier steppe and savannah ecosystems of the interior. Stronger relationships were observed for the superposed epoch analysis than for the linear regressions, supporting the inference that climatic controls are more important in forcing extreme wildfire years than more moderate wildfire years. The current fire-management culture, which emphasizes fuels treatment, does not recognize that climatic variability may cause severe wildfire years. Consequently, fire managers may fail to anticipate extreme wildfire years.

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