Wednesday, 14 November 2001: 10:50 AM
Atmospheric forcing of severe wildfire years in the American Northwest
The association between area burned by wildfire in the Pacific Northwest region of the United States and 500 hPa height anomalies was investigated using multivariate statistical techniques. Region-wide large fire years are associated with prolonged positive 500 hPa anomalies over western North America and the eastern Pacific Ocean. The persistence of these features appears to be more important than either their frequency or total duration in determining the seasonal area burned. When this regional record is decomposed using empirical orthogonal function (EOF) analysis a slightly different perspective emerges. Analysis of seasonal area burned in 20 National Forests since 1948 shows that there are coherent spatial patterns in area burned. These patterns are associated with distinct spatial and temporal atmospheric circulation anomalies. While most severe fire years are associated with summertime high-pressure blocking systems, the specific shape, location and timing of these systems seems to influence the specific region most affected. Additionally, severe wildfire years in the driest forests may be associated with low-pressure anomalies - presumably as a consequence of increased lightning ignitions and strong winds. A comparison of pre- early- and mid-season climatic predictors of wildfire season severity suggests that summertime conditions are much more important than antecedent conditions in forcing severe wildfire years.