The association between circulation anomalies in the mid-troposphere and area burned by wildland fire in Canada

There is evidence that the area burned by wildland fire has increased in ce rtain regions of Canada in recent decades. One cause for this increase is c hanges in the midtropospheric circulation at 500 hPa over northern North Am erica. This study examines the physical links between anomalous mid-troposp heric circulation over various regions of Canada and wildland fire severity . Analysis of monthly and seasonal burned areas for the period 1953 to 1995 reveals a bimodal distribution with distinct low and extreme high burned a rea years. The high/low burned area years coincide with positive/negative 5 00 hPa height anomalies over north-western, western, west-central and east- central Canada. Total area burned and the 500 hPa height anomaly data are a nalyzed for statistical relationships using the Spearman rank correlation n on-parametric measure. Results for the May to August fire season indicate s tatistically significant correlations between regional total area burned an d clusters of anomalous 500 hPa geopotential height values immediately over , and immediately upstream of the affected region. For the north-western an d west-central regions, significantly correlated clusters are found in the central Pacific as well, providing evidence of the influence of a teleconne ction structure on the summer climate of western and north-western North Am erica. Two sample comparison tests show statistically significant differenc es in both the means and variances of the fire data populations during nega tive and positive phases of midtropospheric flow, and the means of the heig ht anomaly populations during extremely high and extremely low area burned seasons. Increases in regional total area burned are related to increases i n mean 500 hPa heights, taken from the significantly correlated clusters of height values, between two successive periods 1953-74 and 1975-95. For Can ada as a whole, the five lowest area burned seasons all occurred during the early period, while the five highest seasons occurred during the later per iod. The difference in the geopotential height fields between the two perio ds identifies an increase in 500 hPa heights over most of Canada with an am plification of the western Canada ridge and an eastward shifted Canadian Po lar Trough (CPT).