Climatology of winter cold spells in relation to mountain pine beetle mortality in British Columbia, Canada

A recent epidemic of mountain pine beetles (MPB) has caused mortality in extensive stands of pine trees in British Columbia, Canada. The epidemic has been attributed, in part, to the recent warming trend in winter in western Canada, as MPB experience mortality during extreme cold spells. This study aimed to clarify the roles of synoptic-scale circulation and large-scale climate modes in these recent trends. Potential cold-mortality events were identified by comparing recorded daily minimum air temperatures with experimentally determined critical thresholds. Annual event frequency has declined over past decades, and between 1998 and 2001 temperatures did not reach the 100% MPB mortality thresholds at the stations analysed. Event frequencies depended on the phase of the teleconnection indices. In particular, after the shift to a predominantly positive phase of the Pacific Decadal Oscillation (PDO) following 1976, cold-mortality events occurred mainly during strongly negative Arctic Oscillation (AO) years. The dominant synoptic-scale circulation pattern causing widespread low temperatures is Arctic Outbreak, although other circulation types can be important, depending on location. The frequencies of these cold circulation types varied with the teleconnection indices. In addition, the conditional probability of temperatures cold enough to cause MPB mortality for a given synoptic type varies with the teleconnection phase, particularly for the Pacific North America circulation pattern and PDO.