EFFECTS OF LARGE-SCALE ATMOSPHERE-OCEAN OSCILLATIONS ON THE NATURE OF AVALANCHE HAZARD IN WESTERN CANADA Bret Shandro and Pascal Haegeli* School of Resource and Environmental Management, Simon Fraser University, Burnaby BC, Canada ABSTRACT: Numerous large-scale atmosphere-ocean oscillations including El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Pacific North American Pattern (PNA) and the Artic Oscil- lation (AO) are known to substantially affect winter weather patterns in western Canada. Several studies have examined the effect of these oscillations on avalanche hazard using long-term avalanche activity records from highway avalanche safety programs. While these studies offer valuable insights, they do not offer a compre- hensive perspective on the influence of these oscillations because the underlying data only represent the conditions at a few locations in western Canada where avalanches are tightly managed. We present a new approach for gaining insight into the relationship between atmosphere-ocean oscillations and avalanche hazard in western Canada that uses information published in public avalanche bulletins. Our approach converts hazard assessments recorded according to the conceptual model of avalanche hazard into an avalanche winter characterization following Shandro and Haegeli (2018) and uses mixed effects models to identify response patterns in the prevalence of typical avalanche hazard situations. Even though our study period is short, the large-scale patterns emerging from our analysis agree reasonably well with the known impacts of the oscillations on winter weather in western Canada. However, we also find numerous smaller scale patterns that indicate that the effects on avalanche hazard are more complicated and regionally variable. KEYWORDS: El Nino-Southern Oscillation, Pacific Decadal Oscillation, Pacific North America Pattern, Arctic Oscillation, avalanche hazard, avalanche climate 1. INTRODUCTION Avalanche hazard conditions continuously evolve over the course of a winter in response to the se- quence of weather events. Much of the existing ava- lanche research is focused on examining the short- term effects of weather on avalanche conditions to improve operational avalanche forecasting. How- ever, examining the relationship between longer- term variations in weather patterns and the nature of avalanche hazard can also offer valuable insight for the development of seasonal avalanche hazard fore- casts and help improve our understanding of the ef- fect of climate change on avalanche hazard. The most prominent and well understood large-scale atmosphere-ocean oscillations affecting the winter weather patterns in western Canada, include El Nino- Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Pacific North American Pat- tern (PNA) and the Artic Oscillation (AO). Interested readers are referred to Fleming et al. (2006), Shabbar and Bonsal (2004) or Stahl et al. (2006) for detailed descrptions. Fitzharris (1987), McClung (2013) and Thumlert et al. (2014) have examined the relationship between ava- lanche hazard and these oscillations using ava- lanche observations from highway avalanche safety programs. In general, these studies found that the ENSO and PDO significantly correlate with overall avalanche activity and the ratio between dry and wet avalanches. While these studies offer valuable in- sight into the effect of these weather patterns on av- alanche hazard, they have considerable limitations. Most importantly, the avalanche observations used to describe the nature of avalanche hazard in these studies (frequency of avalanches, ratio between dry and wet avalanches) only provide a very incomplete characterization of the challenges for avalanche risk management. The existing studies are also unable to provide a comprehensive perspective on the overall effect across western Canada since records from highway programs only offer limited point observa- tions of the experienced avalanche conditions. In ad- dition, it is difficult to conclusively attribute the ob- served patterns to changes in winter weather pat- terns since avalanche observation time series from highway operations are vulnerable to changes in av- alanche control practices. The information included in public avalanche bulle- tins offers a much more regional and richer perspec- tive on the nature of the avalanche hazard than ava- lanche observations at point locations. While the qualitative nature of avalanche bulletin information has prevented its use in quantitative climate analyses in the past, the recent introduction of the conceptual model of avalanche hazard (CMAH; Statham et al. * Corresponding author address: Pascal Haegeli School of Resource & Environ. Management Simon Fraser University 8888 University Drive Burnaby BC V5A 1S6 Canada web: www.avalancheresearch.ca email: pascal_haegeli@sfu.ca phone: +1 778 782 3579 Proceedings, International Snow Science Workshop, Innsbruck, Austria, 2018 1145