Proceedings, International Snow Science Workshop, Banff, 2014...Foundation, and TECTERRA for their support of ASARC. Last, but not least, Scott Thumlert would like to thank the Alberta

RELATING AVALANCHES TO LARGE-SCALE OCEAN – ATMOSPHERIC OSCILLATIONS

Scott Thumlert1, Sascha Bellaire2, Bruce Jamieson1

1 Dept. of Civil Engineering, University of Calgary, AB, Canada 2 Institute of Meteorology and Geophysics, University of Innsbruck, Tyrol, Austria

ABSTRACT: Sea surface temperatures and sea-level pressures in the Pacific and Arctic oceans have been shown to affect weather patterns in western Canada, thus they can affect the snow avalanche ac-tivity. Major oscillations of sea-surface temperature and sea-level pressure have been shown to exist on the 2 to 15 year time-scales. In this paper, avalanche data from over 17,800 avalanches recorded from six different roadways in western Canada were analyzed with respect to several of these oscillations. We studied the El Nino Southern Oscillation, Pacific Decadal Oscillation, Arctic Oscillation, Pacific / North American Pattern, and the North Atlantic Oscillation. Larger and more frequent avalanche activity was found during the Pacific Decadal Oscillation negative phase and during the El Nino Southern Oscillation negative phase (La Niña) for avalanches classified as dry. Conversely, avalanches classified as wet in-creased during the Pacific Decadal Oscillation positive phase and during the El Nino Southern Oscillation positive phase (El Niño). The Artic Oscillation correlated positively with all wet and dry avalanche activity (not significant). Understanding the relationship between avalanche activity in western Canada and these oscillations provides some advanced prediction of general avalanche climate which is helpful for planning avalanche hazard mitigation programs. Finally, global climate change is likely to affect these climate oscil-lations; thus, the relationship between avalanche activity and these climate oscillations provides some insight into how avalanche activity could be affected by climate change.

KEYWORDS: Avalanche forecasting, El Nino Southern Oscillation, Pacific Decadal Oscillation, Arctic Oscillation, Pacific North American Pattern, North Atlantic Oscillation

1. INTRODUCTION

This preliminary analysis explores the effect of several large-scale ocean-atmosphere oscillations on avalanche activity in western Canada.

The most actively researched and studied oscilla-tion, the El Niño Southern Oscillation (ENSO), has been shown to affect global weather patterns (Phi-lander, 1989). The negative phase of ENSO has been shown to increase snowfall (Smith and O’Brien, 2001) and avalanche activity (McClung, 2013) in western Canada. Note, for this study we used the Multivariate El Niño Index (MEI) (Wolter and Timlin, 2011).

The Pacific Decadal Oscillation (PDO) affects western Canada weather similarly to MEI in that the negative phase brings cooler weather and in-creased snowfall (Mantua and Hare, 2002). The Arctic Oscillation (AO) is defined as non-seasonal

sea level pressure variations north of 20 deg lati-tude. The negative phase affects western Canada by pushing the jet stream north into Alaska and thus directing storm tracks away from southwest-ern BC. The negative phase creates a “blocking high” or “Omega block”, whereas the positive phase creates more of a “zonal flow” pattern (Thomson and Wallace, 1998).

The North Atlantic Oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean that mostly affects storm tracks across the North Atlan-tic. The NAO is closely related to the AO (Bjerknes, 1964), therefore we included it in this analysis. The Pacific North American Pattern con-sists of anomalies in geopotential height fields typ-ically between 500 – 700 mb. It is strongly influenced by MEI and affects western Canada climate similarly in that the positive phase brings warmer weather, higher winter precipitation, and higher air pressures (Leathers et al., 1991).

To date, few studies have been conducted on the influence of these large-scale climate oscillations on avalanche activity in western Canada. McClung (2013) showed more snow, more avalanches, and a higher percentage of dry avalanches than wet

* Corresponding author address: Scott Thumlert, University of Calgary, 2500 University Dr. NW, Calgary, AB, T2N 1N4 tel: 403-700-4393 email: [email protected]

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avalanches during the negative phase of ENSO. Bellaire (2013) showed data from Roger’s Pass, a mountainous highway corridor in western Canada, indicating an increasing frequency of avalanche activity with the negative phase of PDO and EN-SO. He also recognized that PDO and ENSO have greater affects on avalanche activity when they are in phase. Mock and Birkeland (2000) showed data from several sites in the central Rocky Moun-tains of USA that indicated relationships between the snow avalanche climate and both the PNA and the PDO.

The oscillations investigated here are actively re-searched and forecasted, thus understanding the connection to avalanche activity yields potential for

advanced prediction of avalanche hazard in west-ern Canada.

2. METHODS

We used a dataset of observed avalanches from the British Columbia Ministry of Transportation and Infrastructure (BCMoTI) for six highway passes. Approximately 17,800 slab avalanches from 30 years were analyzed. By definition, larger ava-lanches have higher destructive potential, thus larger avalanches are more hazardous than smaller ones. To account for this, the observed avalanches were modified by size (Table 1), loosely based on the vulnerability work of Ja-mieson and Jones (2012). An avalanche activity index was created by summing all the modified avalanche observations for each year.

Yearly indices for all the oscillations were created by averaging monthly values. The yearly oscilla-tion indices were then related to the yearly ava-lanche activity using Spearman rank correlations.

Figure 1: Boxplots of the MEI for wet (top) and dry (bottom) avalanche activity. The negative, positive, and neutral classifications were based on ½ a standard deviation of the index. Boxes span the interquartile range. Whiskers extend to the data point closest to 1.5 times the interquar-tile range. Open circles indicate outliers.

Figure 2: Boxplots of the AO for all (top) and dry (bottom) avalanche activity. Classifications and boxplots as in Figure 1.


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Table 2 shows the correlations with significance levels.

Table 1: Modification of avalanche sizes Size Modification

1 0

2 0.01

3 0.2

4 0.5

5 1

3. RESULTS

PDO and MEI showed a significant increase in dry avalanche activity during their negative (cool)

phases, whereas the positives phases produced non significant increases in wet avalanche activity. Interestingly, PDO and MEI correlated significantly with each other.

The AO showed non-significant increases in ava-lanche activity for both wet and dry. Surprisingly the PNA showed no significant correlation with avalanche activity. The NAO showed a significant correlation with wet avalanche activity.

An additive combination of all the indices showed significant relations with avalanche activity. More dry avalanches were found when the combined index was negative and more wet avalanche activ-ity was found when the combined index was posi-tive.

Figures 1 – 4 show boxplots of the important cor-relations.

Figure 3: Boxplots of the PDO for wet (top) and dry (bottom) avalanche activity. Classifications and boxplots as in Figure 1.

Figure 4: Boxplots of the combined index for dry (top) and wet (bottom) avalanche activity. Classifi-cations and boxplots as in Figure 1.


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4. DISCUSSION

The results of the preliminary analysis presented here confirm some relationship between ava-lanche activity and large-scale climate oscillations on a year-to-year scale. This is promising as fore-casted values of these indices are readily availa-ble. Thus, the opportunity for advanced prediction of avalanche activity is available. However, what is not clear is the actual effect on avalanche risk.

The negative phases of MEI and PDO may create winters with higher precipitation from consistent storms that lead to more predictable storm instabil-ities, but few deep weaknesses in the snow cover. Thus, the increased avalanche activity observed may not actually be more hazardous. Conversely, the lower dry avalanche activity observed with the positive phases of PDO and MEI may result from dry spells that create persistent weaknesses in the snow cover. These weaknesses may create more hazardous avalanche conditions overall despite fewer avalanches. We recommend a more in-volved analysis of how the climate oscillations af-fect avalanche character (Atkins, 2004). Such a study would provide more accurate description of how the climate oscillations contribute to ava-lanche risk.

Another limitation of this study is the yearly time scale. The lag time between changing indices and the weather patterns in western Canada is not known. If El Niño (negative MEI) conditions devel-op, how long until the weather patterns across western Canada are influenced, thus affecting avalanche patterns? The indices studied here in-

volved different climatic phenomena, thus the lag times will be different for each oscillation. An anal-ysis with daily or monthly data is required to as-sess the lag times for each individual oscillation. A lag time analysis would likely lead to better corre-lations between avalanching and the oscillations.

Finally, global climate change is likely to affect the climate oscillations and this is a very active mete-orological research topic. Thus, if the link between avalanche risk and the climate oscillations can be established, the opportunity to predict how ava-lanche risk is likely to change with a changing global climate may be possible.

5. ACKNOWLEDGEMENTS

We would like to thank NSERC, Parks Canada, HeliCat Canada, the Canadian Avalanche Associ-ation, Mike Wiegele Helicopter Skiing, Canada West Ski Areas Association, Backcountry Lodges of British Columbia Association, the Association of Canadian Mountain Guides, Teck Mining Compa-ny, the Canadian Ski Guide Association, Back-country Access, the B.C. Ministry of Transportation and Infrastructure Avalanche and Weather Programs, the Canadian Avalanche Foundation, and TECTERRA for their support of ASARC. Last, but not least, Scott Thumlert would like to thank the Alberta Scholarship program for financial support from the Queen Elizabeth II scholarship.

Table 2: Spearman rank correlations between the climate oscillations and avalanche activity (n = 30 years). Stars indicate significance levels which are listed below.


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6. REFERENCES

Atkins, R., 2004. An avalanche characterization checklist for backcountry travel decisions. Pro-ceedings of the 2004 International Snow Sci-ence Workshop, Jackson Hole, Wyoming: pp. 462 – 468.

Bellaire, S., 2013. Relating avalanche activity to climate change and coupled ocean - atmos-pheric phenomena. Davos Atmosphere and Cryosphere Assembly 2013, Davos, Switzer-land. Abstract only.

Bjerknes, J. 1964. Atlantic air–sea interaction. Ad-vances in Geophysics, 10: pp. 1–82.

Daniel L., Yarnal, B., and Palecki, M., 1991. The pacific/north American teleconnection pattern and united states climate. Part i: regional tem-perature and precipitation associations. Journal of Climate, 4: pp. 517–528.

Jamieson, B., and Jones, A., 2012. Vulnerability: caught in an avalanche – then what are the odds? Proceedings of the 2012 International Snow Science Workshop, Anchorage, Alaska: pp. 1 – 8.

Mantua, N., Hare, S., 2002. The Pacific Decadal Oscillation" Journal of Oceanography 58 (1): pp. 35–44.

McClung, D., 2013. The effects of El Niño and La Niña on snow and avalanche patterns in British Columbia, Canada, and central Chile. Journal of Glaciology, 59 (216): pp. 783 – 792.

Mock, C., and Birkeland, K., 2000. Snow ava-lanche climatology of the western United States mountain ranges. Bulletin of the American Me-teorological Society, 81: pp. 2367–2392.

Philander, S., 1989. El Nino, La Nina and the Southern Oscillation. Academic Press; 1 edi-tion.

Smith, S. and O’Brien, J., 2001. Regional snowfall distributions associated with MEI: implications for seasonal forecasting. Bulletin of American Meteorological Society, vol 82 (6): pp. 1179 – 1191.

Thompson, D., and J. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopo-tential height and temperature fields. Geophys-ical Research Letters, 25 (9): pp. 1297-1300.

Wolter, K., and Timlin, M., 2011. El Niño/Southern Oscillation behaviour since 1871 as diagnosed in an extended multivariate MEI index (MEI.ext). International Journal of Climatolo-gy, 31 (7): pp. 1074 – 1087


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Proceedings, International Snow Science Workshop, Banff, 2014...Foundation, and TECTERRA for their support of ASARC. Last, but not least, Scott Thumlert would like to thank the Alberta