Diminished Windstorm Frequency in Southwest British Columbia and a Possible Association With the Pacific Decadal Oscillation Regime Shift of

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1 Diminished Windstorm Frequency in Southwest British Columbia and a Possible Association With the Pacific Decadal Oscillation Regime Shift of Mantua, N. M. Wolf Read PhD Program Forest Science University of British Columbia Green, R. A.

2 Outline Brief background on the PDO Windstorm climatology at CYVR Windstorm climatology at other stations Supporting evidence Planned directions

3 Pacific Decadal Oscillation Warm Phase Cool Phase The PDO index is a measure of monthly sea-surface temperature variability poleward of 20º N (Mantua 2000) Mantua, N. M The pacific decadal oscillation (PDO). January Joint Institute for the Study of the Atmosphere and Ocean, University of Washington. Accessed April 17, 2008 at

4 Pacific Decadal Oscillation Transition Present Strong Warm Phase Slow Cooling Mantua, N. M The pacific decadal oscillation (PDO). January Joint Institute for the Study of the Atmosphere and Ocean, University of Washington. Accessed April 17, 2008 at

5 Pacific Decadal Oscillation Negative Pacific North American Index (PNA) reflects an Aleutian low further west, and a zonal upper-air pattern conducive to winterstorminess in the Puget Lowlands (Ebbesmeyer 1989) Ebbesmeyer, C. E. and Coombs, C Strong, Low-Frequency (Decadal) Environmental Fluctuations During the 20 th Century. Oceans 89 Proceedings, vol. 1, pp

6 Vancouver International (CYVR) Windstorms Discrete Storm Events MSC stations switch From recording wind in mph to knots The Great Calm

7 Vancouver International (CYVR) Windstorms Different ways of characterizing the Great Calm at CYVR

8 A Broader Examination Isolating discrete storms is time-consuming Used a faster method to explore PDO idea before large time investment Method: Isolate windstorm events by day Method imperfect: Two discrete storms can occur in a single day One storm can easily be spread over two days Rarely, a single storm can spread over three days Generally results in over-counting of events Discrete storms are the logical unit of measure and isolation of events a various stations continues

9 Preliminary Multi-Station Analysis Vancouver International Airport : 7.2 events per year : 3.9 events per year Ratio: 1.8

10 Preliminary Multi-Station Analysis Victoria International Airport : 5.0 events per year : 0.8 events per year Ratio: 6.3

11 Preliminary Multi-Station Analysis Toronto International Airport : 12.7 events per year : 13.8 events per year Ratio: 0.9

12 Preliminary Multi-Station Analysis Halifax International Airport : 23.3 events per year : 14.4 events per year Ratio: 1.6

13 The Halifax Problem The Halifax response may be problematical: Could the PDO influence the weather at stations far downstream? If negative PDO (cold-phase) supports troughiness along the west coast, might it also support a trough along the east coast? Example: Average circulation for Oct 1962 (Green 1963) Green, R. A The Weather and Circulation of October 1962, A Warm Month with a Mid- Month Circulation Reversal. Monthly Weather Review, vol. 91, no. 1, Jan 1963, pp

14 Proxy Evidence for PDO Effect Tuller used the geostrophic wind potential based on hourly pressure data from Vancouver, Victoria and Comox Trends in wind speed based on geostrophic wind potential also point to a decrease during the Great Calm This proxy suggests that a natural cause is likely a significant contributor to the observed surface wind speed trends Tuller, S. E Measured Wind Speed Trends on the West Coast of Canada. International Jounral of Climatology, vol. 24, pp

15 Proxy Evidence for PDO Effect More evidence: Ebbesmeyer (1989) looked at wind direction and speed in the Puget Sound Basin Using 5-year averages for winter wind speed parameters and the PNA these correlation coefficients were returned for the time series : Measure Compared to PNA Frequency of southerly ( ) wind speeds Correlation Coefficient Frequency of wind speed >5.7 m/s Frequency of wind speed >8.7 m/s The Tuller and Ebbesmeyer studies support a relationship between PDO and wind speeds in the Puget-Georgia Basin Ebbesmeyer, C. E. and Coombs, C Strong, Low-Frequency (Decadal) Environmental Fluctuations During the 20 th Century. Oceans 89 Proceedings, vol. 1, pp

16 Preliminary Thoughts and Conclusions (PDO) Transition from mph to knots in the timeframe may have had an influence on the wind record: If so, the influence was probably minor Some control stations, especially Toronto, appear to have escaped the Great Calm, and may support the idea that the transition from mph to knots did not have a detectible influence on the wind record The 1989 Ebbesmeyer study found a reduction in wind speed post transition at NWS stations in Puget Sound: These stations have a different history of instrumental and methodological change than Canadian stations If the effect is real, Victoria may have been influenced by the PDO more strongly than Vancouver

17 Questions / Comments