THE WINTER OF JUST HOW WEIRD WAS IT EXACTLY?

Transcription

1 Courtesy Reuters Buffalo New York THE WINTER OF JUST HOW WEIRD WAS IT EXACTLY? Eyad Atallah and John Gyakum McGill University

2 Some Headlines Certainly for large portions of southern Canada and the northern USA, the story of the winter was unusually cold weather resulting in scenes like this from Niagara Falls. (March 2, 2014 High temp -13C, low -19C.

3 Some Headlines In fact, The Great Lakes showed the largest amount of Ice Cover in over 35 years with only Lake Ontario remaining largely ice free

4 Meanwhile on the other side of the Atlantic The United Kingdom was unusually warm with wide swaths where the temperature was over 2 C above normal. And at the same time precipitation amounts were generally % above normal

5 A case of extremes An examination of Climate Division Rankings shows that parts of the Midwest had a top 5 coldest winter while parts of the Southwest had the warmest cool season ever Climate Division Data

6 The extremes were not a local phenomena, but the cold was An examination of the hpa Thickness relative to a 66 year climatology shows a small area of -3C near Hudson Bay and corresponding +3C anomalies in the North Pacific and Eastern Europe

7 The extremes were not a local phenomena, but the cold was The previous picture becomes a little more stark when you consider the ranks of the thickness over the 66 Year period since 1949

8 Hemispheric Average Thickness The negative anomalies from this winter are particularly impressive when you consider the background state of the atmosphere is warming Northern Hemisphere Area Weighted Thickness Ave Min-Ave In fact 2014 had the 4 th warmest average thickness and the coldest average thickness value is also the 4 th t

9 Latitude Band Average Thickness In fact, Polar latitudes experienced the highest average thickness on record consistent with the southern displacemen t of the Polar Vortex Area Weighted Average hpa Thickness 45-60N 75-90N 15-30N Linear (45-60N) Linear (75-90N) Linear (15-30N) y = x R² = y = x R² = y = x R² =

10 Specific Thickness Thresholds (<480 dam)

11 Specific Thickness Thresholds (<498 dam)

12 Specific Thickness Thresholds (<510 dam)

13 Specific Thickness Thresholds The story behind the thickness thresholds indicates that it really wasn t the quality or intensity of the cold air that was at issue, instead it really was the persistence of the pattern.

14 So why so persistent?

15 Persistence The reason for the persistence is somewhat unclear. A popular perception is that weakening zonal winds result in slower propagation/ more blocking.

16 Persistence However, another possibility is the warm pool in the eastern North Pacific. This warm pool ranks as the 1 st ranked warm anomaly relative to an climatology for the 5 month period.

17 Persistence Interestingly, a global view of the SST anomalies from ESRL suggests that the North Pacific warm pool is THE anomaly globally.

18 Finding Analogues The problem with finding analogues for the winter is the overall trend in thickness, particularly at higher latitudes Area Weighted Average hpa Thickness 45-60N 75-90N 15-30N Linear (45-60N) Linear (75-90N) Linear (15-30N) y = x R² = y = x R² = y = x R² =

19 Finding Analogues Analogues are based on the correlations of the thickness anomaly pattern relative to the pattern that we see in 2014

20 Finding analogues However, doing so results in all of the 7 best analogues (outside of 1990) occurring in the past 15 years because of the ubiquitous warm anomalies in Polar Regions. To get around this problem a linear trend in the thickness is calculated at every grid point leading to a detrended anomaly pattern.

21 Detrended Anomaly Basic Anomaly Detrended Anomaly

22 Analogues Detrended Thickness Anomaly SST Anomalies

23 Persistence A similar global pattern of SST anomalies is again found for 1990

24 Persistence SST anomalies for the five best analogues to An inspection of each of the analogues indicates a North Pacific warm pool...

25 But surely this winter will be different

26 Notable points 4 th Warmest Hemispherically Juxtaposition of Coldest and Warmest average thicknesses in 66 years over the North Atlantic Warmest ever average thickness in the Arctic Air not exceptionally cold/warm, but pattern exceptionally persistent Analogues all show significant North Pacific Warm Pool