The blocked summer of 2009 over eastern Canada

1. The blocked summer of 2009 over eastern Canada Environment Canada Meteorological Service of Canada Montréal, Québec

2. Summary Summer of 2009 over eastern Ontario, Quebec and the Atlantic provinces was cool, cloudy and wet. New records were established in several locations for high rainfall, low sunshine hours and number of rainy days. Such weather had significant impacts on agriculture, transportation, tourism and cultural and sporting events, not to mention vacationers who had to cope with disappointing weather for a second summer in a row. Flooded basements and roads were reported on several occasions. In some instances, extensive leeching of soils by heavy rains contaminated waters. These conditions were created by several episodes of blocked atmospheric circulation. During these episodes, the easterly component of the flow at 500 hPa was blocked by upper closed lows or very deep and elongated upper troughs. Near the surface, low pressure systems stayed quasi-stationary for three to seven days at a time, resulting in sequences of cool and wet weather. We present here an analysis of atmospheric blockings of summer 2009, their seasonal signature, and a hypothesis about their cause.

3. Summer 2009 weather Temperature anomalies averaged - 4C during the summer months over Ontario, southern Quebec and N-B., with a maximum anomaly of - 6C over the Great Lakes.

4. Summer 2009 weather Total rainfall was above normal almost everywhere and reached 200% of normal values over south-western Quebec and Nova-Scotia.

5. Summer 2009 weather Geopotential height at 500hPa was below normal from June through August, with a maximum anomaly reaching -8 dam over the Great Lakes in July. Also in July, a strong positive anomaly was observed over Nunavut which, combined with the negative anomaly, created a record breaking gradient. Indeed, never had the 500 hPa height anomalies been so high in the north and, at the same time, so low in the south. This pattern is typical of a NAO-negative height anomaly pattern.

6. Summer 2009 weather Regions having the greatest temperature and rain anomalies at the same time were eastern Ontario and western Quebec. The city of Ottawa located in this region witnessed record-breaking or near record-breaking values of cold, wet and cloudy days for July 2009.

7. Summer 2009 blocking events During the summer of 2009, a total of six blocked lows and two blocked highs (yellow and pink boxes below) affected eastern Canada; the last one starting on Sept 1 (numbers refer to following maps). The blocking having the most impacts is the one extending June 29 through July 7 (no. 4).

8. Summer 2009 blocking events

9. Summer 2009 blocking events

10. Summer 2009 blocking events

11. Blocking index Here is displayed the 92-day time-longitude diagram of the summer. Over eastern Canada (pink frame, 90-60W), four of the six blockings identified previously are pointed-out by arrows (red: early July severe event). Note that summertime values are typically below 5 m/deg-lat, whereas wintertime values reach 20 m/deg-lat.

12. Blocking frequency The comparison of blocked summer days over the period 1979-2009 shows a record number of blocked days during the summer of 2009, e.g. five times more than the 30-year mean value of 2.2. The previous summer (2008) also shows an abnormally high number of blocked days, which is probably why summer 2008 was also cool and wet. (NARR-NCEP data, TM90 index, 60-90W)

13. Blocking frequency versus NAO The North Atlantic Oscillation (NAO), a pattern having considerable impact on the weather of northeastern NA was showed to be strongly correlated with blocking frequency at these longitudes*. Plotting our calculated blocking frequency against the NAO index shows the same result: a relatively good correlation at 0.66. Particularly for summers 2007 through 2009, correlation stands-out even better and constant. * Croci-Maspoli et al. 2007. Atmospheric blockings – their influence on the NAO and PNA. AMS 87th Annual meeting

14. Blocking frequency versus NAO Analyzing further the link NAO - blockings, the following maps show that the 500 hPa height anomaly during summer 2009 is similar to the mean anomaly of summers having NAO index lower than -1.5 SD.

15. Conclusion Atmospheric blockings are important to understand because they create sequences of constant surface conditions which can result in abnormal, sometimes extreme, conditions be it excessive rain or lack of rain, excessive heat or lack of heat or excessive sun or lack of sun. Blockings over eastern Canada during summer of 2009 were abnormally frequent. Blocked lows created six spells of cold and wet weather, and blocked highs created two late spells of hot and dry weather. Seasonally, the impacts of the blocked lows was greatest, creating autumn-like conditions in the heart of the summer. These blockings were possibly the result of a modified circulation by the important negative phase of the North Atlantic Oscillation during summer 2009 but also summer of 2008. The numbers for 2009 and 2008, and most of all the correlation between blocking frequency and NAO index over the past 31 years support this hypothesis.