2014 vs 2012.. A tale of 2 winters..

Comparison of daily temperature anomalies at Winnipeg
during winters of 2011-12 (top) and 2013-14 (bottom)
(images from CPC/NCEP)

The winter of 2013-14 has been the coldest winter in 35 years in Winnipeg and the 3rd coldest in over a century. The winter has been dominated by relentless cold and bitter wind chills that began in early December and hasn't let up through the end of February. Even for hearty Winnipegers who are used to cold winters, this winter has been a difficult one to get through. From bursting water mains to school and highway closures, the cold has taken a toll on everyday life in southern Manitoba. This winter seems especially harsh given how incredibly mild the winter was just two years ago.(see chart left) 

(NOTE: "Winter" is defined as the 3 month period from December through February)
Average winter temperature for Winnipeg : -14.5C (1981-2010 average)

                                             Dec-Feb            Dec-Feb

                                             2011-12              2013-14

Avg temp ........................  - 9.7C .......... -20.3C

diff from normal .................. + 4.8C  .........  -  5.8C
rank out of 141 years......  4th warmest ..... 11th coldest

# of days above normal ........   70    ...........     18

# of days below normal ........   18   ............     66

# of days > 10C below normal    1 .............     25
# of -20C days ....................     24   ............     75  (4th most in 141 winters)

# of -25C days ....................       8   ............     55

# of -30C days ....................       0  .............     27

One can see how this winter has been a complete flip from the winter of 2011-12. This winter is over 10C colder than 2 years ago, with 3.5 times the number of below normal days.  The winter of 2011-12 was the 4th warmest on record in Winnipeg since 1872. This winter has been the 11th coldest, and the 3rd coldest in the last 120 years. Whereas we've seen almost a month's worth of -30C days this winter, there were NONE during the entire winter of 2011-12. Incredibly, there were only 8 days when the temperature went below -25C in 2011-12.. compared to 55 such days this winter. During the winter of 2011-12, there was only 1 day that was abnormally cold (>10C below normal). This winter, we've had 25 such days! It's like we moved 1000 km north this year! What happened?

BLAME IT ON THE JET STREAM

Why the dramatic difference between these two winters?  The simple answer is due to the jet stream. The jet stream is a strong ribbon of air that flows across the mid latitudes and polar regions across the globe, and separates Arctic air to the north from warmer air to the south. It defines the storm track, along which storms are steered as they travel from west to east across the northern hemisphere. In the winter of 2011-12, the jet stream was quite strong but locked in a westerly orientation across the northern Prairies (see right image below), which effectively blocked intrusions of Arctic air from barrelling southward into the Prairies, while also allowing frequent invasions of mild Pacific air to spread east across southern Manitoba. As a result, temperatures were frequently above normal in this setup, with an average of 7 days above normal for every 2 below.  Cold spells were brief and infrequent.

The behaviour of the jet stream dictates what kind of winter
we'll have in southern MB. A wavy jet stream (left) increases
likelihood of a colder winter, while a westerly jet stream (right)
  favours a milder winter with less frequent Arctic outbreaks  

This winter, the jet stream has been locked in a meridional position, that is, north to south instead of west to east (see left image above)  This has allowed frequent invasions of Arctic air southward into the Prairies and eastern North America, while effectively blocking mild Pacific airmasses from moving into southern MB. The result has been the complete opposite of 2011-12, with 7 days below normal this winter for every 2 days above. Cold spells this winter have been frequent and long lasting, while any mild spells have been infrequent and short lived.

So what dictates how the jet stream will set up?   That depends on a number of factors, including large scale climatic oscillations such as the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), Madden-Julian Oscillation (MJO) as well as ENSO phases (El Nino, La Nina) The interplay between these oscillations will favour a stronger westerly (aka "zonal") jet stream or a weaker "wavy" jet stream that can vary season to season, and year to year. In addition, some are saying that the rapid melting of the Arctic ice cap is resulting in a weaker jet stream (due to a weaker thermal gradient between the poles and the equator) which would tend to favour a "wavier" jet stream prone to blocking patterns. It's pretty complicated stuff that all depends on global temperatures, winds, ocean currents and ice cover.. so it's easy to understand why it's so difficult to predict what kind of winter we'll have in any given year. Suffice to say, both setups above can result in prolonged periods of abnormally warm or cold weather.. depending on which side of the jet stream you happen to be on.   And based on what's been happening over the past few years, it's possible these types of extreme weather patterns may become more frequent in the future, leading to a greater number of abnormally mild or cold winters in the years ahead. Predictability however will continue to be a challenge, until a better understanding of these global interactions is realized. Until then, hope for the good side of the jet stream!