Wednesday, January 29, 2014

The Polar Vortex

Can we talk about the weather?
Just like the circus act that juggles plates on a stick, meteorologists juggle two plates – one over both of the earth’s poles. Each of these spinning plates is a circumpolar vortex but the one over the Canadian Arctic has hit the main stream media and become famous as the “polar vortex”. The jet stream wraps around the polar vortex directing Arctic air far to the south.  The larger and stronger the polar vortex, the further south the chilly Arctic air is delivered by the jet stream. The polar vortex is strongest and thus more newsworthy in the winter when there is no sun and 24 hours of darkness. Minus 2 in New Orleans is enough to chill out the preparations for Mardi Gras and six inches of snow has paralyzed Atlanta, Georgia. The blue sky blizzard in southern Ontario might be very rare but still can be attributed to “winter in Canada”.
I expected an El Nino pattern to explain the current polar vortex but the ENSO is apparently in a neutral phase that is expected to last into the summer. The El Nino pattern of a large ridge of high pressure on the west coast and a deep trough over eastern North America would have given a perfect home for the current polar vortex over Hudson Bay and northern Quebec. A fresh supply of fresh, chilly Arctic air would be delivered southward by such a large cyclonic circulation. Alas, El Nino is not to blame.
These large amplitude blocking patterns can also be the result of a weakening jet stream. The mean speeds of the jet stream have reportedly diminished by 15% in the last 15 years – more or less. I partially use these numbers because they are easy to remember. Climate change and the greater warming of the poles as compared to the equatorial regions have diminished the temperature contrast between the two regions. Less temperature contrast results in a weaker jet stream – the simple thermal wind equation. It is paradoxical that warming in the Arctic can create a stronger polar vortex in a higher amplitude atmospheric pattern that delivers the cold air further south.
None of this is new. The term “polar vortex” has been used since the 1950’s and climate change was important from my early days as a meteorologist starting in1977. What is new is to see these changes evolve within one’s own meteorological career. Atmospheric blocking patterns used to be rare. They are much more common now with the meandering and weakening jet stream. I even wrote a COMET module on them included within the Satellite Palette because of their importance to weather prediction. Some notable 2013 severe rain events like those in Boulder, High River and Toronto were all linked to atmospheric blocks. The associated areas of fair weather in the blocks do not make the news.
Meanwhile in the upper ridge portion of this polar vortex pattern, California is hot and dry with record low relative humidity (5%), it is sunny and mild in Vancouver and Alaska is warm with rain. Heavy rains, snows, and warm temperatures helped to trigger a‪ series of huge avalanches that have blocked a 100 km section of the ‬Richardson Highway, the only road into Valdez, Alaska (population 4,000), located about 200 km east of Anchorage.
Atmospheric blocks of a bunged up atmosphere is what all of this is about and just maybe these will be the new words that the media sensationalizes. I can hardly wait. Do not expect the Numerical Weather Predictions to do very well with these patterns.

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