For at least the past one or two decades the adjective extreme has increasingly become used in describing unusual weather. It’s virtually impossible now to escape news of extreme drought, excessive rainfall and floods, record breaking heat waves, cool spells and severe weather outbreaks, etc. which seem to recur year after year around the Northern Hemisphere. This summer was no different except that the behavior and configuration of the polar jet stream, the river of high altitude winds marking the divide between warm and cool air, were rare and mind-boggling.
Instead of meandering as a single stream like it normally does, it transformed into a “dual” jet stream configuration, sometimes transitioning from this dual setup back into a single more coherent stream, back and forth.
The rarity of dual polar jets was highlighted by Professor John Nielsen-Gammon(Texas A&M University) in an article in Popular Mechanics. He pointed out they are something one might see once per decade. From an independent assessment myself, it appears that there are no other polar jet examples comparable to this summer at least as far back as 2000 (the furthest back I’ve looked).
Mostly, the perplexing behavior of the polar jet has been described in befuddling terminology such as weird, mangled, and wobbly. Some have described the jet in a state of disarray, not playing by the so-called rules. Jeff Masters said that in his 30 years doing meteorology, the jet stream has been doing things he’s not seen before.
http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/09/11/summer-2013-weather-extremes-tied-to-extraordinarily-unusual-polar-jet-stream/
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