[youtube]QfQ0kZ13-JA[/youtube]The roots of the recent cold weather in Britain and eastern North America lie in unusual goings on high in the atmosphere above the North Pole, as this animation from NASA’s Earth Observatory demonstrates (full video here: 6MB .mov file). The left hand image shows vorticity (rotation, roughly) and the right the temperature at 20km. As the animation moves through January into February, we see the polar vortex (the red bit in the middle) split into two, and stratosphere temperatures over the Arctic jump by as much as 50C. The Earth Observatory explains:
The big change in the Arctic came when the polar vortex ripped apart. A developing weather system in the lower atmosphere traveled upward into the stratosphere. The disturbance nudged into the center of the Arctic air mass, elongating it and eventually splitting it like a cell in mitosis. By February 2, two air masses existed, each with a jet of wind circling it counterclockwise […]. Warm air filled the gap between the two colder air masses, and temperatures high over the North Pole climbed […]. Now the colder air had shifted farther south over Canada and Siberia. Over North America, this piece of the stratospheric polar vortex had a deep reach into the lower atmosphere (troposphere), which created strong winds from the north that carried cold Arctic air far south into the United States.
In Europe, the split in the air mass actually changed the direction of winds in the lower atmosphere. The second piece of the polar vortex was centered east of Western Europe […], and it too was surrounded by a jet of strong wind moving counterclockwise. Like the segment of the polar vortex over North America, this piece of the polar vortex also had a deep reach into the lower atmosphere. It caused cold continental air to blow in from the east, replacing the warmer air that typically blows in from the west. As the frigid air moved over the North Sea, it picked up moisture, which fell over the United Kingdom and parts of France as heavy snow.
There’s a full explanation of the polar circulation at the Earth Observatory page. Well worth a read. Any meteorologists care to comment on just how unusual a feature this is? Are the large blocking highs that bring cold easterlies to Western Europe often associated with polar vortex splits? This is weather, not climate, but the Arctic is experiencing rapid climate change, and this will be expressed as changing weather patterns. A new paper in Climate Dynamics examines this and found “large increases in the potential for extreme weather events […] along the entire southern rim of the Arctic Ocean, including the Barents, Bering and Beaufort Seas.”
[Calexico]
FWIW, this event seems quite distinct from the ones described in the paper:
“A common feature of these weather types is that they form when cold air masses wander out from over the ice sheets over the warm ocean to be heated from below. In the North Atlantic, such conditions arise frequently along the Gulf Stream and its northern branches. The North-East Atlantic (the Greenland, Iceland, Norwegian and Barents Seas) is particularly prone to marine cold-air outbreaks (MCAOs), as they are referred to in the paper.”
Agreed, Steve. My point was more general; that we both expect and observe change in Arctic weather patterns. But I would really like to know more about polar vortex behaviour in the past…
This temperature jump of 50C over the pole has me curious as to what its effect will be on the 2009 ice melt.
I realize the increased temperature is high in the atmosphere, but if these two air masses remain separated would this not jump start the spring and summer season in the far north?
Well, it’s still very dark and cold up there. Spring will come with the sun, but how the polar vortex behaves/recovers will have an impact on NH weather patterns. One to watch…
Thanks for the link, Gareth. It is certainly interesting.
The weather has been odd up here. Not too bad where I live, since I’m on the coast and that has a moderating effect, but 10 miles inland they had a fall of about 18 inches overnight last week. We’ve also seen temperatures as low as -18C, which is on the chilly side.
For Northern Scotland neither is unusual – we had lower temperatures last winter and heavier snowfalls in 2005 – but it’s rare to get both very low temperatures and heavy snow at the same time (at least it is here). One of the local expressions is “it’s too cold for snow”.
It will be interesting to see how this plays out. At the NSIDC it looks like there hasn’t been a lot of growth in sea ice extent in the last couple of weeks, but I guess that could be a coincidence.
According to some second-hand information I’ve seen, the split seems to be unique in the satellite record (@ 30 years).
A paper from a few months ago (discussed in some detail here) describes a recent marked shift in Arctic winter weather patterns, with (wait for it) the vortex being disrupted by warm air intrusions. Just such a warm air mass is mentioned in the NASA article as being critical to the recent vortex split.
I can’t find a free copy of the paper to see all of the details, but I do wonder to what extent this change is related to the observed poleward shift in the climate zones and jets caused by the expansion of the tropics (review paper). So far the Antarctic jet is the only element that hasn’t gone anywhere, but I have a feeling it’s just a matter of time. Such changes play a critical role in the glacial cycles and could be key to a future global tipping point (article).
FYI on a related topic, this new paper notes a two day shift in the seasonal cycle. As the phrase goes, this can’t be good.
Just to note that the weather effects anticipated in the Climate Dynamics paper linked at the end of the post are also part of the poleward circulation shift. So it sounds as if the implication for Scotland e.g. may be better weather overall but with nasty winter outbreaks.
Steve, thanks for those links. Plenty of reading to do.
I have asked a few people about the polar vortex split, but haven’t got much back yet.
These events not too unusual at all. This is an example of a “stratospheric sudden warming” (SSW), where the polar vortex over the North Pole breaks down spectacularly in late winter/early spring. It is not uncommon for stratospheric temps to rise ~50C in a day or two. These have been known about for decades, and are often triggered by big blocking events in the troposphere (associated with upward transport of energy to the stratosphere). I’m not aware that there are trends observed in the timing or intensity of SSWs in the NH, though a literature search on “sudden warming” might turn something up. The IPCC AR4 (Ch3, page 283) discusses this briefly.
One interesting thing is that NO such events had ever been observed in the Southern Hemisphere – because the polar vortex is so strong, and planetary wave activity is weaker in the south than the north (less land down this way). But, a spectacular SSW event happened in September 2002, unprecedented before or since. What that signifies, who can say…
Thanks, Baldrick, but I think what got this one all the attention is when it happened (mid-winter).