The weather pattern responsible for bringing frigid air to Europe, like
this heavy snow fall on the Colosseum in Rome, is driven in part by a
naturally-occurring pattern known as the Arctic Oscillation. This is a repost from Climate Central
By Andrew Friedman
While the U.S. cruises
through winter with a snow drought and above-average temperatures, much
of Europe and Eurasia are locked in the grips of a deadly cold air
outbreak, with more than 300 people reported dead so far. According to
news reports, entire communities in Italy, Bosnia, and Romania have
become inaccessible due to heavy snowfall and power outages. According
to Sky News,
a dam in Bulgaria burst due to the combination of snowmelt and heavy
rains, killing four people in a village downstream, and other dams in
Southeastern Europe are also being threatened. Bosnia, Serbia, Macedonia
and Turkey are at risk for heavy snows during the next few days.
According to U.S. News and World Report,
the wintry blitz presents a poorly-timed challenge to European
economies, which already are struggling to contain a debt crisis:
"In more robust economic times, the economic effects of bad weather
might be nothing more than bump in the road, but in a place already
teetering on the brink of recession, the stakes are higher."
"[E]ven relatively limited disruption from snow and freezing conditions could very well be enough to tip the balance
towards the economy suffering further contraction in the first quarter
of this year, which would put it officially back into recession," writes
Howard Archer, chief European and UK economist of IHS Global Insight,
in a commentary on the cold snap.
.
According to Bloomberg News,
93 stations from the Czech Hydrometeorological Institute reported the
lowest-ever temperatures for February 5, with one station bottoming out
near -38°F. In Ukraine, the cold has killed at least 131 people, and
nearly 2,000 were hospitalized due to hypothermia.
Ice has forced Austria to close the Danube river to navigation, shutting down access to part of the second-longest river in Europe. In addition, the iconic canals of Venice were reported to be frozen as well.
What's Going On?
 |
| The departures from average of the height of the 500 mb pressure surface from Jan. 28 to Feb. 3, 2012. The red colors near Norway indicate unusually warm air at the mid-levels of the atmosphere (high heights). Dark blue shading across Europe and Eurasia shows telltale signs of colder than average conditions (low heights). Credit: NOAA/ESRL. |
The weather pattern responsible for bringing the frigid air to Europe and
Eurasia, and locking it in place, is being driven in part by a
naturally-occurring pattern of climate variability known as the Arctic Oscillation.
The Arctic Oscillation, or AO, is is a climate index that describes the
characteristics of the atmospheric circulation over the Arctic, and a related index
describes the circulation over the North Atlantic. Depending on whether
it's in a "positive" or "negative" phase, the Arctic Oscillation can
bring warmer or cooler than average wintertime conditions to the U.S.
and Europe.
Right now the Arctic Oscillation is in a negative phase, which tends to
favor colder than average weather in Europe and the U.S. Scientists
don't fully understand what causes the Arctic Oscillation to switch from
one phase to the other, which limits their ability to forecast these
changes ahead of time beyond a week in advance.
Stu Ostro, a senior meteorologist at The Weather Channel who has been
keeping close tabs on trends in weather and climate extremes in recent
years, said the frigid weather in Europe is another in a series of weird
winters that have been related to the Arctic Oscillation.
“It’s interesting that the winter started out the opposite of the
previous two, with an exceptionally positive Arctic Oscillation and
non-blocking pattern, and then it flipped," he said via email.
 |
| Map showing sea level pressure departures from average during the last week in January through early February. The bright red area over Russia and parts of Europe indicate the presence of an unusually strong high pressure area. Credit: NOAA/ESRL. |
 |
| Map showing surface wind departures from average during the same period. The circle and arrows show how the frigid air from Russia has flowed westward, into much of Europe. Credit: NOAA/ESRL. |
"There has been extreme variability of the Arctic Oscillation in recent
years, including a record negative monthly value for December in 2009
(the month of the so-called “Snowpocalypse” in the northeast U.S.), a record negative for any month in February 2010 (“Snowmageddon”),
and a record positive one for April in 2011 (coinciding with the
extraordinary number of tornadoes in the U.S. that month)," Ostro said.
"And this has been occurring in the context of the precipitous decline of Arctic sea ice volume.”
Also noteworthy is a very strong and persistent high pressure area that
has been sprawled across Russia. The airflow around this stubborn high
has been transporting Siberian air into Western Europe, as the maps
below show. (Jeff Masters of WeatherUnderground discusses this in more
detail in a blog post today.)
As Ostro alluded to, in recent years there have been studies examining
how the global warming-related loss of Arctic sea ice might affect
winter weather patterns in the northern hemisphere. Some of this
research shows that sea ice loss may favor winters with predominately
negative phases of the Arctic Oscillation. One potential result of
global warming, referred to as the "Arctic Paradox," is that sea ice
loss can help warm the Arctic during the winter, while setting in motion
a chain reaction of events that make winters colder than they otherwise
would be in Europe and the U.S..
The weather pattern during the past two weeks bears some similarities
to conditions last December, when the Arctic Oscillation was in an
extremely negative phase, and the jet stream helped drive frigid air and
winter storms into both Europe and the U.S. This year, though, only
Europe and Eurasia have been unusually cold, as other factors have
conspired to protect the Lower 48 from Old Man Winter's wrath. At least
for now, anyway, as there are signs this may change during the next few
weeks.
