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January 2012, Week 2

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Fri, 13 Jan 2012 21:48:48 -0500
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Jet Lag: What's Causing One of the Driest, Warmest
Winters in History?

     The jet stream controls winter weather, but strange
     forces are controlling the jet stream this season

By Mark Fischetti  | 
January 12, 2012
Scientific American
http://www.scientificamerican.com/article.cfm?id=whats-causing-dry-winter

A little snow and rain are falling in a few states
today, but the 2011-12 winter has been extremely warm
and dry across the continental U.S. Meteorologists think
they have figured out why.

First, a few records: The initial week of January was
the driest in history. And more than 95 percent of the
U.S. had below-average snow cover-the greatest such
percentage ever recorded-according to some intriguing
data maps generated by the National Oceanic and
Atmospheric Administration. During December,
approximately half of the U.S. had temperatures at least
5 degrees Fahrenheit above average, and more than 1,500
daily record highs were set from January 2 to 8. Europe
has seen similar extremes.

The chief suspect behind the mysterious weather is an
atmospheric pressure pattern called the Arctic
Oscillation, which circles the high Northern Hemisphere.
Its lower edge is known as the North Atlantic
Oscillation (NAO). Together, the related features
influence the path and strength of the jet stream. The
jet itself is an air current that flows west to east
across the northern latitudes of the U.S., Europe and
Asia, altering temperature and precipitation as portions
of it dip southward or crest northward. A strong jet
stream that flows in a somewhat straight line from west
to east, with few southward dips, prevents cold arctic
air from drifting south. "The cause of this warm first
half of winter is the most extreme configuration of the
jet stream ever recorded," according to Jeffrey Masters,
a meteorologist who runs the Weather Underground, a Web
site that analyzes severe weather data.

By "extreme," Masters means that the jet stream was far
north and fairly straight, and stayed that way for an
unusually long time. That position allowed warm southern
air to prevail over the entire U.S., and prevented cold
fronts from descending from the north and clashing with
warm fronts, creating large snow- and rainstorms. The
jet stream has been locked in that position by the NAO
for most of the winter, and Masters says it has
sustained the largest pressure gradient since tracking
began in 1865.

Conversely, December 2010 set record snowfalls in many
parts of the U.S. Sure enough, the NAO at that time had
some of the lowest pressures ever observed, allowing the
jet stream to move south and stay there. Arctic air
descended, picked up moisture or interacted with warm
fronts, and dropped snow. "The December Arctic
Oscillation index has fluctuated wildly over the past
six years," Masters notes, "with the two most extreme
positive and two most extreme negative values on
record." Data for the trends is available at the Weather
Underground site.

Meteorologists are not certain what causes the
oscillations to vary so dramatically. Some scientists
say the loss of Arctic sea ice due to global warming is
causing the Arctic Oscillation to drop in pressure.
Others have noticed a correlation with sunspot activity,
which was very low in December 2010 and very high during
December 2011, although they haven't proposed a
mechanism whereby sunspots would directly alter the
Arctic Oscillation.

Of course, winter has many weeks to go, so the
oscillations, and U.S. weather, could shift. But if
plentiful precipitation does not fall, complications
could arise for many more people than ski resort owners
and their patrons. A small snowpack often leads to
spring droughts in the Midwest and summer water
shortages in the West as well as a longer wildfire
season in the latter because the soil dries out earlier
than usual.

In the meantime if you want snow, hop a flight to
Cordova or Valdez, two towns on Alaska's Pacific coast
that are buried under 4.5 to 5.5 meters of snow-with
more on the way. They, too, can thank the Arctic
Oscillation because, being so high latitude, they lie
within the band the jet stream has been stuck in, not
south of it.

___________________________________________

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