Arctic Oscillation Results in Warm Winter . . . so far

Temperatures at dusk on December 26, across the Florida peninsula.  The green colors indicate cooler air, still held back far to our north.  The yellows and oranges generally indicate temperatures in the 70°s F (22°-25° C).  It was a very warm Christmas Day as well with temperatures that reached into the low 80°s (28° C) across the Florida peninsula.

Warm and Very Dry Conditions Persist

Large portions of the eastern half of the United States have been more than 4° F above average so far in December, with temperatures averaging 8° F above average over portions of North Dakota.  This is quite a switch from the previous two winters, which were both much colder and snowier than average.  All three winters were strongly influenced by La Niña conditions in the Eastern Pacific Ocean, so La Niña does not explain the difference year-to-year.

The likely reason for the December warmth this year and the cold and snowy December of 2010 and 2009 is a weather pattern known as the Arctic Oscillation.

The maps show temperature patterns in late fall/early winter of 2010 (left) and 2011 (right).  Places that were up to 7° F warmer than the 1981-2000 average are dark red, while places that were up to 7° cooler than average are dark blue.  Locations where temperatures were near the long-term average are white.

One of the biggest influences on United States winter climate is the Arctic Oscillation, a natural seesawing of atmospheric pressure between the Arctic and the mid-latitudes of the North Pacific and North Atlantic Oceans.  

The oscillation is not predictable far in advance.

So far this winter, the Arctic Oscillation has been in its positive phase, favoring a milder winter in the eastern United States.  The winter of 2009-2010 had the most extreme negative Arctic Oscillation since record keeping began in 1865.  Similarly, in winter 2010-2011 an extreme negative Arctic Oscillation developed.  This year the pattern is completely opposite.

In many respect the maps indicate opposed conditions.  Last year, the U.S. South and East were cooler than average, while Greenland was unusually warm.  This year the pattern is reversed.  This complete flip-flop in temperature patterns extends across the Atlantic and into Europe.  The arrangement of opposites isn't a coincidence:  it reveals how different locations in the Northern Hemisphere are connected through the Arctic Oscillation.

Temperature forecast for winter 2012.  Most of the southeast USA will likely experience above average temperatures for the remainder of winter.  La Niña conditions in the equatorial Pacific -- present for the 3rd year in a row -- will generally cause the Pacific jet stream to meander north more frequently than normal, guiding winter storms into the norther tier of the country.

Negative Phase; much colder over USA and Europe

In the fall and winter of 2010, the Arctic Oscillation was persistently and often strongly negative -- the lowest it has been in the past 60 years.  During the negative phase of the pattern air pressure and polar jet stream patterns tend to draw Arctic air down into the eastern United States.  With this cold air in place, passing storms are more likely to generate heavy snow and colder air is more likely to reach much further south.  A return flow of milder air from the mid-latitudes then warms Greenland and eastern Canada.  Across the Atlantic, the polar jet stream tends to dip farther south than usual over Europe, favoring chillier than usual winters.

Positive Phase; much warmer over USA and Europe

During the positive phase of the Arctic Oscillation (this winter, so far) air pressure and wind patterns tend to drive winter storms on more northerly tracks.

Typical this winter. . . a weak cool front draped across the Florida peninsula has brought only limited showers to isolated locations. . . and not much cooler air.  Temperatures today north of the cool front were in the high 70°s F (25° C) and no rainfall occurred.

Location is Important

The Arctic Oscillation influences winter temperatures more strongly in some places than in others and big differences from average temperature can occur anywhere for reasons other than changes in the Arctic Oscillation (including El Niño and La Niña, as well as random variation).

For example, the phase of the Arctic Oscillation by itself doesn't have a strong influence on winter temperatures in most of Alaska, but it can modulate the influence of El Niño and La Niña.

Winter precipitation forecast for USA -- strongly influenced by La Niña conditions in the Pacific Ocean.  The peninsula of Florida will likely see worsening drought conditions until the rainy season begins again in June of 2012.  These conditions will likely exacerbate existing drought conditions (dry lakes, water shortages, etcetera). 

Arctic Oscillation Defined

The Arctic Oscillation and its close counterpart -- the North Atlantic Oscillation -- are climate patterns that are defined by fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores High.  The Arctic Oscillation is one of the oldest known climate oscillations.  Seafaring Scandinavians described the weather pattern centuries ago.

The latest computer model predictions show that the positive Arctic Oscillation pattern will continue for at least the next ten days.  Real winter conditions won't arrive in the United States until -- at earliest -- the end of the first week of January.

Temperature departure from average for November 2011.

In the contiguous United States, November ranked as the 25th warmest in the 117-years that records have been kept.  Thirteen states in the Northeast and Upper Midwest recorded a top-ten warmest November, and no states had a top-ten coldest November.  

Brown colors indicate well below normal rainfall predicted for already drought ravaged Texas and Florida.

La Niña continues

A borderline weak/moderate La Niña event continues in the equatorial Pacific Ocean, where sea surface temperatures were approximately 1.0° C below average during the first half of December.  The impacts of La Niña on United States weather are well-defined.  

It is likely that the drought in the South will continue and worsen along with above average temperatures.  The Northwest can expect cooler than average temperatures, as well as the potential for another winter with heavy snowpack across the western United States.

Arctic Sea Ice extent yesterday.

Arctic Sea Ice 3rd Lowest on Record

Arctic sea ice extent was at its third lowest on record in November (behind 2006 and 2010), according to the National Snow and Ice Data Center.  Sea ice records date to 1979.