The U.S. and European cold blast: blame the NAO

The Deep South shivered through another frigid morning today, with low temperature records crumbling again over much of Florida. Lakeland hit 27°F, smashing the old record of 34°F; Melbourne hit 28°F, eclipsing the old record of 32°F; and West Palm Beach bottomed out at 37°F, besting the old record of 38°F. The cold wave is being driven by an unusual sharp and persistent kink in the jet stream that is being blocked from moving by a strong ridge of high pressure over Greenland. As a result, an exceptionally strong surface high pressure of 1055 mb over the North Central U.S. is pushing large amounts of cold, Arctic air southwards from Canada. No coldest January temperature records have been set yet from the cold blast, but the 500 largest U.S. cities have been averaging about 11 new daily low temperature records per day the first five days of January, according to the National Climatic Data Center. The cold will ease Friday in the Deep South, but return with a vengeance Saturday night though Monday morning, as another push of cold air descending from Canada promises to bring a cold wave that will approach the December 1989 and January 1977 cold waves in intensity, and may being some new all-time January low temperature records to the South.

Colder in Florida than Alaska and Greenland
The sharp kink in the jet stream has brought record warm temperatures to a few stations in Alaska and the Pacific Northwest this week, making much of coastal Alaska warmer than Florida. Cold Bay, Alaska, set a record high yesterday of 47°F, after recording a low temperature of 30°F. This made Cold Bay warmer than Pensacola, Florida, which had a high of 47°F and a low of 24°F. In fact, most of Florida--including Jacksonville, Tampa, Melbourne, and Tallahassee--recorded lows at or below the 27°F low recorded in Anchorage, Alaska yesterday. The jet stream kink has also brought temperatures more than 30°F above average to Greenland. The temperature in Narsarsuaq, Greenland at 10am EST today was 46°F, far warmer than most of Florida.


Figure 1. Departure of the surface temperature from average for the first three day of 2010 shows much colder than average conditions were present over the Southeast U.S., much of Europe, and Central Asia. Much warmer than average temperatures were present over the Northwest U.S., Greenland, the Arctic, and Southern Asia. A sharp kink in the jet stream was responsible for the temperature anomaly pattern. Image credit: NOAA/ESRL.

Snow in Florida?
It doesn't snow very often in Florida, and the Wikipedia list of snow events in Florida lists only seven such events over the decade of the 2000s. This weekend's cold wave may be able to generate some snow over isolated regions of Central Florida, though it appears that the odds of this happening are less than 30%. The most widespread snowfall in Florida history occurred on January 19, 1977, when snow fell over much of the state, with flurries as far south as Homestead. Snow flurries also fell on Miami Beach for the only time in recorded history.


Figure 2. North Atlantic Oscillation (NAO) index for September 9, 2009 - January 6, 2010 (black line) and forecast from the GFS model (red lines). The NAO index was strongly negative, near -2.0, for much of December and January. Image credit: NOAA/CPC.

The winter cold blast: blame the North Atlantic Oscillation (NAO)
Why has the winter been so cold over Eastern North America and northern Europe? Well, don't blame El Niño. El Niño winters are rarely this cold. Instead, blame the the North Atlantic Oscillation (NAO). The NAO is a climate pattern in the North Atlantic Ocean of fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores High. It is one of oldest known climate oscillations--seafaring Scandinavians described the pattern several centuries ago. Through east-west oscillation motions of the Icelandic Low and the Azores High,the NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. A large difference in the pressure between Iceland and the Azores (positive NAO) leads to increased westerly winds and mild and wet winters in Europe. Positive NAO conditions also cause the Icelandic Low to draw a stronger south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward. In contrast, if the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), westerly winds are suppressed, allowing Arctic air to spill southwards into eastern North America more readily. Negative NAO winters tend to bring cold winters to Europe, and the prevailing storm track moves south towards the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.

The winter of 2009 - 2010 has seen a very strong negative NAO, causing much of our cold weather over Eastern North America and Europe. The NAO index for the month of December 2009 was -1.93, which is the third lowest NAO index since 1950 for a winter month (December, January, or February). The only winter months with a lower NAO index were February 1978 (-2.20) and January 1963 (-2.12). January 1963 was one of the coldest months on record in the UK and the Eastern U.S.. February 1978 was the coldest February on record for five U.S. states, and featured the historic blizzards in both the U.S. and UK. The NAO so far for January 2010 has continued to stay strongly negative, ranging between -1.5 and -2.1. However, the blocking ridge over Greenland is forecast to weaken next week, allowing the sharp kink in the jet stream to straighten out. This will increase the NAO index to more typical values, allowing a return of more ordinary winter weather to the U.S. and Europe.

Jeff Masters