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Winter Is Finally Releasing Its Chilly Grip on the U.S.

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After more than a month of colder-than-average weather in the U.S., warmer days are finally on the way. According to the Climate Prediction Center, which is part of the National Oceanic and Atmospheric Administration (NOAA), above-average temperatures are expected across the eastern U.S. from April 8-16, and above-average temperatures are also favored in the East, South, and Southwest for the April-June time period. 

Click image to enlarge. Credit: NASA.

The warmth will be especially welcome after an unusually cold March gripped much of the U.S., and affected Europe and parts of Asia as well. The above graphic from NASA tells the story, showing the colder-than-average temperatures between March 14-20 (compared to average of the same dates from 2005 to 2012). The image is based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite, and shows land surface temperature anomalies .

Areas with above-average temperatures appear in red and orange, and areas with below-average temperatures appear in shades of blue. Much of Europe, Russia, and the U.S. saw unusually cool temperatures, while Greenland was surprisingly warm for the time of year.

As Climate Central has reported, the long-lasting cold was related to a strong blocking High pressure system over Greenland, which was associated with a particular configuration of an atmospheric pressure pattern known as the Arctic Oscillation, or AO. The AO is a measure of the difference in relative air pressure between the Arctic and the mid-latitudes, and the configuration of air pressure patterns can have profound impacts on weather patterns in the Northern Hemisphere. 

During March, the AO hit rock bottom.

Plot of the Arctic Oscillation Index during the 2012-2013 winter season.
Credit: Image adapted from NASA's Earth Observatory.

When the AO index is in its “negative” phase, air pressure over the Arctic is higher than average, while pressure over the mid-latitudes is relatively low, and prevailing winds allow extremely cold air to spill out of the Arctic, as if opening the Northern Hemisphere's refrigerator door. 

Warmer times ahead! NOAA's latest temperature outlook for the April-June period. 
Credit: NOAA.

The AO Index fell all the way to -5.6, which was one of its lowest readings on record, dating back to 1950. As this occurred, near record-breaking cold broke out in the U.K., which had its fourth-coldest March since 1962. Germany saw its coldest March since 1883, and Moscow had its coldest March since the 1950s, according to NASA and news reports.

The AO, which is still in negative territory, is expected to return to its positive phase by mid-April, raising the odds of a major weather pattern shift, possibly to warmer-than-average conditions.

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W
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Comments

By Dave (Basking Ridge, NJ 07920)
on April 3rd, 2013

“…as if opening the Northern Hemisphere’s refrigerator door.”

To extend the analogy, when you leave the refrigerator door open the refrigerator also gets warmer.  From the looks of the red/orange over Greenland in the NASA image, it looks like more heat flowed in to that region than usual while cold air moved out.

Reply to this comment

By Andrew
on April 3rd, 2013

Dave,

You’re exactly right. I included more details on the warm Arctic aspect in this news story:
http://www.climatecentral.org/news/from-2012-to-2013-march-blows-hot-then-cold-15807

Reply to this comment

By Camburn (North Dakota)
on April 3rd, 2013

Part of the reason for the AO to be so negative is the phase of the sun.

http://www.agu.org/news/press/pr_archives/2012/2012-39.shtml

Reply to this comment

By Dave (Basking Ridge, NJ 07920)
on April 4th, 2013

Hi Camburn.  I was interested in this a while back too so I read or scanned several papers about it.  The takeaway I got from what seemed like several different trusted sources is that based purely on the correlation between certain data sets there is indeed probably a link between AO/NAO and solar activity impacting regional climate, i.e. that of the North Atlantic region. If you’re interested this is a couple of the papers I read on the topic. They predate your reference, but they are also the actual journal articles.
http://www.researchgate.net/publication/233729987_Solar_modulation_of_Northern_Hemisphere_winter_blocking/file/79e4150ad7dca4bda4.pdf
http://dods.ipsl.jussieu.fr/dssce/public_html/Papier/Swingedouw_et_al_CD_2010.pdf

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