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February 14, 2012
Like El Niño and La Niña, the Arctic Oscillation (AO) is a big picture of atmospheric conditions that influence weather. The AO, which ranges between two distinct modes, describes how pressure patterns are distributed over the Arctic region and the middle latitudes of the Northern Hemisphere.
From November through the first part of January the AO was in a generally positive phase, which tends to bring warm conditions to the United States and Europe, and colder conditions to the Arctic. However, in the middle of the month, the AO shifted back to a negative phase. This shift helped bring cold air outbreaks over middle latitudes, notably a record cold snap throughout much of Europe.
The AO can persist in one phase from anywhere from days to months. When pressure is higher than normal over the Arctic, and lower than normal over middle latitudes, the AO is in its negative mode. When it is positive, air pressure is lower than normal over the Arctic and higher than normal over middle latitudes. Researchers look to the AO to better understand year-to-year variability in climate indicators like the Arctic sea ice cover.
The AO, which had been in its positive phase most of the winter, switched to a negative mode, bringing cold weather to Europe and changing the direction of sea ice movement on the Arctic Ocean. This may explain why sea ice extent remained lower than average in January but why sea ice extent in the Bering Sea was much greater than normal.
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