Arctic oscillation – what is it?

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The extreme or, rather, unusually low temperatures this winter in Europe ad North America is being explained by some as an effect of  “Arctic oscillation”. But what is this phenomenon?
Hang on as we turn to the  National Snow and Ice Data Center (US) for some answers:

The Arctic Oscillation
The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and high latitudes.

The oscillation exhibits a “negative phase” with relatively high pressure over the polar region and low pressure at midlatitudes (about 45 degrees North), and a “positive phase” in which the pattern is reversed. In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general “opposite” to those of the positive phase, as illustrated below.

Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.

Arctic Oscillation

Effects of the Positive Phase      |     Effects of the Negative Phase
of the Arctic Oscillation                    of the Arctic Oscillation

(Figures courtesy of J. Wallace, University of Washington)

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