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The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is an index (which varies over time with no particular periodicity) of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered about 37–45N. The AO is believed by climatologists to be causally related to, and thus partially predictive of, weather patterns in locations many thousands of miles away, including many of the major population centers of Europe and North America. NASA climatologist Dr. James E. Hansen explained the mechanism by which the AO affects weather at points so distant from the Arctic, as follows:
The North Atlantic oscillation (NAO) is a close relative of the AO and there exist arguments about whether one or the other is more fundamentally representative of the atmosphere's dynamics; Ambaum et al. argue that the NAO can be identified in a more physically meaningful way.
Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s the oscillation has trended to more of a positive phase when averaged using a 60-day running mean, though it has trended to a more neutral state in the last decade. The oscillation still fluctuates between negative and positive values on daily, monthly, seasonal and annual time scales, although, despite its stochastic nature, meteorologists have attained high levels of predictive accuracy in recent times, at least for the shorter term forecasts. (The correlation between actual observations and the 7-day mean GFS ensemble AO forecasts is approximately 0.9, a figure at the high end for that statistic.)
The National Snow and Ice Data Center describes the effects of the AO in some detail:
Climatologists are now routinely invoking the Arctic Oscillation in their official public explanations for extremes of weather. The following statement from the National Oceanic and Atmospheric Administration's National Climatic Data Center: State of the Climate December 2010 which uses the phrase "negative Arctic Oscillation" four times, is very representative of this increasing tendency:
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