The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 55 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The southern hemisphere analogue is called the Antarctic oscillation or Southern Annular Mode (SAM). The index varies over time with no particular periodicity, and is characterized by non-seasonal sea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45° N. The North Atlantic oscillation (NAO) is a close relative of the Arctic oscillation. There is debate over whether one or the other is more fundamentally representative of the atmosphere's dynamics. The NAO may be identified in a more physically meaningful way, which may carry more impact on measurable effects of changes in the atmosphere.

Description

The Arctic oscillation appears as a ringlike (or "annular") pattern of sea-level pressure anomalies centered at the poles. The presence of continents and large landmasses disrupts the ringlike structure at the Arctic pole, while anomalies surrounding the Antarctic pole are nearly circular. The Arctic oscillation is believed by climatologists to be causally related to (and thus partially predictive of) global weather patterns. NASA climatologist James E. Hansen explained the mechanism by which the Arctic oscillation affects weather at points so distant from the Arctic, as follows (note, however, that Hansen's explanation is erroneous: pressure in the Arctic is low in the positive AO phase, which configuration also enhances the jet stream): The Arctic oscillation index is defined using the daily or monthly 1000 hPa

geopotential height Geopotential height, also known as geopotential altitude or geopotential elevation, is a vertical coordinate (with dimension of length) representing the work involved in lifting one unit of mass over one unit of length through a hypothetical spac ...

anomalies from latitudes 20° N to 90° N. The anomalies are projected onto the Arctic oscillation loading pattern, which is defined as the first empirical orthogonal function (EOF) of monthly mean 1000 hPa geopotential height during the 1979-2000 period. The time series is then normalized with the monthly mean index's

standard deviation In statistics, the standard deviation is a measure of the amount of variation of the values of a variable about its Expected value, mean. A low standard Deviation (statistics), deviation indicates that the values tend to be close to the mean ( ...

Periodicity

Over most of the past century, the Arctic oscillation alternated between positive and negative phases. Data using a 60-day running mean has implied the oscillation has been trending to more of a positive phase since the 1970s, though it has trended to a more neutral state in the last decade. The oscillation still fluctuates

stochastic Stochastic (; ) is the property of being well-described by a random probability distribution. ''Stochasticity'' and ''randomness'' are technically distinct concepts: the former refers to a modeling approach, while the latter describes phenomena; i ...

ally between negative and positive values on daily, monthly, seasonal and annual time scales, although meteorologists have attained high levels of predictive accuracy for shorter term forecasts. The correlation between actual observations and the 7-day mean

Global Forecast System The Global Forecast System (GFS) is a global numerical weather prediction system containing a global computer model and variational analysis run by the United States' National Weather Service (NWS). Operation The mathematical model is ...

ensemble AO forecasts is approximately 0.9. This zonally symmetric seesaw between sea level pressures in polar and temperate latitudes was first identified by

Edward Lorenz Edward Norton Lorenz (May 23, 1917 – April 16, 2008) was an American mathematician and meteorologist who established the theoretical basis of weather and climate predictability, as well as the basis for computer-aided atmospheric physics and m ...

and named in 1998 by David W.J. Thompson and John Michael Wallace.

Impacts

The

National Snow and Ice Data Center The National Snow and Ice Data Center (NSIDC) is a United States information and referral center in support of polar and cryospheric research. NSIDC archives and distributes digital and analog snow and ice data and also maintains information ab ...

describes the effects of the Arctic oscillation in some detail. In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

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and

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, as well as drier conditions to the western

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and the

Mediterranean The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...

. In the positive phase, frigid winter air does not extend as far into the middle of

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as it would during the negative phase of the oscillation. This keeps much of the United States east of the

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warmer than normal, but leaves

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and

Newfoundland Newfoundland and Labrador is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region of Labrador, having a total size of . As of 2025 the population ...

colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase. 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 The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with Weather forecasting, forecasting weather, monitoring oceanic and atmospheric conditions, Hydrography, charting the seas, ...

National Climatic Data Center The United States National Climatic Data Center (NCDC), previously known as the National Weather Records Center (NWRC), in Asheville, North Carolina, was the world's largest active archive of weather data. In 2015, the NCDC merged with two other ...

: ''State of the Climate December 2010'' which uses the phrase "negative Arctic Oscillation" four times, is very representative of this increasing tendency:
:"Cold arctic air gripped western Europe in the first three weeks of December. Two major snowstorms, icy conditions, and frigid temperatures wreaked havoc across much of the region...The harsh winter weather was attributed to a negative Arctic Oscillation, which is a climate pattern that influences weather in the Northern Hemisphere. A very persistent, strong ridge of high pressure, or 'blocking system', near Greenland allowed cold Arctic air to slide south into Europe. Europe was not the only region in the Northern Hemisphere affected by the Arctic Oscillation. A large snow storm and frigid temperatures affected much of the Midwest United States on December 10–13...."

Events of 2010

A further, quite graphic illustration of the effects of the negative phase of the oscillation occurred in February 2010. In that month, the Arctic oscillation reached its most negative monthly mean value at about −4.266, in the entire post-1950 era (the period of accurate record-keeping). That month was characterized by three separate historic snowstorms in the

mid-Atlantic region The Mid-Atlantic is a region of the United States located in the overlap between the nation's Northeastern and Southeastern states. Traditional definitions include seven U.S. states: New York, New Jersey, Pennsylvania, Delaware, Maryland, Virgi ...

of the United States. The first storm precipitated on

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on February 5–6, and a second storm precipitated on February 9–10. In New York City, a separate storm deposited on February 25–26. Another snowstorm swept

Catalonia Catalonia is an autonomous community of Spain, designated as a ''nationalities and regions of Spain, nationality'' by its Statute of Autonomy of Catalonia of 2006, Statute of Autonomy. Most of its territory (except the Val d'Aran) is situate ...

as well as neighbouring French departments (

Languedoc-Roussillon Languedoc-Roussillon (; ; ) is a former regions of France, administrative region of France. On 1 January 2016, it joined with the region of Midi-Pyrénées to become Occitania (administrative region), Occitania. It comprised five departments o ...

, Midi-Pyrenées) on March 8, depositing 60 cm of snow in

Girona Girona (; ) is the capital city of the Province of Girona in the autonomous community of Catalonia, Spain, at the confluence of the Ter, Onyar, Galligants, and Güell rivers. The city had an official population of 106,476 in 2024, but the p ...

. This kind of snowstorm activity is considered to be highly anomalous, and as extreme as the negative Arctic oscillation value itself. These negative values of the AO during 2010 and following into the next winter allowed colder air to penetrate much further south than usual into sub-tropical South Florida which brought record breaking low temperatures and months registered in many locations and the coolest average monthly minimum temperatures for February, March and December that year in the tropical beach getaway of

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, up to over 4C below the climate period averages. The greatest negative value for the Arctic oscillation since 1950 in January was −3.767 in 1977, which coincided with the coldest mean January temperature in New York City, Washington, D.C., Baltimore, and many other mid-Atlantic locations in that span of time, although the January Arctic oscillation has been negative only 60.6% of the time between 1950 and 2010, nine of the ten coldest Januarys in New York City since 1950 have coincided with negative Arctic oscillations.National Weather Service Climate Prediction Center and NWS Forecast Office, as of 2010

References

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