The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 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 Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45° N. The

North Atlantic oscillation The North Atlantic Oscillation (NAO) is a weather phenomenon over the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High. Through fluctuations in the ...

(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 James Edward Hansen (born March 29, 1942) is an American adjunct professor directing the Program on Climate Science, Awareness and Solutions of the The Earth Institute, Earth Institute at Columbia University. He is best known for his research ...

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 or geopotential altitude is a vertical coordinate referenced to Earth's mean sea level, an adjustment to geometric height (altitude above mean sea level) that accounts for the variation of gravity with latitude and altitude. Thu ...

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 In statistics and signal processing, the method of empirical orthogonal function (EOF) analysis is a decomposition of a signal or data set in terms of orthogonal basis functions which are determined from the data. The term is also interchangeab ...

(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.

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 stochastically 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 run f ...

ensemble Ensemble may refer to: Art * Architectural ensemble * Ensemble (album), ''Ensemble'' (album), Kendji Girac 2015 album * Ensemble (band), a project of Olivier Alary * Ensemble cast (drama, comedy) * Ensemble (musical theatre), also known as the ...

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 ...

and named in 1998 by David W.J. Thompson and

John Michael Wallace John Michael Wallace (born October 28, 1940), is a professor of Atmospheric Sciences at the University of Washington, as well as the former director of the Joint Institute for the Study of the Atmosphere and Ocean (JISAO)—a joint research ventur ...

Impacts

The National Snow and Ice Data Center describes the effects of the Arctic oscillation in some detail. In the positive phase, higher pressure at

midlatitudes The middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) are a spatial region on Earth located between the Tropic of Cancer (latitudes 23°26'22") to the Arctic Circle (66°33'39"), and Tropic of Capric ...

drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

and

Scandinavia Scandinavia; Sámi languages: /. ( ) is a subregion#Europe, subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, ...

, as well as drier conditions to the western

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

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 north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...

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

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

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

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

warmer than normal, but leaves

Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...

and Newfoundland 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 (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

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. Starting as a tabulation unit in New Orl ...

: ''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 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 (; ca, Catalunya ; Aranese Occitan: ''Catalonha'' ; es, Cataluña ) is an autonomous community of Spain, designated as a ''nationality'' by its Statute of Autonomy. Most of the territory (except the Val d'Aran) lies on the north ...

as well as neighbouring French departments (

Languedoc-Rousillon Languedoc-Roussillon (; oc, Lengadòc-Rosselhon ; ca, Llenguadoc-Rosselló) is a former administrative region of France. On 1 January 2016, it joined with the region of Midi-Pyrénées to become Occitania. It comprised five departments, and ...

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

Girona Girona (officially and in Catalan language, Catalan , Spanish: ''Gerona'' ) is a city in northern Catalonia, Spain, at the confluence of the Ter River, Ter, Onyar, Galligants, and Güell rivers. The city had an official population of 103,369 in ...

. 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

Cancún Cancún ( ), often Cancun in English (without the accent; or ) is a city in southeast Mexico on the northeast coast of the Yucatán Peninsula in the Mexican state of Quintana Roo. It is a significant tourist destination in Mexico and the seat ...

, 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

External links