TheArctic oscillation (AO) orNorthern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 55 degrees latitude. It is an importantmode of climate variability for the Northern Hemisphere. The southern hemisphere analogue is called theAntarctic oscillation or Southern Annular Mode (SAM). The index varies over time with no particular periodicity, and is characterized by non-seasonalsea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45° N.[1]
TheNorth 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.[2]
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 climatologistJames E. Hansen explained the mechanism by which the Arctic oscillation affects weather at points so[clarification needed] 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 degree to which Arctic air penetrates into middle latitudes is related to the AO index, which is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is high in the polar region. This helps the middle latitude jet stream to blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative, there tends to be low pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes."[4]
The Arctic oscillation index is defined using the daily or monthly 1000 hPageopotential height anomalies from latitudes 20° N to 90° N. The anomalies are projected onto the Arctic oscillation loading pattern,[5] which is defined as the firstempirical 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'sstandard deviation.
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,[citation needed] though it has trended to a more neutral state in the last decade. The oscillation still fluctuatesstochastically 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 meanGlobal Forecast Systemensemble AO forecasts is approximately 0.9.[6]
This zonally symmetric seesaw between sea level pressures in polar and temperate latitudes was first identified byEdward Lorenz[7] and named in 1998 by David W.J. Thompson andJohn Michael Wallace.[8]
TheNational Snow and Ice Data Center describes the effects of the Arctic oscillation in some detail. In the positive phase, higher pressure atmidlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather toAlaska,Scotland andScandinavia, as well as drier conditions to the westernUnited States and theMediterranean. In the positive phase, frigid winter air does not extend as far into the middle ofNorth America as it would during the negative phase of the oscillation. This keeps much of the United States east of theRocky Mountains warmer than normal, but leavesGreenland andNewfoundland 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 theNational Oceanic and Atmospheric Administration'sNational 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:
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).[10] That month was characterized by three separate historic snowstorms in themid-Atlantic region of the United States. Thefirst storm precipitated 25 inches (640 mm) onBaltimore, Maryland on February 5–6, and asecond storm precipitated 19.5 inches (500 mm) on February 9–10. In New York City, aseparate storm deposited 20.9 inches (530 mm) on February 25–26. Another snowstorm sweptCatalonia as well as neighbouring French departments (Languedoc-Roussillon,Midi-Pyrenées) on March 8, depositing 60 cm of snow inGirona.[11] 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[12] and the coolest average monthly minimum temperatures for February, March and December that year in the tropical beach getaway ofCancún, up to over 4C below the climate period averages.[13]
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.[14]
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