Arctic oscillation
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
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.[2]
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)
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 hPa geopotential 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 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.
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,[citation needed] 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 ensemble AO forecasts is approximately 0.9.[6]
This zonally symmetric seesaw between sea level pressures in polar and temperate latitudes was first identified by
Impacts
The
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:
- "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...."[9]
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).
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]
See also
References
- ^ "Arctic Oscillation (AO) time series, 1899 – June 2002". Archived from the original on 2013-05-18. Retrieved 2012-01-21.
- ^ Ambaum et al. "Arctic Oscillation or North Atlantic Oscillation?"
- ^ "CPC - Teleconnections: Arctic Oscillation Loading Pattern".
- ^ Hansen, James; Reto Ruedy; Makiko Sato; Ken Lo (2009). "If It's That Warm, How Come It's So Damned Cold?" (PDF). Retrieved 2013-07-17.
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: Cite journal requires|journal=
(help) - ^ "CPC - Teleconnections: Arctic Oscillation Loading Pattern".
- ^ "CPC – Monitoring & Data: Daily Arctic Oscillation Index". Archived from the original on 2010-12-30. Retrieved 2012-01-29.
- ISSN 1520-0469.
- . Retrieved 2010-08-28.
- ^ "State of the Climate | December 2010". Retrieved 2012-01-29.
- S2CID 122729063.
- .
- ^ 2010 South Florida Weather in Review (PDF)
- ^ SERVICIO METEOROLÓGICO NACIONAL NORMALES CLIMATOLÓGICAS, archived from the original on 2015-07-05
- ^ National Weather Service Climate Prediction Center and NWS Forecast Office, as of 2010
External links
- Overview of Arctic Climate Indices
- Monthly Northern Hemisphere Annular Mode (NAM) Index or Arctic Oscillation (AO) Index 1850–2013
- Daily Northern Hemisphere Annular Mode (NAM) Index or Arctic Oscillation (AO) Index 1948–2013
- NOAA 3-month running mean AO Index 1950–2009
- Arctic roots of 'upside-down' weather BBC news, Tuesday, 5 January 2010