Wikipedia

North Atlantic oscillation

For the rock band, seeNorth Atlantic Oscillation (band).
Not to be confused withAtlantic multidecadal oscillation.

TheNorth Atlantic Oscillation(NAO) is a weather phenomenon over the North Atlantic Ocean of fluctuations in the difference ofatmospheric pressure at sea level (SLP)between theIcelandic Lowand theAzores High.Through fluctuations in the strength of the Icelandic Low and the Azores High, it controls the strength and direction of westerlywindsand location ofstormtracks across the North Atlantic.[1]

The NAO was discovered through several studies in the late 19th and early 20th centuries.[2]Unlike theEl Niño–Southern Oscillationphenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.[3]

The North Atlantic Oscillation is closely related to theArctic oscillation(AO) (or Northern Annular Mode (NAM)), but should not be confused with theAtlantic multidecadal oscillation(AMO).

Definition

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The NAO has multiple possible definitions. The easiest to understand are those based on measuring the seasonal average air pressure difference between stations, such as:

These definitions all have in common the same northern point (because this is the only station in the region with a long record) inIceland;and various southern points. All are attempting to capture the same pattern of variation, by choosing stations in the "eye" of the two stable pressure areas, theAzores Highand theIcelandic Low(shown in the graphic).

A more complex definition, only possible with more complete modern records generated bynumerical weather prediction,is based on the principalempirical orthogonal function(EOF) of surface pressure.[4]This definition has a high degree of correlation with the station-based definition. This then leads onto a debate as to whether the NAO is distinct from the AO/NAM, and if not, which of the two is to be considered the most physically based expression of atmospheric structure (as opposed to the one that most clearly falls out of mathematical expression).[5][6]

Description

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Winter index of the NAO based on the difference of normalizedsea level pressure(SLP) betweenGibraltarandIcelandsince 1823, with aloess smoothing(black)

Westerly winds blowing across the Atlantic bring moist air into Europe. In years whenwesterliesare strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading toheat waves,deep freezes and reduced rainfall.[7][8]

A permanent low-pressure system overIceland(theIcelandic Low) and a permanent high-pressure system over theAzores(theAzores High) control the direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the NAO. A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (NAO-), westerlies are suppressed, northern European areas suffer cold dry winters and storms track southwards toward theMediterranean Sea.This brings increased storm activity and rainfall to southern Europe and North Africa.

Especially during the months of November to April, the NAO is responsible for much of the variability of weather in the North Atlantic region, affecting wind speed and wind direction changes, changes in temperature and moisture distribution and the intensity, number and track of storms. Research now suggests that the NAO may be more predictable than previously assumed and skillful winter forecasts may be possible for the NAO.[9]

There is some debate as to how much the NAO impacts short term weather over North America. While most agree that the impact of the NAO is much less over the United States than for Western Europe,[10]the NAO is also believed to affect the weather over much of upper central and eastern areas of North America.[10]During the winter, when the index is high (NAO+), the Azores High draws a stronger south-westerly circulation over the eastern half of the North American continent which prevents Arctic air from plunging southward (into the United States south of 40 latitude). In combination with theEl Niño,this effect can produce significantly warmer winters over theupper MidwestandNew England,but the impact to the south of these areas is debatable. Conversely, when the NAO index is low (NAO-), the upper central and northeastern portions of the United States can incur winter cold outbreaks more than the norm with associated heavy snowstorms. In summer, a strong NAO- is thought to contribute to a weakened jet stream that normally pulls zonal systems into the Atlantic Basin contributing significantly to excessively long-lasting heat waves over Europe, however, recent studies do not show the evidence of these associations.[10]

More recent studies have shown that the components (pressure centers strength, and locations) of the NAO are more powerful to investigate the relationships to seasonal and sub-seasonal climate variability over Europe, North America and the Mediterranean region.[10][11][12]

Effects on North Atlantic sea level

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Under a positive NAO index (NAO+), regional reduction in atmospheric pressure results in a regional rise in sea level due to the 'inverse barometer effect'. This effect is important to both the interpretation of historic sea level records and predictions of future sea level trends, as mean pressure fluctuations of the order of millibars can lead to sea level fluctuations of the order of centimeters.

North Atlantic hurricanes

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See also:Pre-1600 Atlantic hurricane seasons

By controlling the position of the Azores High, the NAO also influences the direction of general storm paths for majorNorth Atlantic tropical cyclones:a position of the Azores High farther to the south tends to force storms into theGulf of Mexico,whereas a northern position allows them to track up the North American Atlantic Coast.[13]

Aspaleotempestologicalresearch has shown, few majorhurricanesstruck the Gulf coast during 3000–1400 BC and again during the most recent millennium. These quiescent intervals were separated by a hyperactive period during 1400 BC – 1000 AD, when the Gulf coast was struck frequently by catastrophic hurricanes and their landfall probabilities increased by 3–5 times.[14][15][16]

Ecological effects

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Until recently, the NAO had been in an overall more positive regime since the late 1970s, bringing colder conditions to the North-West Atlantic, which has been linked with the thriving populations ofLabrador Seasnow crabs,which have a low temperature optimum.[17]

The NAO+ warming of theNorth Seareduces survival ofcodlarvae which are at the upper limits of their temperature tolerance, as does the cooling in the Labrador Sea, where the cod larvae are at their lower temperature limits.[17]Though not the critical factor, the NAO+ peak in the early 1990s may have contributed to thecollapse of the Newfoundland cod fishery.[17]

In southwestern Europe, NAO- events are associated with increased aeolian activity.[18]

On the East Coast of the United States an NAO+ causes warmer temperatures and increased rainfall, and thus warmer, lesssalinesurface water. This prevents nutrient-richupwellingwhich has reduced productivity.Georges Bankand theGulf of Maineare affected by this reduced cod catch.[17]

The strength of the NAO is also a determinant in the population fluctuations of the intensively studiedSoay sheep.[19]

Strangely enough, Jonas and Joern (2007) found a strong signal between NAO and grasshopper species composition in the tall grass prairies of the midwestern United States. They found that, even though NAO does not significantly affect the weather in the midwest, there was a significant increase in abundance of common grasshopper species (i.e.Hypochlora alba, Hesperotettixspp.,Phoetaliotes nebrascensis, M. scudderi, M. keeleri, and Pseudopomala brachyptera) following winters during the positive phase of NAO and a significant increase in the abundance of less common species (i.e.Campylacantha olivacea, Melanoplus sanguinipes, Mermiria picta, Melanoplus packardii, and Boopedon gracile) following winters during a negative phase of the NAO. This is thought to be the first study showing a link between NAO and terrestrial insects in North America.[20]

The NAO's ecological effects extend as far as theTibetan Plateau,where increases in aridity resulting in significant forest mortality and intensification of dust storms have been linked to NAO- events.[21]

Winter of 2009–10 in Europe

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Thewinter of 2009–10 in Europewas unusually cold. It is hypothesized that this may be due to a combination of low solar activity,[22]a warm phase of the El Niño–Southern Oscillation and a strong easterly phase of theQuasi-Biennial Oscillationall occurring simultaneously.[23]TheMet Officereported that the UK, for example, had experienced its coldest winter for 30 years. This coincided with an exceptionally negative phase of the NAO.[24]Analysis published in mid-2010 confirmed that the concurrent 'El Niño' event and the rare occurrence of an extremely negative NAO were involved.[25][26]

However, during thewinter of 2010–11 in Northern and Western Europe,theIcelandic Low,typically positioned west of Iceland and east of Greenland, appeared regularly to the east of Iceland and so allowed exceptionally cold air into Europe from the Arctic. A strong area of high pressure was initially situated overGreenland,reversing the normal wind pattern in the northwestern Atlantic, creating ablockingpattern driving warm air into northeastern Canada and cold air into Western Europe, as was the case during the previous winter. This occurred during a La Niña season, and is connected to the rareArctic dipole anomaly.[27]

In the north western part of the Atlantic, both of these winters were mild, especially 2009–2010, which was the warmest recorded in Canada. The winter of 2010-2011 was particularly above normal in the northern Arctic regions of that country.[28]

The probability of cold winters with much snow in Central Europe rises when the Arctic is covered by less sea ice in summer. Scientists of the Potsdam Research Unit of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association have decrypted a mechanism in which a shrinking summertime sea ice cover changes the air pressure zones in the Arctic atmosphere and effects on European winter weather.

If there is a particularly large-scale melt of Arctic sea ice in summer, as observed in recent years, two important effects are intensified. Firstly, the retreat of the light ice surface reveals the darker ocean, causing it to warm up more in summer from the solar radiation (ice–albedo feedbackmechanism). Secondly, the diminished ice cover can no longer prevent the heat stored in the ocean being released into the atmosphere (lid effect). As a result of the decreased sea ice cover the air is warmed more greatly than it used to be particularly in autumn and winter because during this period the ocean is warmer than the atmosphere.

The warming of the air near to the ground leads to rising movements and the atmosphere becomes less stable. One of these patterns is the air pressure difference between the Arctic and mid-latitudes: theArctic oscillationwith the Azores highs and Iceland lows known from the weather reports. If this difference is high, a strong westerly wind will result which in winter carries warm and humid Atlantic air masses right down to Europe. In the negative phase when pressure differences are low, cold Arctic air can then easily penetrate southward through Europe without being interrupted by the usual westerlies. Model calculations show that the air pressure difference with decreased sea ice cover in the Arctic summer is weakened in the following winter, enabling Arctic cold to push down to mid-latitudes.[29]

Winter of 2015–16 in Europe

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Despite one of the strongestEl Niñoevents recorded in the Pacific Ocean, a largely positive North Atlantic Oscillation prevailed over Europe during the winter of 2015–2016. For example,Cumbriain England registered one of the wettest months on record.[30]TheMaltese Islandsin the Mediterranean registered one of the driest years ever recorded up to beginning of March, with a national average of only 235 mm and some areas registering less than 200 mm.[31]

See also

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References

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  2. ^Stephenson, D.B., H. Wanner, S. Brönnimann, and J. Luterbacher (2003), The History of Scientific Research on the North Atlantic Oscillation, in The North Atlantic Oscillation: Climatic Significance and Environmental Impact, edited by J.W. Hurrell, Y. Kushnir, G. Ottersen, and M. Visbeck, pp. 37-50, American Geophysical Union, Washington, DC,doi:10.1029/134GM02
  3. ^Hurrell, James W. (1995). "Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation".Science.269(5224): 676–679.Bibcode:1995Sci...269..676H.doi:10.1126/science.269.5224.676.PMID17758812.S2CID23769140.
  4. ^abHurrell, Jim."NAO/NAM Climate Indices".CGD's Climate Analysis Section.Archived fromthe originalon 28 March 2010.
  5. ^Bjerknes, J (1964)."Atlantic air-sea interaction".Adv. Geophys.Advances in Geophysics.10:1–82.Bibcode:1964AdGeo..10....1B.doi:10.1016/S0065-2687(08)60005-9.ISBN9780120188109.
  6. ^Cook, E. R.; D'Arrigo, R. D.; Briffa, K. R. (1998). "A reconstruction of the North Atlantic Oscillation using tree-ring chronologies from North America and Europe".Holocene.8(1): 9–17.Bibcode:1998Holoc...8....9C.doi:10.1191/095968398677793725.S2CID128944923.
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  25. ^Pamela Rutherford (2 September 2010)."Huge snowfall caused by rare clash of weather events".BBC News.BBC News Online.Retrieved2 December2010.
  26. ^R. Seager; Y. Kushnir; J. Nakamura; M. Ting & N. Naik (July 2010)."Northern hemisphere winter snow anomalies: ENSO, NAO and the winter of 2009/10".Geophysical Research Letters.37(14): L14703.Bibcode:2010GeoRL..3714703S.doi:10.1029/2010GL043830.Retrieved2 December2010.
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  30. ^"Record breaking December rainfall".28 December 2015.
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