North Atlantic oscillation

Under the North Atlantic Oscillation ( NAO) is understood in meteorology, the fluctuation of the pressure ratio between the Iceland low and the Azores high in the north to the south over the North Atlantic. This term was coined in the 1920s by Sir Gilbert Walker.

  • 2.1 consequences of a positive NAO
  • 2.2 consequences of a negative NAO

NAO index

A situation created for the NAO index, the NAO index indicates the NAO under the changing strength of the west wind drift, which influences the weather in Europe ( especially in winter). He is currently based usually on the difference of the standardized air pressure anomalies between Ponta Delgada ( Azores ) and Reykjavik ( Iceland ); there are or were, however, other reference stations in use. The NAO index changed significantly over periods of 30 years or more, as well as the climate in Europe is seriously affected.

Positive

A positive NAO index both Azores high and Iceland low are well trained. This leads in most cases to a strong west drift, resulting mild and humid air to Europe. In extreme cases, this even numerous storms brings. Thus resulted the winter storms and hurricanes 1999 ( Anatol, Lothar, Martin) from such a situation.

Negative

A negative NAO index, the action centers are weak, so the West Drift " asleep ". Thus, frequent surges of cold air from the north-east in central Europe again and again according to cold winters. The weakened westerlies shifted southward and leads in the Mediterranean to damp weather.

Strongly negative ( Reversal )

Has the Azores High taken the place of Iceland lows, and vice versa, the NAO index is strongly negative. In the professional world we often refer to this as a high -over -low position. Cold, continental air, starting from the Asian high, which is colloquially known as the Siberia high can penetrate far into this case to Central Europe and brings man in the truest sense of the word " Siberian cold ."

Fluctuation periods

The NAO index varies greatly over time. Based on the statistical data can be derived different types of temporal changes. Thus, besides the short-term fluctuations in the range of two to five years, yet the superimposed periodic fluctuations with a periodicity ranging from 12 to 15 ( decadal oscillation) and about 70 years ( Atlantic Multi- decadal Oscillation, AMO).

The consequences for the environment and climate

Depending on whether the NAO is positive or negative, result in different consequences.

Consequences of a positive NAO

Atmosphere:

  • In Greenland, the polar air determined the weather, so it is particularly cold and dry.
  • The jet stream, a wind system that blows several kilometers above the ground at about 30 ° to 60 ° north latitude around the globe, is influenced by the Iceland low so that the areas of low pressure formed over the Atlantic Ocean directly reach Northern Europe with its strong storms. Heavy precipitation and mild temperatures are the result in our latitudes.
  • Meanwhile achieve cold foothills of the Russia- highs often in the Mediterranean. Accordingly, it is there, colder and drier than usual.
  • About North Africa reinforce the trade winds.

Ocean:

  • The ice export from the Arctic seems to be larger than normal due to the cold weather in Greenland.
  • The surface temperature shows a three-phase structure: Due to the cold winter storms, the Labrador Sea is particularly cold, the Gulf Stream region, however, heats up when the Gulf Stream (and its offshoot, the North Atlantic current) northward transports more hot water.
  • The reinforced trade winds lead to a cooling of the equatorial Atlantic through increased buoyancy from the deeper ocean.

Biological processes:

  • In Scandinavia, the flora and fauna around 20 days more time to flourish until the frost prevails.
  • In the Labrador Sea, it comes through the descent of the cold surface water to a decline in fish stocks (especially cod and cod ).
  • The strong trade winds cause the sand of the Sahara blowing far out over the Atlantic Ocean. Not entirely clear is the impact of this dust on the biology of the ocean.
  • The weggetriebene from the African coast surface water is replaced by nutrient- richer deep water from the deeper ocean, so there are a lot of fish, the fishermen brought a good catch.

Consequences of a negative NAO

Atmosphere:

  • In Greenland it is relatively warm, since the weak low is not against the warm air currents from the American mainland.
  • Through a weaker air pressure opposites westerly winds over the Atlantic are weaker. You barely reach northern Europe, but rather the Mediterranean.
  • In the Mediterranean, the air flow from the Atlantic for increased precipitation provides.
  • Northern Europe in the winter gets increasingly under the influence of cold - highs over Asia. The consequences are weather conditions with low temperatures and little rainfall.

Ocean:

  • The ice in the Arctic seem to retreat, just as the glaciers on Greenland. But this is much slower than the NAO fluctuates.
  • The three-part structure in the North Atlantic has turned around: the warmer continental air in the Labrador ensures that this area will be warmer. Off the east coast of the U.S., it remains rather cool, because now the weaker Gulf Stream shoveling less warm water to the north.
  • The trade winds along the equator are weaker, so that the cooling is less there.

Biological processes:

  • The colder water in the area of the Gulf Stream can grow there the stock of mussels.
  • In the Labrador Sea fish stocks off at higher temperatures.
  • Increased precipitation in the Mediterranean favored wine and olive harvests.

The NAO as an influencing factor for the deep-water transport

Taking place in both the Labrador Sea and in the Norwegian Sea in the North Atlantic vertical water exchange is significantly influenced by fluctuations in the NAO. By the last two decades mainly positive NAO documents, there has been an increased deep water formation in the North West Atlantic. This is correlated with the relatively cold winter temperatures that occur due to the positive NAO location on the east coast of Canada. The convective renewal of intermediate and deep water layers of the Labrador Sea to the production and transport of North Atlantic Deep Water helps and keeps the global thermohaline circulation in progress. The intensity of taking place in the North Atlantic convection is thereby less affected by seasonal fluctuations than by the NAO, which extend over decades. So that takes place in the Labrador Sea convection in the late 1960s has been much weaker and thus flatter pronounced than is the case today. Since then, the waters of the Labrador Sea with The sharp rise in convection currents up to about 2300 meters depth has become colder and less salt a lot. In contrast, the convection is rather suppressed in the Norwegian Sea in recent years and is characterized by warmer and salt -rich deep waters.

Remote effects

The North Atlantic Oscillation and the Arctic Oscillation (AO) are very similar in space and therefore can not be considered separately. Similarly, a relationship between the NAO and the Pacific Decadal Oscillation ( PDO ) seems to exist. Decades with high NAO index wintry fall with decades of high PDO index together. This means that with many La Niña events can be expected in the decades of harsh winters in Europe. The period 1945 to 1970 supports this thesis. Whether there is a possible cause that can affect both oscillations crucial remains to be examined. However, it is important that the Southern Oscillation (SO) appears to have no direct influence on the NAO. Investigations of the correlation between the winter in Europe and the SO show that the SO only temperature changes of less than 1 /10 ° C are to be counted. In the summer, this is still far lower. The SO is thus rather a large relative of the NAO.

Cyclone Anatol El Niño–Southern Oscillation#Southern Oscillation
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