North Atlantic oscillation
All are attempting to capture the same pattern of variation, by choosing stations in the "eye" of the two stable pressure areas, the Azores High and the Icelandic Low (shown in the graphic).
A more complex definition, only possible with more complete modern records generated by numerical weather prediction, is based on the principal empirical orthogonal function (EOF) of surface pressure.
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 the Mediterranean Sea.
In combination with the El Niño, this effect can produce significantly warmer winters over the upper Midwest and New 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.
[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.
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] 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 of Labrador Sea snow crabs, which have a low temperature optimum.
[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.
[20] The NAO's ecological effects extend as far as the Tibetan Plateau, where increases in aridity resulting in significant forest mortality and intensification of dust storms have been linked to NAO- events.
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 feedback mechanism).
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.
[29] Despite one of the strongest El Niño events recorded in the Pacific Ocean, a largely positive North Atlantic Oscillation prevailed over Europe during the winter of 2015–2016.
