Definition
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:
- Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland
- Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland
- Azores (1865–2002), Gibraltar (1821–2007), and Reykjavik, Iceland
These definitions all have in common the same northern point (because this is the only station in the region with a long record) in Iceland; 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: 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. 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).
Read more about this topic: North Atlantic Oscillation
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