The positive and negative phases of the North Atlantic Oscillation are defined by the differences in pressure between the persistent low over Greenland and Iceland and the persistent high off the coast of Portugal. During a positive NAO, both systems are stronger than usual. That is, the low has a lower atmospheric pressure and the high has a higher atmospheric pressure. During the negative phase of the NAO, both systems are weaker, lowering the difference in pressure between them. (Images by Robert Simmon)

http://earthobservatory.nasa.gov/Study/NAO/NAO_2.html archive copy at the Wayback Machine

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