A large-scale alternation of atmospheric mass between the Icelandic low- and the Azores high-pressure area. When the pressure difference is large, with a deep Icelandic low and a strong Azores high, the NAO is said to be high or positive, and said to be negative when the pressure difference is less (or occasionally negative) as a result of persistent blocking in the Iceland–Scandinavia area. Strong positive phases of the NAO tend to be associated with above-normal temperatures in the eastern USA and across northern Europe, and below-normal temperatures in Greenland and across south-eastern Europe and the Middle East. Opposite patterns are observed during negative phases (Perry (2000) PPG 24, 2—an excellent source). Kingston et al. (2006) PPG 30, 2 find generally positive NAO/Arctic Oscillation streamflow relationships in north-west Europe and north-east USA, and positive and negative streamflow relationships in parts of eastern Canada. Kuszmina et al. (2005) Geophys. Res. Letts. 32, L04703–L04703 suggest that the NAO may intensify with further increases in greenhouse gas concentrations.

The name Arctic Oscillation was introduced to highlight the fact that the pressure anomalies associated with the NAO span most of the Arctic. But ‘oscillation’ is something of a misnomer, as the northern and southern annular modes oscillate regularly in time. For this reason, in recent years the NAO/AO nomenclature has been increasingly supplanted in the dynamical literature by the phrase Northern Annular Mode (NAM). Annular denotes the longitudinal scale of the pattern, and annular mode suggests the NAM reflects dynamical processes that transcend a particular hemisphere or, for that matter, planet.