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) PPG24, 2—an excellent source). Kingston et al. (2006) PPG30, 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.
Subjects: Meteorology and Climatology.