Hurrell's North Atlantic Oscillation (NAO) Index from NCAR

The North Atlantic Oscillation (NAO) is based on the difference of normalized sea level pressure (SLP) between Lisbon, Portugal or Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland since 1864. The SLP anomalies at each station were normalized by division of each seasonal pressure by the long-term (1864--1983) standard deviation. The station data were obtained from the World Monthly Surface Station Climatology: http://rda.ucar.edu/datasets/ds570.0/ Station data includes: Extended Winter (Dec - Mar) index of the NAO based on the difference of normalized sea level pressure (SLP) between Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland since 1864. Seasonal (DJF,MAM,JJA,SON) indices of the NAO are based on the difference of normalized sea level pressure (SLP) between Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland since 1864. Annual: The annual index of the NAO is based on the difference of normalized sea level pressure (SLP) between Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland since 1864. The values in the above table differ slightly from those in Hurrell (1995) because of continual updates to the data and a change in the base period. The SLP anomalies at each station were normalized relative to the 120-year period 1864--1983. Hurrell (1995) normalized relative to 1864--1994.