Latest update (see below for earlier updates):
The North Atlantic Oscillation (NAO) is one of the major modes of variability of the Northern Hemisphere atmosphere (Hurrell, 1995). It is particularly important in winter, when it exerts a strong control on the climate of the Northern Hemisphere (Osborn, 2011). This is also the season that exhibits strong interdecadal variability (Osborn, 2004). For winter, the difference between the normalised sea level pressure over Gibraltar and the normalised sea level pressure over Southwest Iceland is a useful index of the strength of the NAO. Jones et al. (1997) used early instrumental data to extend this index back to 1823.
The NAO is traditionally defined as the normalized pressure difference between a station on the Azores and one on Iceland. An extended version of the index can be derived for the winter half of the year by using a station in the southwestern part of the Iberian Peninsula (Hurrell, 1995). Here we give data for SW Iceland (Reykjavik), Gibraltar and Ponta Delgada (Azores). The NAO calculated from Gibraltar and SW Iceland is also given (Jones et al., 1997).
The standard NAO index data file has been provided in the same format since 1998. This was supplemented by a version stored with 3 decimal places in 2017. The extra precision is provided for users who are making further calculations where rounding to the appropriate precision is best done at the end of the calculations.
Main reference to cite when using this NAO index:
Related references:
File format (NAO index) |
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for year = 1821 to endyear format(i5,12f7.2,f8.2) year, 12 * monthly value, annual value |
Missing values represented by -99.99 |
File format (NAO index with 3 decimal places) |
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for year = 1821 to endyear format(i5,12f8.3,f8.3) year, 12 * monthly value, annual value |
Missing values represented by -99.99 |
File format (Station sea-level pressure) |
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format(i1,i5,i5,a) ?,startyear,endyear,name for year = startyear to endyear format(i5,12i6) year, 12 * monthly value |
Missing values represented by -10
Divide by 10 for millibars |
Filename | Size | Description |
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nao.dat | 19 kilobytes | NAO index |
nao_3dp.dat | 22 kilobytes | NAO index (stored with 3 decimal places) |
nao_azo.dat | 10 kilobytes | Sea-level pressure data for Ponta Delgada, Azores |
nao_gib.dat | 15 kilobytes | Sea-level pressure data for Gibraltar |
nao_ice.dat | 15 kilobytes | Sea-level pressure data for South-West Iceland |
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