Monthly and Seasonal, raw and normalized, indices of the NAO are based on the difference in sea level pressure between Ponta Delgada, Azores (38°N, 26°W) and Akureyri, Iceland (66°N, 18°W) from 1874-2000. Raw indices are obtained as the simple monthly sea level pressure anomaly difference between the two stations, biased toward the Arctic where the standard deviation in pressures is approximately twice that of the subtropical Azores location. Normalized indices are obtained by first obtaining the monthly standardized sea level pressures [monthly anomaly from the long-term (1874-1999) mean divided by long term monthly standard deviation]. The standardized values at the Azores is then subtracted from that at Iceland. It is these normalzied data that were first publised in Rogers (1984; his Figure1), and updated here. The station data were obtained from the World Monthly Surface Station Climatology (WMSSC) and updated recently from the publication "Monthly Climatic Data for the World".
The Azores/Iceland index is a simplification of the original NAO index devised by Sir Gilbert Walker in the 1920s which incorporated pressure, air temperatures, and precipitation at several Atlantic coastal stations. The notion that an NAO index could be simplified to subtropical and subpolar pressure differences was suggested by Dr. Jerome Namias, who informed me in the early 1980s that he always understood the NAO to be the pressure variation between the Azores and Iceland - the subsequent basis for this index. It is clear by all accounts dating back to Walker however, that an index of the NAO always be a descriptor of the strength of the North Atlantic westerlies.
The earliest data (1874-1906) at Ponta Delgada appear in the publication "Monthly Mean Values of Barometric Pressure for 73 Selected Stations over the Earth's Surface" by the Solar Physics Commission under the direction of Norman Lockyer (1908). This publication is available in the Atmospheric Environment Service Library in Toronto, Canada. The monthly pressures were in millimeters of mercury and were converted to millibars by multiplying by 1.3333 exactly. These monthly mean data overlap the previously mentioned WMSSC data (in millibars) during 1894-1906. Comparison of the datasets in this overlap period showed a consistent monthly bias toward lower values in the Lockyer dataset by 0.9 mb, give or take 0.01-0.03 mb. Thusly the 1874-1893 Azores data are from Lockyer and are 0.9 mb higher in all months than would be obtained using a standard conversion between millimeters of mercury and millibars.
This paragraph describes other details regarding replacement of missing data values. The following adjustments were made for missing mean monthly SLP values at Ponta Delgada.
1. The average gridded pressure at 25°W, 35°N and 25°W, 40°N are used for the following months: July and December 1961; October through December 1979; September 1980 and 1982; June 1981 and 1983 and October and November 1985. The gridded data are part of the historical sea level pressure dataset available at the National Center for Atmospheric Research.
2. Beginning with November 1989 and in many months since then, Ponta Delgada data are missing and are replaced by sea level pressures at either Horta, Azores or Santa Maria, Azores - which currently seems to be the only meteorological station on the island reporting to the WMO.
At Akureyri, Iceland:
3. Missing data from October 1918 through November 1919 were substituted with data from Angmagssalik, Greenland, located at nearly the same latitude and obtained from the same dataset, and the same applies for September and October 1980. No other attempts to replace recent missing Akureyridata have been made.