NAO indices and related time series

Here is a collection of on-line time series that can be used as indices of the North Atlantic Oscillation. The data is FREELY available thanks to the generosity of the people who have taken their time to provide it. Such freedom of information is vital to the progress of research in climate so please ALWAYS reference the source of this data by citing the appropriate authors when you have used this data. Indices based on pressure measured at 2 stations

The NAO index is often defined as the difference of sea-level pressure between 2 stations situated close to the "centres of action" over Iceland and the Azores.

Stykkisholmur (Iceland) is invariably used as the northern station, whereas either

  • Ponta Delgada (Azores)
  • Lisbon (Portugal)
  • Gibraltar
are used as the southern station.

Many previous studies have used Ponta Delgada (e.g. Rogers 1997). More recently Jim Hurrell has used Lisbon, and Jones et al. have used Gibraltar for various reasons. The choice of southern station can make some differences especially in seasons other than winter (see Jones et al. 1997).

It is also sometimes possible to use only the pressure in Iceland as a proxy index for the NAO because of it being so strongly correlated with pressure at the southern stations (Besse et al. 1999).

NCAR climate analysis centre 1864-1998 Jim Hurrell's monthly, seasonal and annual indices based on pressures at Lisbon (Portugal) and Stykkisholmur (Iceland).
Uni. Washington, Seattle 1864-1995 Jim Hurrell's Dec-March winter mean NAO index in Matlab format.
Climate Research Unit, University of East Anglia 1821-Oct 1999 Jones et al. monthly mean index based on pressures at Gibraltar and Stykkisholmur (Iceland).
NAO thematic web site
(David B. Stephenson)
1821-1998 Raw sea-level pressure series used in making NAO indices



Multivariate indices

It is possible to combine multiple time series together to construct more robust NAO indices that are less sensitive to displacements in centres of action. Mention Walker's index and our PC one.

Northern hemisphere teleconnection pattern indices from NOAA's NCEP/CPC
1950-one month ago Rotated principal components obtained from NCEP analyses



Proxy indices inferred from NAO impacts

Quantitative historical measurements of meteorological variables rarely extend further back than the beginning of the 19th century. To go further it is necessary to infer the NAO index from either written accounts, biological records (e.g. tree rings), or geological records. From these sources, it is possible to construct "proxy" indices that can help describe some of the past behaviour in the NAO.


NCAR climate analysis centre 1864-1998 Jim Hurrell's monthly, seasonal and annual indices based on pressures at Lisbon (Portugal) and Stykkisholmur (Iceland).




  • Rogers, J.C., 1997: North Atlantic storm track variability and its association to the North Atlantic Oscillation and climate variability of Northern Europe. Journal of Climate 10(7), 1635-1647.
  • Hurrell, J.W., 1995: Decadal trends in the North Atlantic Oscillation and relationships to regional temperature and precipitation. Science 269, 676-679.
  • Jones, P.D., Jónsson, T. and Wheeler, D., 1997: Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South-West Iceland. Int. J. Climatol. 17, 1433-1450.
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