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20CR Climate Indices: North Atlantic Oscillation (NAO)

This mode describes variability over the northern Atlantic ocean, eastern North America and western Europe. The NAO represents the relative strength of the Icelandic low and the Azores high. It is typically defined as the pressure difference between Lisbon, Portugal and Reykjavík, Iceland though there are other definitions. A positive index is associated with a stronger westerlies and warmer winters/cooler summers in Europe and warmer winter temperatures in Eastern North America while a negative index is associated with higher amplitude temperature anomalies leading to heat waves and cold snaps in Europe. A negative index is also associated with a more southerly storm track bring storms to southern Europe and Northern Africa.
Positive and negative NAO spatial patterns based on compositing Januaries with large negative and positive values of the NAO index.
Calculation Method:
We used a station based definition (rather than an EOF based one) similar to that of Hurrell (see other links below). We took as our pressure dataset, gridded 20th Century Reanalysis V2 sea level pressure (SLP) values. Two timeseries starting in 1871 for the closest 20CR grids to Lisbon, Portugal [37.71,9.14W uses 38N,10W] and Reykjavík, Iceland [64.13N,21.93W uses 64N, 22W] were extracted. For each month of the year, we calculated a climatology based on 1871-1980 for each of the locations. The monthly climatology was then subtracted from each corresponding month of the whole timeseries to form anomalies. The standard deviation of each set of monthly anomalies was calculated using the years 1871-1980. Each monthly anomaly was divided by its corresponding standard deviation to form standardized data for Lisbon and for Reykjavik.
Time Interval: Monthly
Time Coverage: 1871 to 2011
Update Status: none

Get Data:
NAO index: Standard PSD Format (What is standard format?)
NAO index: Standard PSD Format (1948 onwards version)(What is standard format?)
The calculation method was applied at NOAA/ESRL PSD using their 20CR V2 monthly means.
  1. Compo, G.P., J.S. Whitaker, P.D. Sardeshmukh, N. Matsui, R.J. Allan, X. Yin, B.E. Gleason, R.S. Vose, G. Rutledge, P. Bessemoulin, S. Brönnimann, M. Brunet, R.I. Crouthamel, A.N. Grant, P.Y. Groisman, P.D. Jones, M. Kruk, A.C. Kruger, G.J. Marshall, M. Maugeri, H.Y. Mok, Ř. Nordli, T.F. Ross, R.M. Trigo, X.L. Wang, S.D. Woodruff, and S.J. Worley, 2011: The Twentieth Century Reanalysis Project. Quarterly J. Roy. Meteorol. Soc., 137, 1-28. DOI: 10.1002/qj.776 (Free and Open Access.)
Other Links
  1. Climatic Research Unit (CRU) NAO Data page.
  2. NOAA/NCEP Climate Prediction Center (CPC) NAO monitoring page including links to a monthly and daily NAO timeseries. Note that they use a EOF-based definition.
  3. NCAR Hurrell North Atlantic Oscillation (NAO) index (station-based).
  4. Wikipedia NAO article.
  5. Daily NAO Index forecast webpage based on the GEF2 Reforecast2 dataset.