20CR Climate Indices:AO

Note: data has been updated through 2012 (with very small changes in values for 1871-2012). Method now standardizes on the 1981-2010 time period.
This mode describes low frequency variability of the Northern Hemisphere extratropics. A positive index (low polar pressure) is associated with a stronger, more zonal winds in the extratropics. A negative value is associated with less zonal winds and more incursions of Arctic air into the Northern Hemisphere.
Spatial pattern of the AO. EOF is based on 1981-2010 monthly anomalies of SLP from the 20CR V2 dataset.
Calculation Method:
We used the definition in Thompson and Wallace 2000 (references) but with different years. Our EOF is calculated using SLP from 20N to the pole from 1981 to 2010.Anomalies are calculated for each month based on a 1981-2010 climatology and all months are used in the calculation of the EOF. The grids are area-weighted via the cosine of latitude. The data is then projected back onto the EOF and standardized to produce the principal component (time-series) using the data from 1981-2010.
Time Interval: Monthly
Time Coverage: 1871 to 2012
Update Status: none

Get Data:
AO index: Standard PSD Format (What is standard format?)
AO index: Standard PSD Format (1948 onwards version)(What is standard format?)
Data is calculated at PSD using method in paper below.
  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.
  2. Thompson D. W.J. and J.M. Wallace:2000: Annular Modes in the Extratropical Circulation. Part I: Month-to-Month Variability: J. of Climate, 13, 1000-1016.
  3. See annular modes webpage at Colorado State
Other Links
  1. Daily AO values using the 20CR.
  2. NOAA/NCEP Climate Prediction Center (CPC) AO monitoring page