20CR Climate Indices:AO

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. To get daily values, we projected the daily anomalies of SLP from 1871-2012 onto the EOF's (anomalies were relative to 1981-2010). This time series is now standardized by the daily standard devation from 1981-2010. The corelation with one obtained using the NCEP R1 data (1950-2010) was .993 and with the CPC AO was ~.9 though they used 1000mb height and a different climatology time period.
Time Interval: Daily
Time Coverage: 1871 to 2012
Update Status: none

Get Data
AO index daily values 1871-2012
EOF is calculated at PSL using method in paper below and entire time period of SLP is projected onto the EOFs.
  1. 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.
  2. See annular modes webpage at Colorado State
Other Links
  1. Daily AO values from CPC.
  2. Monthly AO values using the 20CR.
  3. NOAA/NCEP Climate Prediction Center (CPC) AO monitoring page