The ESRL website will be unavailable for 24 hours starting Friday, March 6th at 5:00pm MT due to building maintenance.

20CR Climate Indices:AAO

This mode describes low frequency variability of the Southern 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 Antarctic air into the Southern Hemisphere. It is also referred to as the "Southern Annular Mode" (SAM).
Spatial pattern of the AAO. 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. We also used SLP and not 850mb geopotential heights. An 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 and a covarinace matrix is used. 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 not standardized.
Time Interval: Daily
Time Coverage: 1871 to 2012
Update Status: none

Get Data
AAO index daily values 1871-2012
EOF is calculated at PSD 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 AAO values from CPC.
  2. Monthly AAO values using the 20CR.
  3. NOAA/NCEP Climate Prediction Center (CPC) AAO monitoring page