Indices of the annular modes
In general, indices of the annular modes are based on either
Daily and monthly resolution, updated in real-time.
1) the leading principal component (PC) time series of gridded geopotential height anomalies at a given pressure level or
2) approximations of the leading PC time series of geopotential height anomalies using differences between
sea level pressure anomalies at stations in middle and high latitudes.
Daily and monthly resolution indices of the annular modes are available from the NOAA CPC.
The data is available 1958-present for the Northern Annular Mode and 1979-present for the Southern Annular Mode.
Indices preceding the reanalysis era.
Link to real-time indices of the annular modes from the NOAA CPC
Indices for the Northern Annular Mode/North Atlantic Oscillation are available back to the middle 1800s based on station data over the Atlantic sector.
The most widely used index is based on surface data from Iceland and Portugal or the Azores. An excellent source for historical indices of the NAM is maintained by Jim Hurrell at:
Indices for the SAM preceding the satellite era (that is: preceding 1979) are based on very limited observations: there are only a handful of stations
over the high latitudes of the SH, and there are no continuous data records over Antarctica prior to ~1958. Below are listed three different SAM indices available for the period
preceding 1979. All are generated on the basis of midlatitude stations and/or tree ring records.
1) Data provided by Gareth Marshall (British Antarctic Survey):
2) Tree ring based reconstructions can be obtained from Martin Widmann at GKSS (firstname.lastname@example.org) and
Julie Jones at Univ. Sheffield (Julie.Jones@Sheffield.ac.uk)
3) Midlatitude station-based values of the SAM available from Martin Visbeck at the University of Kiel: http://www.ifm-geomar.de/~SAM