The Southern Annular Mode (SAM) is a large scale mode of atmospheric variability and arguably the most significant forcing of intraseasonal to decadal climate variability in the middle to high latitude Southern Hemisphere. Statistical analysis revealed a largely zonally-symmetric mass sea-saw between the subtropical latitudes centered at 45ºS and the Antarctic continent which is characteristic of the strength and position of the circumpolar westerlies (Hall and Visbeck, 2002; Visbeck and Hall, 2004).
A thematic website for annular modes can be found at http://ao.atmos.colostate.edu/
Atmospheric pressure observations from the Southern Hemisphere are used to estimate a monthly index of the Southern Annular Mode back to 1884. The analysis groups all relevant observations in four regions, one for Antarctica and three in the subtopical belt. By the end of the 19th century pressure observations are available at a number of locations in the subtropical regions. Systematic observations in the subpolar regime around the Antarctic continent began only during the 1940-1960 period seriously compromising the length of a traditional SAM index. Here estimates of Antarctic sea level pressure anomalies are provided based on the concept of atmospheric mass conservation poleward of 20ºS. A detailed description of the methods used is given in a paper submitted to the Journal of Climate.
Hall, A. and M. Visbeck, 2002: Synchronous Variability in the Southern Hemisphere Atmosphere, Sea Ice, and Ocean Resulting from the Annular Mode. J. Climate, 15, 3043-3057.
Visbeck, M., and A. Hall, 2004: RESPONSE To - Comments on "Synchronous Variability in the Southern Hemisphere Atmosphere, Sea Ice, and Ocean Resulting from the Annular Mode." J. Clim., 17(11), 2255-2258.
Visbeck, M., 2009: A station-based Southern Annular Mode index from 1884 to 2005. J. Climate, 22, 940-950. (pdf)