SAM (Southern Annular Mode) Forecasts
The SAM index is derived from the leading Empirical Orthogonal Function (EOF) of observed (NCEP Reanalysis) zonal-mean sea level pressure in the region from 25°S to 75°S. Forecasts of the SAM index are computed by projecting forecast sea level pressure onto the EOF derived from observed data.
Details of the derivation are provided in Marshall et al. (2011, Climate Dynamics). A high positive value of the SAM index is associated with anomalously high pressure centred near 45°S, and anomalously low pressure south of about 55°S, with a concomitant poleward shift of the midlatitude westerly jet. The opposite occurs when the SAM index is negative. Impacts of the SAM on Australian rainfall and temperature are provided in Hendon et al. (2007, Journal of Climate). In the above figure a 3-day running mean smoother is applied to the forecast data (but not to the observations).
The impact that the SAM has on rainfall varies greatly depending on season and region. If Australia were a few degrees further south, then the impact of changes in SAM would be much more pronounced. The diagram below describes the average impact on rainfall during a "positive" (westerly winds further south) SAM event.
The SAM also has an impact on temperatures. In general, in areas where rainfall is increased, temperature is decreased whilst where rainfall is decreased, temperature is increased.
The diagram above shows the impact that a "positive" SAM event (decreased westerly winds) has on Australian rainfall. Shading indicates daily rainfall anomaly in mm/day for each of the seasons. (Source: Hendon et al. (2007, Journal of Climate).