Chapter 11 - Southern Annular Mode

The Southern Annular Mode (SAM) is an important pattern of Southern Hemisphere circulation variability: it accounts for between 15 and 30 or more percent of the total variability of the Southern Hemisphere extratropical circulation on daily to annual time scales and significantly modulates weather variability across the middle and higher latitudes. It is an expression of feedback between the mean westerly flow over the Southern Ocean and its associated storm track. The discovery in the late 1990s of a trend in SAM behavior led to the realization that stratospheric ozone depletion and greenhouse gas increase both influence SAM behavior, and will do through coming decades. This chapter surveys the mechanism of the SAM, the history of our understanding of the SAM, its impacts upon surface climate, and the outlook for the SAM’s future.