Abstract: The Southern Annular Mode (SAM) is the leading mode of atmospheric interannual variability over the entire Southern Hemisphere, yet the impact of the SAM and its trend on Southern Ocean circulation and biogeochemistry is largely unknown. Using satellite observations and a numerical model, this dissertation investigates the role of the SAM in driving variability and trends in the circulation, biological activity, and carbon cycling in the Southern Ocean. A large fraction of the variability in the Southern Ocean circulation is found to be linked to the SAM. Positive phases of the SAM are associated with a poleward contraction of the atmospheric westerly winds, increasing the rate of upwelling, Ekman transport, and meridional overturning in the Southern Ocean. This circulation variability causes fluctuations in surface temperature and, through modifications in light and nutrient availability, biological activity. The SAM also drives a large fraction of the variability in the flux of CO₂ between the atmosphere and Southern Ocean. The primary cause for the flux variations are anomalies in the surface concentration of dissolved inorganic carbon, due to ocean circulation changes. The long term-trend in the SAM is found to contribute to a reduction in the strength of the Southern Ocean carbon sink over the past fifty years.