| Description: |
The Southern Annular Mode (SAM) influences Southern Hemisphere temperature and precipitation, ocean circulation, carbon cycling and the Antarctic cryosphere. In this Review, we examine the dynamics, projections and effects of the SAM, focusing on future implications for the Southern Ocean and Antarctica. The SAM is the leading mode of atmospheric variability in the Southern Hemisphere extratropics, associated with variations in the mid-latitude westerly jet strength and position. The SAM is primarily an internally driven atmospheric process, for which anomalies dissipate in 1–2 weeks; however, sustained SAM anomalies can also be forced by stratospheric processes and tropical Pacific variability. Ozone depletion during the 1970s–1990s contributed to large positive trends in austral summer. The SAM is now in its most positive mean state in over 1,000 years, and a year-round positive trend in the SAM is projected to continue throughout the twenty-first century in response to increasing greenhouse gases. Given the importance of SAM effects on Southern Ocean circulation, carbon cycling, and Antarctic ice mass balance for future climate and sea level rise projections, it is crucial that the effects of SAM are better modelled and understood, including accounting for the influence of the shifting seasonality of positive SAM trends and its increasing asymmetry. |