Forcing of Ocean and Sea Ice Variability by the Southern Hemisphere Annular Mode

Zonally-symmetric fluctuations of the mid-latitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons. This is true not only in observations, but also in an unforced 15,000 yr integration of a coarse resolution (R15) coupled ocean-atmosphere model. Here we document how this mode of atmospheric variability, known as the Southern Annular Mode (SAM), generates ocean circulation and sea ice variations in the model integration on interannual to centennial time scales that are tightly in phase with the SAM. The positive phase of the SAM is associated with an intensification of the surface westerlies over the circumpolar ocean (around 60S), and a weakening of the surface westerlies further north. This induces Ekman drift to the north at all longitudes of the circumpolar ocean, and Ekman drift to the south at around 30S. Through mass continuity, the Ekman drift generates anomalous upwelling along the margins of the Antarctic continent, and downwelling around 45S. The anomalous flow diverging from the Antarctic continent also increases the vertical tilt of the isopycnals in the Southern Ocean, so that a more intense circumpolar current is also closely associated with positive SAM. In addition, the anomalous divergent flow advects sea ice further north, resulting in an increase in sea ice coverage. Finally, positive SAM drives increases in poleward heat transport at about 30S, while decreases occur in the circumpolar region. Ocean and sea ice anomalies of the opposite sign occur when the SAM is negative. The ocean and sea ice fluctuations associated with the SAM constitute a significant fraction of simulated ocean variability poleward of 30S all year round. The robustness of the mechanisms relating the SAM to oceanic variability suggests that the SAM is likely an important source of large-scale variability in the real southern hemisphere ocean.