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

• Main Authors: Alex Hall, Martin Visbeck
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: If the earth’s surface were perfectly uniform; Antarctic; Southern Ocean; The Antarctic; Antarc*; Sea ice
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.528.2683; http://rainbow.ldeo.columbia.edu/papers/SAM_ocn_JClim.pdf

submitted to Journal of Climate 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 60ÆS), 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 30ÆS. Through mass continuity, the Ekman drift generates anomalous upwelling along the margins of the Antarctic continent, and downwelling around 45ÆS. 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