Antarctic Bottom Water formation in the Southern Ocean: Concepts and new results E. Fahrbach, Alfred-Wegener-Institut für Polar- und Meeresforschungin der Helmholtz-Gemeinschaft, Bremerhaven, Germany The Southern Ocean contributes through atmosphere-ice-ocean interaction processes to the variability of the global climate system. Atmosphere-ice-ocean interactions, occurring in the open ocean and on the shelves, lead to water mass conversions which result in the formation of Antarctic Bottom Water. A major contribution to the global deep and bottom water formation occurs in the Weddell Sea. It is controlled by the transport of source waters into the Weddell Sea, transformation processes within the Weddell Sea, and the transport of modified water out of the Weddell Sea. In the Weddell Sea, Circumpolar Deep Water enters from the north and circulates as Warm Deep Water in intermediate layers within the large-scale cyclonic gyre. Recent observations indicate that the water mass properties of the Warm Deep Water are subject to significant variations. After an initial warming and salinity increase observed during the nineties a cooling followed during the last years. The variations are most likely due to changes in the inflow from the circumpolar water belt, in combination with changes in the ice-ocean-atmosphere interaction in the Weddell Sea induced by changes in the atmospheric forcing conditions. Whereas the properties of the Weddell Sea Deep Water have remained essentially constant, the Weddell Sea Bottom Water has been subject to significant changes as well. Since the Warm Deep Water is the source water of bottom water, the variations of the two water masses are likely to be related through the formation process.