Two key oceanic processes are involved in climate variability: hemispheric mass and heat exchanges associated to the meridional overturning circulation, and momentum and heat-flux exchanges at the air-sea interface. Those can drive variability at timescales from synoptic to decadal and centennial, as well as act through their impact on the circulation on biogeochemical properties of the ocean (e.g. oxygen minimum zones) and feedback on the cycle of greenhouse gases. These processes are also likely to be sensitive to change in mean state associated to global warming. A proper understanding of these processes requires an accurate estimation of surface energy and mass fluxes for the closure of the ocean energy balance, as well as of the exchanges of heat between ocean basins. The latter has remained uncertain due to the paucity of comprehensive datasets in particular in the Southern Ocean. While recent technological developments (including autonomous floats and gliders, animal-borne sensors, satellite systems) are beginning to give unprecedented insight into the role of the Southern Ocean in the global climate system, progresses in regional and global modeling has allowed to resolve a wide range of phenomena thought to be important for modulating both the meridional circulation and the air-sea fluxes, including mesoscale activity and its feedback with the overlying atmosphere. This session will bring together modellers and observers to assess the current state of knowledge of the role of the Southern Ocean in the global climate system as well as of the radiative heat budget at the air-sea interface at the regional and global scales.