The Southern Ocean (SO) not only constitutes a geographical link between the world ocean basins but also is a very active climate sub-system in terms of air–sea exchanges, water mass formation and transformation and biogeochemical processes and budgets. Its role in climate is central and its sensitivity to climate change is shown to be extremely important. For example, the Southern Ocean is a major CO₂ sink via dynamical processes such as convection/subduction and biological carbon pumping through the export of particulate organic matter.

Prior to 1990, SO observations were sparse. Since then, an intense monitoring effort has been undertaken for two of the three SO choke points, Drake Passage and south of Australia. The one south of Africa, which is the largest, has been undersampled despite its suspected importance in the Meridional Overturning Circulation (MOC). There, the SO plays a unique role in providing the export channel for North Atlantic Deep Water to the global ocean and by importing heat, salt and biogeochemical tracers from the Indian and Pacific oceans. This region is influenced by the largest turbulence ever observed and by very efficient air–sea exchanges. Here the atmosphere and ocean are equally dynamic: the complex fronts, eddies and filaments in the ocean are analogous to the rapid frontal movements, storm passages, and dry air intrusions in the atmosphere. Transfer of heat to the atmosphere from the ocean north of the polar front contributes to the dominance of cumuliform cloud types in this region. These local small-scale processes and the derived meridional fluxes constitute the major link between the Atlantic and the other ocean basins.

The BONUS-GoodHope (BGH) project is based on a multi-disciplinary oceanographic cruise that took place during the International Polar Year (13 February–22 March 2008) sailing from Cape Town, South Africa, to 57°S along the Greenwich Meridian in the Southern Ocean. BGH is part of an international effort on ocean exploration on the regional scale (polar regions, IPY) and on the global scale (IMBER, GEOTRACES) and has three main objectives:

• understanding of large-scale inter-ocean exchanges between the Indian, Atlantic and Southern oceans;
• characterising the biogeochemical processes involved in the internal cycling of carbon, trace elements and isotopes;
• assessing the impact of coastal–open-ocean exchanges on geochemical properties of the water-column and biogeochemical cycles.

During BGH, observations on ocean and atmosphere dynamics were coupled with biogeochemical observations covering the full water column, with particular focus on transfer pathways of trace elements and isotopes from atmosphere to seafloor. Significant progress was achieved toward a synthetic understanding of physical, geochemical and biological processes as well as air–sea and inter-ocean exchanges. The BG–OS special issue will integrate a collection of papers presenting these achievements.