The Southern Ocean regulates the global climate by controlling heat and carbon exchanges between the atmosphere and the ocean. The overall objective of SO-CHIC is to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. SO-CHIC considers the Atlantic sector of the Southern Ocean as a natural laboratory both because of its worldwide importance in water-mass formation and because of the strong European presence in this sector already established at national levels, which allow to best leverage existing expertise, infrastructure, and observation network, around one single coordinated overall objective. SO-CHIC also takes the opportunity of the recent re-appearance of the Atlantic Sector Weddell Polynya to unveil its dynamics and global impact on heat and carbon cycles.