Deep water formed around the Antarctic continent drives the world ocean circulation. 50-70% of this deep water is formed within only about 10% of the Antarctic circumpolar band: the Weddell Sea. Subtle changes in the circulation of the Weddell Sea can lead to major changes in floating ice-shelves, with critical implications for global sea-level, the production of deep water and the global ocean overturning circulation. Despite these critical climate implications, the Antarctic shelf circulation remains poorly understood. I propose an ambitious project at the crossroads of experimental and numerical oceanography. By drawing on the strengths of each discipline I will explore the regional water-mass pathways in the Weddell Sea: an unchartered cornerstone for understanding the polar ocean circulation and its links to global climate. A key issue facing climate scientists will be addressed: “What sets the tridimensional water-mass structure and pathways in the Weddell Sea and modulates the flow of deep waters between the Antarctica ice-shelves and the global ocean circulation?”To address this question I propose to investigate several key aspects of the Weddell Sea system: the dynamical forcing of the Weddell gyre and its response to atmospheric variability; the forcing and the circulation on the continental shelf and its interaction with the gyre; and the time-scale and mixing associated with bottom water sinking along the continental shelf. WAPITI approaches these objectives through a series of innovations, including (i) an ambitious field experiment to investigate the shelf circulation and processes, (ii) a powerful conceptual framework applied for the first time to a realistic eddy-resolving model of the Weddell gyre, and (iii) a novel instrument that will be developed to directly observe the sinking of deep water into the abyssal ocean for the first time. Collectively, the project will contribute a new insight into global climate feedbacks.