The origins of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC) are traced with backward-in-time Lagrangian trajectories, quantifying the partition of volume transport between different routes of entry from the Indo-Pacific sector into the Atlantic. Particles are advected by a three-dimensional, incompressible velocity field from a recent release of “Estimating the Circulation and Climate of the Ocean’ (ECCOv4). This time-variable velocity field is a dynamically consistent interpolation of over one billion oceanographic observations collected between 1992 and 2015. Of the 13.6 Sverdrups (1Sv = $10^6$ m$^3$/s) of upper and intermediate water flowing northward across 6\mdeg S, 15% enters the Atlantic from Drake Passage, 35% enters from the straits between Asia and Australia, termed the Indonesian Throughflow, and 49% comes from the region south of Australia, termed the Tasman Leakage. The salinity budget shows that the AMOC exports freshwater out of the Atlantic.