The origin of the Mediterranean Outflow is investigated by deploying six millions virtual Lagrangian parcels at the Strait of Gibraltar, and tracing them backward in time using velocity estimates from an eddy-permitting reanalysis. The Lagrangian parcels are followed until they intercept one of three sections. The hypothesis is that each section is associated with distinct water masses: the Gulf of Lions, related to Western Mediterranean Deep Water and Western Intermediate Water, carries 86\% of the Outflow’s transport; the Northern Tyrrhenian, related to Tyrrhenian Deep and Intermediate Waters, carries 1\% of the transport; the Strait of Sicily, related to Levantine Intermediate Waters, carries 13\% of the transport. The median transit times from the sections to the Strait of Gibraltar range from 5 years (Gulf of Lions) to 8 years (Strait of Sicily).

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.