In order to determine the origin of water composing the upper branch of the CB we computed (backward in time) individual trajectory of hundred of thousands of particles belonging to the northward flowing upper layers of the equatorial Atlantic section. We stopped each trajectory when it reached one of the sections closing the Indo-Atlantic sector, i.e., the Drake Passage (DRAKE), the Indonesian Passage (ITFL), the segment linking Australia to Antarctica at the Tasmania longitude (TAS) and the equatorial Atlantic (EQATL).

The Lagrangian computations are achieved with a monthly varying velocity field as the results showed to be sensitive to a larger time sampling.

The simulated total northward flow at EQATL amounts to 45 Sv, the largest fraction of which is a recirculation within the Indo-Atlantic domain (27 Sv meander in the upper layers and 2.2 Sv upwell from NADW). The mass flux that originates outside the Indo-Atlantic sector amounts to 15.8 Sv and its vertically integrated path is shown below.

While the larger fraction of water comes from the two classical Cold-DRAKE and Warm-ITFL Routes with equivalent intenstities (6.8 and 5.8 Sv, respectively), the model suggests interestingly an additional source of water, just south of Tasmania, that accounts for 3.2 Sv.