In order to determine the origin of water composing the upper branch of the CB we computed (backward in time) individual trajectories of hundred of thousands of particles belonging to the northward flowing upper layers of the equatorial Atlantic section. We stopped the integration for each particle when it reaches one of the section closing the Indo-Atlantic sector, that is 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 were shown 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 of water 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 sums to 15.8 Sv and its vertically integrated path is shown below (see Blanke et al., 1999 for description of the particle-related streamfunction computation).

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