Abstract. The northward flow of the upper limb of the
Atlantic Meridional Overturning Circulation (AMOC) is fed by waters entering
the South Atlantic from the Indian Ocean mainly via the Agulhas Current (AC)
system and by waters entering from the Pacific through Drake Passage (DP),
commonly referred to as the “warm” and “cold” water routes, respectively.
However, there is no final consensus on the relative importance of these two
routes for the upper limb's volume transport and thermohaline properties. In
this study we revisited the AC and DP contributions by performing Lagrangian
analyses between the two source regions and the North Brazil Current (NBC) at
6∘ S in a realistically forced high-resolution (1∕20∘)
ocean model. Our results
agree with the prevailing conception that the AC contribution is the major
source for the upper limb transport of the AMOC in the tropical South
Atlantic. However, they also suggest a non-negligible DP contribution of
around 40 %, which is substantially higher than estimates from
previous Lagrangian studies with coarser-resolution models but now better
matches estimates from Lagrangian observations. Moreover, idealized analyses
of decadal changes in the DP and AC contributions indicate that the ongoing
increase in Agulhas leakage indeed may have induced an increase in the AC
contribution to the upper limb of the AMOC in the tropics, while the DP
contribution decreased. In terms of thermohaline properties, our study
highlights the fact that the AC and DP contributions cannot be unambiguously
distinguished by their temperature, as the commonly adopted terminology may
imply, but rather by their salinity when entering the South Atlantic. During
their transit towards the NBC the bulk of DP waters experiences a net density
loss through a net warming, whereas the bulk of AC waters experiences a
slight net density gain through a net increase in salinity. Notably, these
density changes are nearly completely captured by Lagrangian particle
trajectories that reach the surface mixed layer at least once during their
transit, which amount to 66 % and 49 % for DP and AC
waters, respectively. This implies that more than half of the water masses
supplying the upper limb of the AMOC are actually formed within the South
Atlantic and do not get their characteristic properties in the Pacific and
Indian Oceans.
Highly variable upper and abyssal overturning cells in the South Atlantic
