A state estimate of the routes of the upper branch of the Atlantic
Meridional Overturning Circulation
Abstract
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.