Cold route versus warm route in the Meridional Overturning Circulation according to ECCO4

Understanding the pathways of the meridional overturning circulation (MOC) is of major importance because the MOC redistributes heat and salt in the global ocean. The Atlantic component of the MOC (AMOC), is characterized by an upper limb of northward-flowing warm and salty water, balanced by a lower limb of southward-flowing colder waters. Quantifying the sources of the upper limb and their transport and analyzing characteristics of the water masses entering the South Atlantic (SA) Ocean is essential to evaluate the redistribution of mass, heat and salt in the global ocean. In this study, we use backwards-in-time trajectories of particles with different origins entering the upper layer of the MOC in the SA Ocean. Particles are advected using the particle-tracking package from MITgcm and the velocity field in version 4 of Estimating the Circulation & Climate of the Ocean (ECCO4). Preliminary results indicate that the largest contributor (about 45%) to the incoming transport into the SA is the upper 1000 m of the Agulhas Current (the “warm route”) entering westward around South Africa. Our estimate shows a relatively small and shallow (between 0 and 100 m depth) contribution from the flow through Drake passage (the “cold route”), amounting to almost 4% of the total transport. The remaining transport either comes from the North Atlantic Ocean (about 20%) or recirculates within the SA subtropical gyre (about 30%). A large part of the waters flowing in the Agulhas Current originates from the SA itself: they are originally transported by the Antarctic Circumpolar Current (ACC) and then recirculate within the Indian Ocean before heading back to the SA through the Agulhas Current. The Indonesian throughflow is shown to contribute more to the overall transport than the Tasman Leakage, in contrast to previous estimates. The time-scales, temperature and salinity of the water masses entering the SA through this transport are quantified and discussed.