Response of the South Atlantic circulation to an abrupt Atlantic THC collapse

The South Atlantic response to a North Atlantic thermohaline circulation (THC) collapse is investigated in the ECHAM5/MPI-OM climate model. A slightly reduced Agulhas leakage (about 2 .2 Sv; 1 Sv= 106m3 . s−1 ) is found to be associated with a somewhat weaker Southern Hemisphere (SH) supergyre
and Indonesian throughflow (ITF). These changes are due to reduced wind stress curl over the SH
supergyre, associated with a weaker Hadley circulation and a weaker SH subtropical jet. The northward cross-equatorial transport of thermocline and intermediate waters is much more strongly reduced in relation with a THC collapse. A cross-equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous SST gradient associated with reduced ocean heat transport. This cross-equatorial gyre completely blocks the transport of thermocline waters from the South to the North Atlantic. The waters originating from Agulhas leakage flow somewhat deeper and most of it recirculates in the South Atlantic subtropical gyre, leading to a gyre intensification. This intensification is
consistent with the anomalous surface cooling over the South Atlantic. Most changes in South Atlantic
circulation due to global warming, featuring a reduced THC, are qualitatively similar to the response to a THC collapse, but smaller in amplitude. However, the increased northward cross-equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of a THC collapse.