Atlantic South Current

Antarctic Intermediate Water enters the South Atlantic from the upper circumpolar region near the Drake Passage and the Falkland Islands. It is one of several water masses that meet in the southern basin. South Atlantic Central Water originates in the anti-cyclonic subtropical gyre, and both systems receive some contribution from the Indian Ocean. North Atlantic Deep Water also passes southward beneath the subtropical thermocline. The result is a vertical exchange between warm upper water, cooler intermediate water and a deep return flow.

The Drake Passage itself is a geological gateway. Around fifty million years ago, changes in the relative motion of South America and Antarctica began opening it. Small ocean basins first formed and a shallow gateway appeared; a deeper seaway developed between thirty-four and thirty million years ago. That transition coincided with climatic deterioration and the growth of the Antarctic ice sheet.

The South Atlantic basin was formed still earlier by the Early Cretaceous breakup of West Gondwana. The Mid-Atlantic Ridge then divides the basin lengthwise and is interrupted by major transform faults. Secondary ridges form smaller deep basins, while the Walvis Ridge and Rio Grande Rise obstruct currents. Salty Indian Ocean water enters through Agulhas leakage and rings. Thus intermediate water from the far south moves through a basin whose submerged ridges, deep passages and long history of opening continue to shape Atlantic circulation.