AABW

Antarctic Bottom Water.

This is water that has descended from the surface to the sea floor around Antarctica. From there it can flow along the sea floor to many parts of  the ocean where access isn't blocked by major ridges.

Ocean currents tend to move around the world in rather fixed vertical layers, with not much mixing. So for example the Antarctic Bottom Water moves up into the Pacific, Atlantic & Indian oceans,moving along the sea floor, while other currents move above it.

ABW is some of  the coldest and densest water in the ocean, and also it is the most oxygen rich water on the sea floor, having carried oxygen with it from the surface. It has thus been labelled the ventilation of the deep ocean. Read more about it here:

Definition based on (revised from) IPCC AR4.