AMOC stands for Atlantic Meridional Overturning Circulation. It's slowing down significantly, and people in the know are pretty worried about it.
According to Professor Stefan Rahmstorf, AMOC is "a near-surface northward flow along the Atlantic, from the far south up to the high north. There the water gives off its heat to the atmosphere, sinks down, and returns south at a 2-3 km depth." Here's a schematic animation from NASA, titled, The Thermohaline Circulation aka The Great Ocean Conveyor Belt:
"The AMOC is part of the global overturning circulation driven by density differences. It supplies the deeper waters with oxygen. Schematic from my 2002 Nature article; yellow dots is where the water sinks into the deep."
