The ocean is naturally divided into deep basins and there are currents of dense water that flow around and between them. The sources of these currents are sinking regions near the poles, including the Nordic Seas north of Iceland and Scotland. As the dense water exits the sinking regions and moves from basin to basin, it often spills over topographic sills in the manner of flow over a dam. These overflows can become very turbulent and can mix rapidly with the surround fluid. Sills also represent choke points that are ideal for monitoring slow climate change.
At the other end, the fluid must somehow return to the surface, possibly through the upslope flows that have been observed in abyssal canyons.