The effect of salinity and density on water in the oceans leads to both horizontal and vertical motions. The most important of these is the global conveyor belt, or thermohaline circulation. Water at the high density areas you found on the last map, sinks and drives a large, slow global current. The whole circuit can take 1,000 years to cycle around once.
Source: The Thermohaline Circulation - The Great Ocean Conveyor Belt | NASA