Salt ions have a greater mass than water molecules, so anything that increases the concentration of salt in water, such as evaporation, increases the density of that water. As salinity increases, the density of that water increases, causing it to sink below less dense water. Density differences drive deep ocean mixing.
In the North Atlanic, cold, dense water sinks to deep ocean basins, then slowly migrates to the tropics where it warms and upwells. This warmer, less dense water is then carried back toward the pole to replace the dense, sinking water at the surface. A simplified version of this is often referred to as the ocean conveyor belt.
