Ocean Circulation Demonstration and Interactive
Dr. Susan Humphris describes how differences in temperature and saltiness cause different water masses to form layers and flow at different depths.
Temperature and salinity affect the density of the water. Cold water is more dense than warm water. Salty water is more dense than fresh water. When waters of different density flow in the ocean, they order themselves from less dense near the surface to more dense at depth.
Seawater has salinities of about 35 parts per thousand (ppt) - that is 35 grams of salt in 1000 grams of seawater, but it can vary.
Fresh water has a density of 1 gram per cubic centimeter (g/cc). Seawater has very slightly higher densities of about 1.02-1.03 grams per cubic centimeter. Even though these are very small differences, they are enough to drive deep ocean circulation.