What causes currents in the ocean?

Ocean stratification results when ocean water with different densities forms layers due to their properties. In this video, I show how differences in salinity can create ocean stratification which may result in ocean currents.
In the salinity demonstration, I have a container divided into two halves. On one side I have fresh water and the other I have very salty water. The two types of water are separated by a divider. When I lift the divider the salty water flows under the warm water and creates a current.