6. ocean currents
By now you should know about the movements of waves and what they look like. You may think that ocean waves are the only movements in the ocean, but there's more! Ocean currents are hidden under the waves and are caused by wind and different water densities. There are two different kinds of currents: surface and deep. I'm going to explain them both!
These currents are mainly caused by wind and are around 200-500 meters deep. Some currents travel for long distances (long-distance surface currents) and some currents don't travel very long at all (short distance currents). Most currents move in circular motions, and it depends on the circulation of wind in that particular hemisphere. All currents carry things with them and when they are done "spinning" those materials are deposited in a rip current (type of undertow).
These currents are mainly caused by different water densities. These densities are caused by different temperatures of water (cold water is more dense than warm water). Also, the more salinity in water, the more dense it will be. If cold water with a high salinity is carried into a place with warm water and low salinity, it will cause a deep current. When the 2 different kinds of water meet, the cold water is forced upward. That is known as upwelling.
The following is a video clip from a very popular movie called Finding Nemo. Even though this is a cartoon, it has a very good/accurate example of a current. Pay attention to the circular motion inside and outside of the current.