On shore we see waves caused by the wind. Their size depends on whether they are driven by winds from storms nearby or from far across the ocean. Earthquakes and shifts in the sea bottom often set in motion huge waves that move several hundred kilometres an hour. This type of wave is often called a tidal wave , even though it is not caused by the tide. Scientists call these waves tsunamis .
Water also move about in the ocean in streams known as ocean currents.
The wind systems of the Earth set the ocean currents in motion. Great circular currents move clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. The Earth's rotation makes the currents flow in this way. The strength of the ocean currents changes with the seasons.