Warm water floats on cold water. Water moves horizontally and vertically. Horizontally, it moves mostly west at the equator until it runs into the continents. Circulates clockwise in N hemisphere, counterclockwise in southern. Also moves vertically in less obvious fashion.
Cold water sinks off Greenland, circulates in a complex worldwide pattern.
Important because these vertical movements eventually bring nutrients from the deep to the shallow ocean. Salinity Salinity of ocean varies, depending on evaporation and runoff from land. Oxygen Water salt or fresh, holds much less oxygen than air. In ocean, oxygen is usually highest near the surface. Too little light below that, but there is life even at 10,000 m.