The presence of salt in sea water allows ice to float on top of it. These gases enter the ocean from the atmosphere and from a variety of organic processes. Sea water changes its density with variations in temperature, salinity, and ocean depth. Sea water is least dense when it frozen at the ocean surface and contains no salts. Highest sea water densities occur at the ocean floor.
Atmospheric circulation drives the movement of ocean currents.
Within each of the three ocean basins, the patterns of these currents are very similar. In each basin, the ocean currents form several closed circulation patterns known as gyres. A large gyre develops at the subtropics centered at about 30 degrees of latitude in the Southern and Northern Hemisphere. In the Northern Hemisphere, several smaller gyres develop with a center of rotation at 50 degrees. Similar patterns do not develop in the middle latitudes of the Southern Hemisphere.