Currents

Deep water currents move very slowly, usually around 0.8-1.2 in (2-3 cm) per second. They dominate approximately 90% of the oceans' circulation. Water circulation of this type is called thermohaline circulation. Basically, these currents are caused by variations in water density, which is directly related to temperature and salt level, or salinity. Colder and saltier water is heavier than warmer, fresher water. Water gets denser in higher latitudes due to (1) the cooling of the atmosphere and (2) the increased salt levels, which result from the freezing of surface water. (Frozen water normally contains mostly freshwater, leaving higher concentrations of salt in the water that remains liquid.) Differences in water density generate slow moving currents, due to the sinking of the colder, saltier water into deeper parts of the oceans' basins and the displacement of lighter, fresher currents.