After hydrogen and oxygen, the two most abundant or "common" elements dissolved in seawater are sodium and chloride, which combine to form salt. The salinity of seawater is measured by grams of salt per liter of water, with a typical reading of approximately 35 grams per liter, or 35 percent.
The Atlantic Ocean is the saltiest of Earth's five oceans. A decrease in salinity occurs near the equator and at both poles for different reasons. For water near the equator, the addition of consistent freshwater rain near the tropics causes a decrease in salinity of the surface water. For water near both poles, rain decreases; with less rain and increased sunshine, evaporation also increases, causing the decrease in salinity.