Have an average depth of 3,800 m. Have a present volume of about 1.35 billion cubic kilometers, but the volume fluctuates with the growth and melting of glacial ice. Salinity , a measure of amount of dissolved ions in the oceans, ranges between 33 and 37 parts per thousand. Seawater would contain higher concentrations of dissolved ions if some were not removed by chemical precipitation, plants and animals, and absorption onto clay minerals. Salinity is lower near the equator because precipitation is higher.
Salinity is low near the mouths of major rivers because of input of fresh water.
The temperature of surface seawater varies with latitude, from near 0 o C near the poles to 29 o C near the equator. But restricted areas can have temperatures up to 37 o C. Properties of seawater also vary with depth. The density and salinity of seawater increase with depth. Temperature decreases with depth. Ocean Circulation Surface Ocean currents are result of drift of the upper 50 to 100 m of the ocean due to drag by wind.