Salinity and the Water Cycle

Salinity, the concentration of dissolved salts in seawater, averages around 35 parts per thousand across the world's oceans, though it varies measurably from place to place depending on the local balance between evaporation, precipitation, river input, and sea-ice formation or melting. Salinity tends to be highest in subtropical regions, roughly between 20 and 30 degrees latitude, where high evaporation rates under persistent high-pressure systems concentrate dissolved salts, and in semi-enclosed seas with limited exchange and high evaporation, such as the Red Sea, where salinity can approach 40 parts per thousand. It tends to be lowest near the mouths of major rivers, where large volumes of fresh water dilute the surrounding seawater, and in polar regions, where melting ice and lower evaporation reduce salt concentration. With increasing depth, surface salinity generally gives way to a more uniform deep-ocean value through a transitional layer called the halocline, a zone of relatively rapid change in salinity with depth, analogous to the thermocline for temperature; the halocline plays an important role in ocean stratification, since it affects water density and can inhibit vertical mixing between surface and deep layers.