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Why does seawater salinity increase in places with high evaporation?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

Seawater salinity increases in areas of high evaporation because the process removes fresh water from the ocean surface while leaving the dissolved salts behind.1

In the hydrosphere, the concentration of salt in the ocean is primarily determined by local surface processes that add or remove fresh water. When evaporation occurs, the water molecules transition into the atmosphere, but the salts remain in the remaining liquid, effectively increasing the ratio of salt to water. This effect is particularly pronounced in subtropical zones—often called the horse latitudes—where high evaporation rates lead to elevated surface salinities. The phenomenon is further accentuated in enclosed or isolated bodies of water, such as the Red Sea, where limited exchange with the open ocean prevents the dilution of the increasingly saline water.12