Throughout Earth's history, the weathering of rocks has delivered minerals, including salt, into the ocean. Over decades, this amount of salt in ocean basins has been relatively stable. The water cycle operates on much faster time scales, however, causing changes in salinity patterns. For example, freshwater enters the ocean from rivers, melting ice, rain, and snow. These processes tend to make the ocean less salty. On the other hand, processes that cause freshwater to exit the ocean—such as evaporation and formation of sea ice—make the ocean saltier.

Differences in salinity play a major role in moving seawater around the globe. Below the wind-blown ocean surface, salinity and temperature drive circulation by controlling the “density” (mass per unit volume) of seawater. The weight of dissolved salt increases water's density and when salinities are equal, colder water is denser than warmer water. In addition to moving salt and water, density-driven circulation is crucial to moving heat around our planet. The movement of heat by our ocean—for example, from the equator to the poles—is crucial to keeping Earth's climate “just right.”

These maps are made from satellite observations which cover the globe every week. The black areas show where data were not available, usually close to the coasts where strong signals from the land can interfere with accurate measurements.