Three major factors influence salinity (salt concentration) in Pacific Ocean waters: precipitation, evaporation and winds. Precipitation brings freshwater into the ocean, diluting its salt concentration. The rate of evaporation from the oceanís surface waters is also important because it removes water molecules, leaving the salt behind. In regions where the evaporation is high, due to winds and high temperatures, the concentration of salt in the water increases. As a result, areas of water exposed to the strong trade-winds generally have higher salinity values.

The salinity of ocean water is measured in parts per thousand. High salinity values are over 35 parts per thousand, while values below this are considered low. Around the equator, where there is large amounts of rainfall, surface salinity rarely exceeds 34 parts per thousand. The lowest salinity occurs in the extreme northern regions of the Pacific, near the Bering Sea, where concentrations are often less than 32 parts per thousand.