Source: WeatherSTEM
The average salinity of the ocean is 35 parts per thousand, which means for every 1000 grams of water, there are about 35 grams of salt. Variations in salinity are caused by evaporation and precipitation. If there is more precipitation than evaporation, ocean salinity will be less. If there is more evaporation and precipitation, ocean salinity will be more. This is because as evaporation occurs over salt water, only the water molecules escape to become a gas. The salt ions stay behind in solution.