Why does seawater salinity vary from place to place?

Seawater salinity varies primarily due to local processes at the Earth's surface that add or remove fresh water, altering the concentration of dissolved salts.1

The primary drivers of these variations include:

• Evaporation and Precipitation: In regions where evaporation exceeds precipitation, such as the subtropical horse latitudes, surface salinity increases. Conversely, in areas with high precipitation, such as near the equator, surface salinity is depressed.12
• Freshwater Runoff: Nearshore regions and enclosed seas often experience lower salinity due to the influx of fresh water from large rivers. For example, the Baltic and Black seas have significantly lower salinity compared to the open ocean.12
• Ice Dynamics: At high latitudes, the seasonal formation of sea ice increases the salinity of the remaining water, while the melting of icebergs and sea ice introduces fresh water, which lowers local salinity.1
• Ocean Currents: Currents can transport water masses with distinct salinity characteristics across large distances, influencing the salinity profiles of different regions.2

While these surface processes cause local fluctuations, the composition of sea salt remains remarkably uniform throughout the open ocean.1