_http://science.nasa.gov/earth-science/oceanography/physical-ocean/salinity/ said:
Although everyone knows that seawater is salty, few know that even small variations in Sea Surface Salinity (SSS) can have dramatic effects on the water cycle and ocean circulation. SSS tells us the about the concentration of dissolved salts in the upper centimeter of the ocean surface. Throughout Earth's history, certain processes have served to make the ocean salty. The weathering of rocks delivers minerals, including salt, into the ocean. Evaporation of ocean water and formation of sea ice both increase the salinity of the ocean. However these "salinity raising" factors are continually counterbalanced by processes that decrease salinity such as the continuous input of fresh water from rivers, precipitation of rain and snow, and melting of ice.
Salinity & The Water Cycle
Understanding why the sea is salty begins with knowing how water cycles among the ocean's physical states: liquid, vapor, and ice. As a liquid, water dissolves rocks and sediments and reacts with emissions from volcanoes and hydrothermal vents. This creates a complex solution of mineral salts in our ocean basins. Conversely, in other states of ocean water such as vapor and ice, water and salt are incompatible: water vapor and ice are essentially salt free.
Since 86% of global evaporation and 78% of global precipitation occur over the ocean, SSS is the key variable for understanding how fresh water input and output affects ocean dynamics. By tracking SSS we can directly monitor variations in the water cycle: land runoff, sea ice freezing and melting, and evaporation and precipitation over the oceans.
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Salinity & Ocean Circulation
Changes in salt concentration at the ocean surface affect the weight of surface waters. Fresh water is light and floats on the surface, while salty water is heavy and sinks. Together, salinity and temperature determine seawater density and buoyancy, driving the extent of ocean stratification, mixing, and water mass formation. Greater salinity, like colder temperatures, results in an increase in ocean density with a corresponding depression of the sea surface height. In warmer, fresher waters, the density is lower resulting in an elevation of the sea surface. These height differences are related to the circulation of the ocean. The changes in density bring warm water poleward on the surface to replace the sinking water driving the global thermohaline (heat & salt) circulation within the ocean called the Global Conveyor Belt.
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Salinity & Climate
The Global Conveyor Belt is the principal mechanism by which the oceans store and transport heat. The ocean stores more heat in the uppermost 3 meters than that of the entire atmosphere and acts as a "global heat engine." Since salinity is a key ingredient in the global thermohaline circulation, SSS will help us discover how climate variation induces change in global ocean circulation.
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By tracking SSS we can directly monitor variations in the water cycle: land runoff, sea ice freezing and melting, evaporation and precipitation over the oceans. These changes may reflect changes in the global Earth system or other natural or human-induced changes.
To track changes in SSS patterns over time, scientists monitor the relationship between two primary processes in the oceans: evaporation, which controls the loss of water; and precipitation which governs the gain of water.