NASA: Aquarius Mission Web Site

The water molecule, H20, is unique in that it can naturally exist as a gas (steam), liquid (water), and solid (ice) within the relatively small range of air temperatures and pressures found at Earth's surface. Ancient Greeks, including Homer and Plato, knew that water continually circulates from the ocean to the atmosphere to the land and back again to the ocean. Today's scientists know that Earth's "water cycle" is dominated by exchanges between the ocean and atmosphere. In fact, 86% of global evaporation and 78% of global precipitation occur over the ocean.

Sea Surface Salinity (SSS) is a key tracer for understanding the fresh water cycle in the ocean. This is because whereas some parts of the water cycle increase salinity, other parts decrease it (see diagram, below):

An illustration of Earth's water cycle

Global SSS patterns are governed by geographic differences in the "water budget." Like on continents, some latitudes of the ocean are "rainy" whereas others are arid and "desert-like." In general, latitude zones dominated by precipitation have low SSS and those dominated by high evaporation have high SSS:

Lowest SSS occurs in temperate latitudes (40 - 50 degrees North and South), near coasts and in equatorial regions.
Highest SSS occurs at about 25 - 30 degrees North and South latitude, at ocean centers and in enclosed seas.

Ocean currents can also modify SSS patterns by transporting surface waters - and their SSS "signature" -- across latitude belts. The Gulf Stream for example, transports warm, high salinity water from the tropics to Europe along the Atlantic Ocean's western boundary.

To track changes in SSS patterns over time, scientists monitor the relationship between two primary processes in the oceans: 1) evaporation ("E") which controls the loss of water; and 2) precipitation ("P") which governs the gain of water. After the launch of Aquarius in 2010, scientists will be able to produce accurate maps of global (E - P). Thus, for the first time we will observe how the ocean responds to variability in the water cycle, from season-to-season and year-to-year.

(E - P)	SSS change	Examples of how change in SSS can affect the environment
Positive: Evaporation exceeds precipitation	Increase	Reduction in seawater's ability to absorb carbon dioxide, a greenhouse gas. Scientists have seen evidence of such change off the coast of Hawaii.
Negative: Precipitation exceeds evaporation	Decrease	The "freshening" of seawater decreases its density, reducing its ability to sink. Maintaining density-controlled thermohaline circulation is key to keeping ocean heat transport - and Earth's climate - in balance.

(E - P)

SSS change

Examples of how change in SSS can affect the environment

Positive: Evaporation exceeds precipitation

Increase

Reduction in seawater's ability to absorb carbon dioxide, a greenhouse gas. Scientists have seen evidence of such change off the coast of Hawaii.

Negative: Precipitation exceeds evaporation

Decrease

The "freshening" of seawater decreases its density, reducing its ability to sink. Maintaining density-controlled thermohaline circulation is key to keeping ocean heat transport - and Earth's climate - in balance.