The water molecule, H2O, exists 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 some parts of the water cycle
increase salinity, and parts decrease it (see diagram, below).
Global SSS patterns are governed by geographic differences in the "ocean water budget." Like on continents, some latitudes of the ocean are
"rainy" whereas others are arid and "desert-like." In general:
Lowest SSS occurs at latitudes dominated by precipitation: Equatorial regions, 40-50 degrees North and South latitude, and near coasts.
Highest SSS occurs at latitudes dominated by high evaporation: At ocean centers, in enclosed seas, and between 25-30 degrees North and South latitude.
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.