Evaporation is the process where liquid water transforms into water vapor and rises into the atmosphere. Salinity measures all dissolved salts, primarily sodium chloride, in a body of water, typically expressed in parts per thousand (ppt). The average ocean salinity is about 35 ppt. Evaporation removes pure water from the surface, leaving dissolved salts behind, which fundamentally increases the salinity of the remaining water. This relationship drives many global ocean dynamics.
The Physical Process of Salt Concentration
The scientific basis for increased salinity is rooted in the phase transition of water molecules. When surface water absorbs heat energy, primarily from solar radiation, water molecules vibrate faster. Molecules gaining sufficient velocity escape the liquid’s surface tension and enter the air as vapor.
These escaping water molecules are pure, leaving behind all dissolved solids, which are the salts. Evaporation is a natural purification process for the water turning into vapor, but simultaneously concentrates the salt remaining in the liquid.
The greater the rate of evaporation relative to freshwater input, the higher the concentration of dissolved ions. This localized increase in salt content raises the density of the surface water.
Areas of Intense Evaporation
The highest surface salinities are found in regions where evaporation significantly exceeds precipitation. These areas are characterized by high solar radiation and dry, descending air masses, such as the subtropical zones around 25° north and south latitude. Atmospheric conditions in these zones remove great volumes of water vapor from the ocean surface.
Semi-enclosed water bodies demonstrate this effect intensely due to limited exchange with the open ocean. The Red Sea and the Mediterranean Sea both exhibit notably high salinity levels. For instance, the Red Sea approaches 40 ppt because it lies in an arid zone with high temperatures and very little freshwater inflow. The constant removal of pure water through intense evaporation in these basins makes them much saltier than the vast, well-mixed open ocean.
How Salinity Drives Global Ocean Currents
The concentration of salt by evaporation plays a primary role in driving deep-ocean circulation patterns. Increased salinity makes seawater denser, and this density difference, along with temperature, powers the thermohaline circulation. This circulation system is often referred to as the Global Conveyor Belt, which moves water, heat, and nutrients around the planet.
The process begins when warm, salty surface water from the tropics is transported toward the poles, often aided by currents like the Gulf Stream. As this water reaches high latitudes, it cools significantly, further increasing its density. In regions such as the North Atlantic near Greenland, this cold, highly saline water becomes dense enough to sink to the ocean floor.
This sinking action initiates the deep-ocean current, pulling surface water to replace it and driving the global loop. The resulting deep currents flow along the ocean bottom before slowly rising back to the surface elsewhere. This continuous, density-driven movement regulates climate by distributing heat from the equator toward the polar regions.
Processes That Dilute Salinity
While evaporation concentrates salt, several processes dilute ocean salinity, maintaining a dynamic balance in the global water cycle. The most significant diluting factor is precipitation, as rain and snow add freshwater directly to the ocean surface. Regions of heavy rainfall, like tropical convergence zones, often exhibit lower surface salinity.
Another source of dilution is continental runoff, the flow of freshwater from rivers into the sea. Large rivers, such as the Amazon, significantly lower salinity in coastal areas. Furthermore, the melting of sea ice and discharge from glaciers also introduce freshwater, lowering surface salinity in polar regions.
Ocean salinity levels result from the interplay between concentrating forces, such as evaporation, and diluting forces like precipitation and runoff. This balancing act ensures the overall salt content remains relatively stable over long time scales, while regional variations drive complex circulation patterns.