Salinity in the ocean is constantly changing due to a dynamic interplay of processes that add or remove freshwater, primarily driven by the global water cycle.
Understanding Ocean Salinity
Ocean salinity refers to the total amount of dissolved salts in seawater. While the primary component is sodium chloride (table salt), seawater also contains other dissolved ions like magnesium, sulfate, calcium, and potassium. Salinity is a crucial property of seawater, influencing its density, temperature, and the circulation patterns of ocean currents, which in turn affect global climate. The average ocean salinity is about 35 parts per thousand (ppt), or 3.5%.
Key Processes Driving Salinity Changes
The balance between freshwater input and removal dictates how salinity changes in different parts of the ocean. These processes are largely controlled by atmospheric conditions and geographical features.
Factors That Increase Ocean Salinity
Certain natural processes remove freshwater from the ocean, leaving the dissolved salts behind and consequently increasing the salinity of the remaining water.
Evaporation
When solar energy heats the ocean's surface, water molecules transform into vapor and ascend into the atmosphere, a process known as evaporation. Since salt does not evaporate with the water, the concentration of salt in the remaining ocean water increases. This effect is most pronounced in warm, arid regions with high temperatures and low humidity, such as the subtropical gyres and enclosed seas like the Red Sea.
Formation of Sea Ice
As seawater freezes to form sea ice, salt does not typically incorporate into the ice crystals. Instead, the salt is "brine rejected," meaning it is pushed out into the surrounding unfrozen seawater. This expulsion of salt into the residual water increases its salinity and density. This process is significant in polar regions where extensive sea ice forms annually.
Factors That Decrease Ocean Salinity
Conversely, several processes introduce freshwater into the ocean, diluting the existing saltwater and lowering its salinity.
Precipitation
The input of freshwater from precipitation, including rain and snow, directly reduces the salinity of the surface ocean waters. Areas with high rates of rainfall, such as the equatorial regions and some high-latitude zones, typically exhibit lower surface salinities.
Freshwater Input from Rivers
Rivers and streams continually discharge vast amounts of freshwater from land into the ocean. This inflow is particularly effective at reducing salinity in coastal areas, estuaries, and near the mouths of major rivers. For instance, the Baltic Sea, which receives significant freshwater runoff from surrounding landmasses, has a much lower average salinity than the open ocean.
Melting of Ice
The melting of glaciers, ice sheets, and icebergs releases freshwater directly into the ocean, thereby decreasing salinity. This process is particularly significant in polar and subpolar regions, where large quantities of ice melt seasonally or due to climatic warming trends.
Regional Variations in Ocean Salinity
The combined effect of these factors creates distinct regional patterns of ocean salinity:
- Equatorial Regions: High rainfall (low salinity) often balances high evaporation (high salinity), leading to moderate salinities.
- Subtropical Gyres (around 20-30° N/S): High evaporation and low precipitation lead to the highest ocean salinities globally.
- High Latitudes (near Poles): While sea ice formation increases salinity, significant precipitation and ice melt (from glaciers/icebergs) often lead to lower surface salinities overall.
- Coastal Areas/Estuaries: Freshwater input from rivers dramatically lowers salinity, creating a gradient from fresh to saline water.
Summary of Salinity Modifiers
| Process | Effect on Salinity | Mechanism |
|---|---|---|
| Evaporation | Increases | Removes freshwater, leaving dissolved salts behind. |
| Sea Ice Formation | Increases | Brine rejection expels salt into surrounding water. |
| Precipitation | Decreases | Adds freshwater, diluting existing saltwater. |
| River Runoff | Decreases | Discharges freshwater from land into the ocean. |
| Melting of Ice (Glaciers, Icebergs) | Decreases | Releases freshwater into the ocean. |
Why Ocean Salinity Matters
Understanding how salinity changes is critical for oceanographers and climate scientists because salinity directly influences ocean density. Denser water sinks, driving global thermohaline circulation (the "ocean conveyor belt"), which distributes heat and nutrients around the planet. Variations in salinity can alter these currents, affecting marine ecosystems, weather patterns, and global climate.
