Editing Physical oceanography (section)

=== Salinity ===
Salinity, a measure of the concentration of dissolved salts in seawater, typically ranges between 34 and 35 parts per thousand (ppt) in most of the world’s oceans. However, localized factors such as evaporation, precipitation, river runoff, and ice formation or melting cause significant variations in salinity. These variations are often most evident in coastal areas and marginal seas.

# '''Surface Salinity''': In the open ocean, salinity is generally highest in subtropical regions where high evaporation rates dominate, and lowest in regions of high precipitation or freshwater influx from rivers, such as the mouths of the Amazon and Congo Rivers. Tropical and subtropical seas, such as the Red Sea and the Mediterranean, can experience salinities as high as 40-41 ppt due to intense evaporation and restricted water exchange with the open ocean.
# '''Halocline''': The halocline is a layer within the ocean where salinity changes rapidly with depth. This stratification can be influenced by surface processes like evaporation (which increases salinity) and freshwater input (which decreases it). The halocline often coincides with the thermocline, particularly in tropical and subtropical regions, contributing to overall water column stability.
# '''Polar Regions''': In polar areas, surface salinity is generally lower due to freshwater input from melting ice. During sea ice formation, however, brine rejection increases the salinity of surrounding waters, contributing to the sinking of dense water masses and the formation of deep ocean currents that drive global circulation patterns.
The salt in the oceans originates from runoff from terrestrial sources as well as hydrothermal vents.<ref>https://oceanservice.noaa.gov/facts/whysalty.html</ref> It has been estimated that the salinity of oceans was greater in the distant past than it is today.<ref>https://news.yale.edu/2021/12/13/spice-world-earths-early-oceans-may-have-been-heavy-salt</ref>