Editing Physical oceanography (section)

==Temperature, salinity and density==
<!-- These two images cause clutter at present; section expansion may improve their relevance [[File:WOA05 sea-surf TMP AYool.png|thumb|200px|[[World Ocean Atlas|WOA]] surface temperature.]]
[[File:WOA09 sea-surf SAL AYool.png|thumb|200px|[[World Ocean Atlas|WOA]] surface salinity.]] -->
[[File:Annual mean sea surface sea water density (World Ocean Atlas 2009).png|thumb|200px|[[World Ocean Atlas|WOA]] surface density.]]

Because the vast majority of the world ocean's volume is deep water, the mean temperature of seawater is low; roughly 75% of the ocean's volume has a temperature from 0° – 5&nbsp;°C (Pinet 1996). The same percentage falls in a salinity range between 34 and 35 ppt (3.4–3.5%) (Pinet 1996). There is still quite a bit of variation, however. Surface temperatures can range from below freezing near the poles to 35&nbsp;°C in restricted tropical seas, while salinity can vary from 10 to 41 ppt (1.0–4.1%).<ref name="Marshak2001">{{cite book |title=Earth: Portrait of a Planet |last=Marshak |first=Stephen |year=2001 |publisher=W.W. Norton & Company |location=New York |isbn=0-393-97423-5 |url-access=registration |url=https://archive.org/details/earthportraitofp00mars }}</ref>

The vertical structure of the temperature can be divided into three basic layers, a surface [[mixed layer]], where gradients are low, a [[thermocline]] where gradients are high, and a poorly stratified abyss.

In terms of temperature, the ocean's layers are highly [[latitude]]-dependent; the [[thermocline]] is pronounced in the tropics, but nonexistent in polar waters (Marshak 2001). The [[halocline]] usually lies near the surface, where evaporation raises salinity in the tropics, or meltwater dilutes it in polar regions.<ref name="Marshak2001" /> These variations of salinity and temperature with depth change the density of the seawater, creating the [[pycnocline]].<ref name="Pinet1996" />

=== Temperature ===
The temperature of ocean water varies significantly across different regions and depths. As mentioned, the vast majority of ocean water (around 75%) lies between 0° and 5°C, mostly in the deep ocean, where sunlight does not penetrate. The surface layers, however, experience far greater variability. In polar regions, surface temperatures can drop below freezing, while in tropical and subtropical regions, they may reach up to 35°C. This thermal stratification results in a vertical temperature gradient that divides the ocean into distinct layers.

# '''Surface Mixed Layer''': This uppermost layer is well-mixed due to wind and wave action, resulting in minimal temperature variation with depth. The thickness of this layer varies depending on location and season but can extend from 50 to 200 meters.
# '''Thermocline''': Below the mixed layer lies the thermocline, a zone where temperature decreases rapidly with increasing depth. The thermocline is especially pronounced in tropical and temperate regions but is absent in polar waters where surface temperatures are already near freezing. The depth and sharpness of the thermocline can shift with seasonal changes and ocean currents, playing a critical role in regulating heat exchange between the ocean and the atmosphere.
# '''Abyssal Zone''': Beneath the thermocline is the deep ocean or abyssal zone, where temperatures remain relatively uniform, hovering just above freezing (0°-3°C). This cold, dense water originates from polar regions, where surface water cools, sinks, and spreads towards the equator along the ocean floor, forming the deep ocean circulation system.

=== 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>

=== Density and the Pycnocline ===
The combination of temperature and salinity variations leads to changes in seawater density. Seawater density is primarily influenced by both these factors—colder, saltier water is denser than warmer, fresher water. This variation in density creates stratification in the ocean and is key to understanding ocean circulation patterns.

# '''Pycnocline''': The pycnocline is a layer within the ocean where the density increases rapidly with depth. It typically coincides with the thermocline and halocline in tropical and subtropical waters, forming a sharp boundary between the less dense surface waters and the denser deep ocean waters. This density stratification acts as a barrier to vertical mixing, limiting the exchange of heat, gases, and nutrients between the surface and deep ocean layers.
# '''Thermohaline Circulation''': Density differences drive the thermohaline circulation, also known as the global "conveyor belt," which plays a crucial role in regulating Earth's climate. Cold, dense water formed in the polar regions sinks and moves along the ocean floor toward the equator, while warmer surface waters flow poleward to replace it. This global circulation helps redistribute heat and maintains the ocean's dynamic equilibrium.
# '''Regional Variations''': In areas of upwelling or downwelling, the density structure of the water column can be disrupted. Upwelling brings cooler, nutrient-rich water to the surface, while downwelling pushes surface waters to greater depths, impacting local ecosystems and global climate.

Understanding the complex interactions between temperature, salinity, and density is essential for predicting ocean circulation patterns, climate change effects, and the health of marine ecosystems. These factors also influence marine life, as many species are sensitive to the specific temperature and salinity ranges of their habitats.<!-- This looks like a refugee from an earlier version; commenting out for now; these topics should be properly worked into the article instead; in the meantime, I've copied them to the See also section
===Density===
* [[Thermohaline circulation]] (Density-driven)
'''See also:'''
* [[Downwelling]]
* [[Hydrothermal circulation]]
* [[Ocean current]]
* [[Upwelling]]
{{clear}} 
-->