Halocline

In oceanography, a halocline (from Greek hals, halos 'salt' and klinein 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water.^[1] Because salinity (in concert with temperature) affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m³ results in an increase of seawater density of around 0.7 kg/m³.

Description

In the midlatitudes, an excess of evaporation over precipitation leads to surface waters being saltier than deep waters. In such regions, the vertical stratification is due to surface waters being warmer than deep waters and the halocline is destabilizing. Such regions may be prone to salt fingering, a process which results in the preferential mixing of salinity.

In these regions, the halocline is important in allowing for the formation of sea ice, and limiting the escape of carbon dioxide to the atmosphere.

In certain high latitude regions (such as the Arctic Ocean, Bering Sea, and the Southern Ocean) the surface waters are actually colder than the deep waters and the halocline is responsible for maintaining water column stability, isolating the surface waters from the deep waters.^[2]

Haloclines are also found in fjords, and poorly mixed estuaries where fresh water is deposited at the ocean surface.^[3]

A halocline can be easily created and observed in a drinking glass or other clear vessel. If fresh water is slowly poured over a quantity of salt water, using a spoon held horizontally at water-level to prevent mixing, a hazy interface layer, the halocline, will soon be visible due to the varying index of refraction across the boundary.

A halocline is most commonly confused with a thermocline – a thermocline is an area within a body of water that marks a drastic change in temperature. A halocline can coincide with a thermocline and form a pycnocline.^[4]

Haloclines are common in water-filled limestone caves near the ocean. Less dense fresh water from the land forms a layer over salt water from the ocean.^[1] For underwater cave explorers, this can cause the optical illusion of air space in caverns. Passing through the halocline tends to stir up the layers.

Movement of the Halocline

The location (depth of upper limit) of the Halocline is dependent upon ocean circulation, water sources, and atmospheric conditions. Changes in the halocline's position and strength can significantly impact ocean mixing, nutrient transport, and hypoxia levels in bottom waters (Väli et al., 2013). In each ocean, its water sources impact the advection of saline which is responsible for transforming into the halocline layers through means such as ice melting and surface cooling (Rudels et al., 2004). Winds and river runoff can also impose variations on the depth and stability of the halocline (Väli et al., 2013).

Salinity and density

Salinity is directly responsible for density distribution and stratification within the ocean. This stratification is necessary for the formation of sea ice for example in high-latitude oceans the halocline prevents warm saline Atlantic waters from mixing with the cold surface layer allowing for ice growth (Metzner & Salzmann, 2023). The Arctic halocline has a distinct low-salinity layer that limits deep convection and maintains the stability of the water column (Rudels et al., 2004). Conversely, there are places (like the Baltic Sea) where the halocline depth and salinity are affected by river runoff. Here fresh water is introduced into the upper layers and mixed by wind which influences density gradients and deepwater ventilation (Väli et al., 2013).

Global and regional halocline depths

Global and regional differences in salinity, circulation, and temperature result in halocline depth variations of the different oceans. In the Arctic Ocean, the halocline is between 50 and 250 meters deep. This can vary depending on whether the source water comes from the Atlantic or Pacific (Metzner & Salzmann, 2023). On a global scale, haloclines are common in areas with low surface salinity, such as the tropics and subpolar regions (Ueno et al., 2022). In shallow seas (like the Baltic) halocline depths range from 60 to 80 meters, but they exhibit significant variability due to annul changes in wind forcing and river discharge (Väli et al., 2013).

Graph

In the graphical representation, three layers can be discerned:

About 50 m (160 ft) of low salinity water "swimming" on top of the ocean. The temperature is −1.8 °C (28.8 °F), which is very near to the freezing point. This layer blocks heat transfer from the warmer, deeper levels into the sea ice, which has considerable effect on its thickness.
About 150 m (490 ft) of steeply rising salinity and increasing temperature. This is the actual halocline.
The deep layer with nearly constant salinity and slowly decreasing temperature.^[6]

Other types of clines

Thermocline – A cline based on difference in water temperature,
Chemocline – A cline based on difference in water chemistry,
Pycnocline – A cline based on difference in water density.

References

^ ^a ^b White, William B; Culver, David C (2012). Encyclopedia of Caves. Academic Press. p. 157. ISBN 978-0-12-383832-2.
^ Sprintall, J.; Cronin, M.F. (2001). "Upper Ocean Vertical Structure" (PDF). Encyclopedia of Ocean Sciences: 3120–3128. doi:10.1006/rwos.2001.0149.
^ Svensson, Torbjörn (6 February 1981). "Water Exchange and Mixing in Fjords" (PDF). www.chalmers.se. Chalmers University of Technology. p. 159. Retrieved 13 July 2020.
^ Garrison, Tom (2006). Enhanced Essentials of Oceanography. Cengage Learning. p. 115. ISBN 0-495-11372-7.
^ US Department of Commerce, National Oceanic and Atmospheric Administration. "Mapping the Uncharted Diversity of Arctic Marine Microbes: Microbe-metal Interactions in the Central Arctic Ocean: NOAA Office of Ocean Exploration and Research". oceanexplorer.noaa.gov. Retrieved 15 February 2025.
^ ^a ^b "U.S. National Oceanographic Data Center: Global Temperature–Salinity Profile Programme. June 2006. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Oceanographic Data Center, Silver Spring, Maryland, 20910". 25 November 2020.

[caves-1] White, William B; Culver, David C (2012). Encyclopedia of Caves. Academic Press. p. 157. ISBN 978-0-12-383832-2.

[Halocline_water_column_stability-2] Sprintall, J.; Cronin, M.F. (2001). "Upper Ocean Vertical Structure" (PDF). Encyclopedia of Ocean Sciences: 3120–3128. doi:10.1006/rwos.2001.0149.

[chalmers-3] Svensson, Torbjörn (6 February 1981). "Water Exchange and Mixing in Fjords" (PDF). www.chalmers.se. Chalmers University of Technology. p. 159. Retrieved 13 July 2020.

[ocean-4] Garrison, Tom (2006). Enhanced Essentials of Oceanography. Cengage Learning. p. 115. ISBN 0-495-11372-7.

[Artic_Ocean_Layers-5] US Department of Commerce, National Oceanic and Atmospheric Administration. "Mapping the Uncharted Diversity of Arctic Marine Microbes: Microbe-metal Interactions in the Central Arctic Ocean: NOAA Office of Ocean Exploration and Research". oceanexplorer.noaa.gov. Retrieved 15 February 2025.

[gtsppdata-6] "U.S. National Oceanographic Data Center: Global Temperature–Salinity Profile Programme. June 2006. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Oceanographic Data Center, Silver Spring, Maryland, 20910". 25 November 2020.

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