A halocline (or salinity chemocline), from the Greek words ''hals'' (salt) and ''klinein'' (to slope), refers to a layer within a body of water ( water column) where there is a sharp change in

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

(salt concentration) with depth. Haloclines are typically found in oceans or large

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

and it is a type of chemical stratification that is most commonly found in places where freshwater from rivers or melting ice, mixes with salty ocean water.

Description

In the midlatitudes, an excess of

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

over

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

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 The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions. It spans an area of approximately and is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...

Bering Sea The Bering Sea ( , ; rus, Бе́рингово мо́ре, r=Béringovo móre, p=ˈbʲerʲɪnɡəvə ˈmorʲe) is a marginal sea of the Northern Pacific Ocean. It forms, along with the Bering Strait, the divide between the two largest landmasse ...

, and the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

) 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. Haloclines are also found in fjords, and poorly mixed

where fresh water is deposited at the ocean surface. 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. Haloclines are common in water-filled

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

caves near the ocean. Less dense fresh water from the land forms a layer over salt water from the ocean. 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. Arctic sea temperature salinity plot

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. 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. Winds and river runoff can also impose variations on the depth and stability of the halocline.

Salinity and density

Salinity is directly responsible for density distribution and stratification within the ocean (as seen in the pycnocline). 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. The Arctic halocline has a distinct low-salinity layer that limits deep convection and maintains the stability of the water column. 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.

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 deep. This can vary depending on whether the source water comes from the Atlantic or Pacific. On a global scale, haloclines are common in areas with low surface salinity, such as the tropics and subpolar regions. In shallow seas (like the Baltic) halocline depths range from , but they exhibit significant variability due to annul changes in wind forcing and river discharge.

Other types of clines

* Thermocline – A cline based on difference in water

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

, * Chemocline – A cline based on difference in water

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, * Pycnocline – A cline based on difference in water

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

References