Halocline
Encyclopedia
In oceanography
, a halocline is a subtype of chemocline
caused by a strong, vertical salinity
gradient
within a body of water
. 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.
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 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. 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. Haloclines are also found in fjords, and poorly mixed estuaries 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.
Haloclines are common in water-filled caves near the ocean. Less dense fresh water from the land forms a layer over salt water from the ocean. Underwater cave explorers passing through the halocline stir up the layers.
Oceanography
Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...
, a halocline is a subtype of chemocline
Chemocline
A chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and cooler waters meet in an ocean, sea, lake, or other body of water...
caused by a strong, vertical salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
within a body of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. Because salinity (in concert with temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
) affects the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
, it can play a role in its vertical stratification
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.
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
Salt fingering
Salt fingering is a mixing process that occurs when relatively warm, salty water overlies relatively colder, fresher water. It is driven by the fact that heated water diffuses more readily than salty water...
, a process which results in the preferential mixing of salinity.
In certain high latitude regions (such as the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...
, Bering Sea
Bering Sea
The Bering Sea is a marginal sea of the Pacific Ocean. It comprises a deep water basin, which then rises through a narrow slope into the shallower water above the continental shelves....
, and the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
) 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. 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. Haloclines are also found in fjords, and poorly mixed estuaries where fresh water is deposited at the ocean surface.
Description
In the plot, one can discern three layers:- About 50 m of low salinity water "swimming" on top of the ocean. The temperature is -1.8 °C, which is very near to the freezing point. This layer blocks heat transfer from the warmer, deeper levels into the ice sheet, which has considerable effect on its thickness.
- About 150 m of steeply rising salinity and increasing temperature. This is the actual halocline.
- The deep layer with nearly constant salinity and slowly decreasing temperature.
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
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
- a thermocline is an area within a body of water that marks a drastic change in temperature.
Haloclines are common in water-filled caves near the ocean. Less dense fresh water from the land forms a layer over salt water from the ocean. Underwater cave explorers passing through the halocline stir up the layers.
Types of clines
- ThermoclineThermoclineA thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
- A cline based on difference in water temperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, - ChemoclineChemoclineA chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and cooler waters meet in an ocean, sea, lake, or other body of water...
- A cline based on difference in water chemistryChemistryChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, - PycnoclinePycnoclineA pycnocline is the cline or layer where the density gradient is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, terms of temperature and salinity differences, wind, Coriolis effect, and tides caused by the gravitational pull of the Moon and the...
- A cline based on difference in water densityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
.
See also
- Hypersaline lakeHypersaline lakeA hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride or other mineral salts, with saline levels surpassing that of ocean water . Specific microbial and crustacean species thrive in these high salinity environments that are otherwise...
- IsopycnalIsopycnalAn isopycnal is a surface of constant potential density of water. In the ocean, as the depth increases, so too does the density. Varying degrees of salinity and temperature act to modify the density of water, and the denser water always lies below the less dense water. Because of the action of...
- Thermohaline circulationThermohaline circulationThe term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
- Osmotic power