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The Halocline Layer

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In oceanography the term “cline” is used to describe a thin and typically horizontal layer within a fluid with greatly varying properties over a short vertical distance. When this thin layer or cline has a strong vertical salinity gradient within the body of water, then it is known as Halocline. The halocline plays a significant role in the vertical stratification. Along with the temperature, the salinity also contributes to the density of seawater. This can be understood by the example that if there is 1 kg/m3 increase in salinity, then the sea-water density increases by around 0.7 kg/m3. This affects the seawater environment and has a profound effect on the life of organisms. 


Effects of the Halocline Layer

From the halocline definition it is clear that the layer is high in salt content. The effects of such a layer because of the conditions as described in the halocline definition are more commonly found in places with high temperatures such as mid-latitudes. Because of the excess of evaporation and over precipitation in the middle-latitudes the surface waters have more salt contents as compared to the deep waters. In such regions, the surface waters are relatively warmer than the deep waters and because of that there is a certain destabilizing effect in the vertical stratification of the halocline layer. This destabilization leads to a process of mixing known as salt fingering, that results in the mixing of the salinity across the vertical stratification. 

Another significant role played by the halocline layer is in the high latitude region. In such regions for example, in regions of the Arctic Ocean, Bering Sea, and the Southern Ocean, the water present on the surface is actually colder than the water present in the depth. Here, the condition of increased density in the specific layer of salinity as identified by the halocline definition helps in keeping these waters separate and isolate the deep water from the surface water. Because of this separation, ice is formed on the surface of the sea level as it is cold and does not melt away because of heat transition from the warmer waters present in the depth. The halocline layer also limits the escape of the carbon dioxide gas into the atmosphere in the high latitude regions. 

The halcolines are also found in fjords and poorly mixed estuaries, the place where the freshwater from rivers and streams gets mixed with ocean surface. 

As a small experiment, it is possible to create and see a halocline in a drinking glass or any transparent vessel. For this experiment, the fresh water is to be slowly poured over a significant amount of salt water, with the help of a spoon that is held horizontally at the water-level for preventing the mixing of the two different types of waters. As the water is being poured or after it has been poured, for some amount of time, a hazy interface of salt layer can be easily seen. This hazy salty layer is the halocline. It becomes visible because of the varying refractive index across the boundary. 

Other common occurrences of haloclines are the water-filled limestone caves that are near the ocean. The less denser fresh water from the land forms a layer over the salt water from the ocean. For any of the underwater cave explorers, such as halocline divers that cause an optical illusion of air space present in the caverns. When the halocline divers pass through the halocline layer, the layers get perturbed. 


Different Types of Clines

There are different types of clines. Some of them along with their descriptions are listed below:

  • Chemocline: This is the layer under which different types of clines are classified. It is a cline based upon the chemistry of the particular layer. 

  • Thermocline: This layer is the cline which is identified on the basis of the water temperature. 

  • Pycnocline: It is the layer which is identified on the basis of the water density. 

The halocline is a subtype of chemocline. There are possible combinations of the clines as well. For example, sometimes thermocline and halocline exist together as a vertical gradient. And as mentioned above thermocline and halocline i.e. effect of increased temperature and increased salinity leads to an increase in the density of that layer which is the pycnocline layer. Thus, thermocline haloclines together form a pycnocline layer and have the corresponding effect on the surrounding environment. 

FAQ (Frequently Asked Questions)

1. What is the Halocline Layer?

Ans: Halocline is a vertically stratified zone of oceanic water column of salinity. The salinity increases as it goes down rapidly with the depth and is located below the well-mixed and uniformly saline surface of the water layer. Along with the increase of salinity when there is an increase in the temperature it leads to an increase in the density of that particular layer in water. This affects the surrounding for example in high altitude regions, the surface water is very cold as compared to the water present at the depth. The transfer of heat from the lower water level to the surface level is decreased significantly by the halocline layer which allows the formation of ice on the surface of the seas and oceans. 

2. What is the Difference Between Halocline and Thermocline?

Ans: Cline is the term in oceanography used to describe a thin and horizontal layer within the fluid with significantly varying properties (vertically) over a short distance. There are different types of clines such as chemocline, thermocline and pycnocline. A chemocline is a layer that is the layer of water defined on the basis of the chemistry, the thermocline is the layer of water defined on the basis of the temperature and the pycnocline is the layer of water identified based on the density of the water in the layer. In a subtype of chemocline is the halocline which is a layer that is identified on the base of increasing salinity. Thus, halocline and thermocline are different on the basis of their properties, one being increase in salinity and another one being temperature.