Thermocline
| Aquatic layers | |
| Pelagic | |
| Photic | |
| Epipelagic | |
| Aphotic | |
| Mesopelagic | |
| Bathypelagic | |
| Abyssopelagic | |
| Hadopelagic | |
| Demersal | |
| Benthic | |
| Stratification | |
| Pycnocline | |
| Isopycnal | |
| Chemocline | |
| Halocline | |
| Thermocline | |
| Thermohaline | |
| Marine habitats | |
| Lake stratification | |
| Aquatic ecosystems | |
| Wild fisheries | |
A thermocline (sometimes metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below. Depending largely on season, latitude and turbulent mixing by wind, thermoclines may be a semi-permanent feature of the body of water in which they occur or they may form temporarily in response to phenomena such as the radiative heating/cooling of surface water during the day/night. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude and local environmental conditions, such as tides and currents.
Most of the heat energy of sunlight is absorbed in the first few centimeters at the ocean's surface, which heats during the day and cools at night as heat energy is lost to space by radiation. Waves mix the water near the surface layer and distribute heat to deeper water such that the temperature may be relatively uniform in the upper 100 m (300 ft), depending on wave strength and the existence of surface turbulence caused by currents. Below this mixed layer, the temperature remains relatively stable over day/night cycles. The temperature of the deep ocean drops gradually with depth. As saline water does not freeze until it reaches −2.3 °C (colder as depth and pressure increase) the temperature well below the surface is usually not far from zero degrees.
The thermocline varies in depth. It is semi-permanent in the tropics, variable in temperate regions (often deepest during the summer) and shallow to nonexistent in the polar regions, where the water column is cold from the surface to the bottom. A layer of sea ice will act as an insulation blanket.
In the open ocean, the thermocline is characterized by a negative sound speed gradient, making the thermocline important in submarine warfare because it can reflect active sonar and other acoustic signals. Technically, this effect stems from a discontinuity in the acoustic impedance of water created by the sudden change in density.
When scuba diving, a thermocline where water drops in temperature by a few degrees Celsius quite suddenly can sometimes be observed between two bodies of water, for example where colder upwelling water runs into a surface layer of warmer water. It gives the water an appearance of wrinkled glass that is often used to obscure bathroom windows and is caused by the altered refractive index of the cold or warm water column. These same schlieren can be observed when hot air rises off the tarmac at airports or desert roads and is the cause of mirages.
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Wikipedia