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A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g.
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
, as in an ocean or lake; or air, e.g. an
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
) in which temperature changes more drastically with depth than it does in the layers above or below. In the
ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
, the thermocline divides the upper mixed layer from the calm deep water below. Depending largely on
season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and pol ...
,
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
, and
turbulent In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
mixing by
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
, thermoclines may be a semi-permanent feature of the
body of water A body of water or waterbody (often spelled water body) is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as p ...
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
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...
s and
currents Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
.


Oceans

Most of the heat energy of the sunlight that strikes the Earth 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 Waves most often refers to: *Waves, oscillations accompanied by a transfer of energy that travel through space or mass. *Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (band) ...
mix the water near the surface layer and distribute heat to deeper water such that the temperature may be relatively uniform in the upper , depending on wave strength and the existence of surface turbulence caused by currents. Below this
mixed layer The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...
, 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 (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 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. The first accurate global measurements were made during the oceanographic expedition of HMS Challenger. In the open ocean, the thermocline is characterized by a negative
sound speed gradient In acoustics, the sound speed gradient is the rate of change of the speed of sound with distance, for example with depth in the ocean, or height in the Earth's atmosphere. A sound speed gradient leads to refraction of sound wavefronts in the direct ...
, making the thermocline important in submarine warfare because it can reflect active
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
and other acoustic signals. This stems from a discontinuity in the
acoustic impedance Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal-second per cub ...
of water created by the sudden change in density. In
scuba diving Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for "Self-Contained Underwater Breathing Apparatus", was coined by Chris ...
, 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, the kind often used in bathroom windows to obscure the view, and is caused by the altered
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
of the cold or warm water column. These same
schlieren Schlieren ( ; , ) are optical inhomogeneities in transparent media that are not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of such inhomogeneities. These inhomogeneiti ...
can be observed when hot air rises off the tarmac at airports or desert roads and is the cause of
mirage A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the Latin ''mirari'', meanin ...
s.


Thermocline seasonality

The thermocline in the ocean can vary in depth and strength seasonally. This is particularly noticeable in mid-latitudes with a thicker mixed layer in the winter and thinner mixed layer in summer. The cooler winter temperatures cause the thermocline to drop to further depths and warmed summer temperatures bring the thermocline back to the upper layer. In areas around the tropics and subtropics, the thermocline may become even thinner in the summer than in other locations. At higher latitudes, around the poles, there is more of a seasonal thermocline than a permanent one with warmer surface waters. This is where there is a dichothermal layer instead. In the Northern hemisphere, the maximum temperatures at the surface occur through August and September and minimum temperatures occur through February and March with total heat content being lowest in March. This is when the seasonal thermocline starts to build back up after being broken down through the colder months. A permanent thermocline is one that is not affected by season and lies below the yearly mixed layer maximum depth.


Other water bodies

Thermoclines can also be observed in lakes. In colder climates, this leads to a phenomenon called
stratification Stratification may refer to: Mathematics * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Data stratification in statistics Earth sciences * Stable and unstable stratification * Stratification, or st ...
. During the summer, warm water, which is less dense, will sit on top of colder, denser, deeper water with a thermocline separating them. The warm layer is called the
epilimnion The epilimnion or surface layer is the top-most layer in a thermally stratified lake. It sits above the deeper metalimnion and hypolimnion. It is typically warmer and has a higher pH and higher dissolved oxygen concentration than the hypolimnion ...
and the cold layer is called the
hypolimnion The hypolimnion or under lake is the dense, bottom layer of water in a thermally- stratified lake. The word hypolimnion is derived from the Greek "limnos" meaning "lake". It is the layer that lies below the thermocline. Typically the hypolimni ...
. Because the warm water is exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs, particularly in calm weather. One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline as the water below the thermocline never circulates to the surface and organisms in the water deplete the available oxygen. As winter approaches, the temperature of the surface water will drop as nighttime cooling dominates heat transfer. A point is reached where the density of the cooling surface water becomes greater than the density of the deep water and overturning begins as the dense surface water moves down under the influence of gravity. This process is aided by wind or any other process (currents for example) that agitates the water. This effect also occurs in Arctic and Antarctic waters, bringing water to the surface which, although low in oxygen, is higher in nutrients than the original surface water. This enriching of surface nutrients may produce blooms of
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Ph ...
, making these areas productive. As the temperature continues to drop, the water on the surface may get cold enough to freeze and the lake/ocean begins to ice over. A new thermocline develops where the densest water () sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C. During this transition, a
thermal bar A thermal bar is a hydrodynamic feature that forms around the edges of holomictic lakes during the seasonal transition to stratified conditions, due to the shorter amount of time required for shallow areas of the lake to stratify. Description Du ...
may develop. Waves can occur on the thermocline, causing the depth of the thermocline as measured at a single location to oscillate (usually as a form of
seiche A seiche ( ) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors, caves and seas. The key requirement for formation of ...
). Alternately, the waves may be induced by flow over a raised bottom, producing a thermocline wave which does not change with time, but varies in depth as one moves into or against the flow.


Atmosphere

The lower atmosphere also typically contains a boundary between two distinct regions (the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
and
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
), but that boundary (the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...
) displays quite different behavior. However, atmospheric thermoclines, or ''inversions'', can occur, e.g. as nighttime cooling of the Earth's surface produces cold, dense, often calm air adjacent to the ground. The coldest air is adjacent to the ground, with the air temperature increasing with height. At the top of this nighttime boundary layer (which may be only a hundred meters thick), the normal adiabatic temperature profile of the troposphere (i.e. temperature decreasing with altitude) is again observed. The thermocline or inversion layer occurs where the temperature profile changes from positive to negative with increasing height. The stability of the night-time inversion is usually destroyed soon after sunrise as the sun's energy warms the ground, which in turn warms the air in the inversion layer. The warmer, less dense air then rises, thereby destroying the stability that characterizes the nightly inversion. This phenomenon was first applied to the field of
noise pollution Noise pollution, also known as environmental noise or sound pollution, is the propagation of noise with ranging impacts on the activity of human or animal life, most of them are harmful to a degree. The source of outdoor noise worldwide is main ...
study in the 1960s, contributing to the design of urban highways and
noise barrier A noise barrier (also called a soundwall, noise wall, sound berm, sound barrier, or acoustical barrier) is an exterior structure designed to protect inhabitants of sensitive land use areas from noise pollution. Noise barriers are the most effecti ...
s.


See also

* * * * * * * * *
Thin layers (oceanography) Thin layers are concentrated aggregations of phytoplankton and zooplankton in coastal and offshore waters that are vertically compressed to thicknesses ranging from several centimeters up to a few meters and are horizontally extensive, sometim ...


References


External links


"Temperature of Ocean Water", University Corporation for Atmospheric Research, University of Michigan.
{{authority control Anti-submarine warfare Oceanography Physical oceanography Aquatic ecology Fisheries science Limnology