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Encyclopedia > Thermohaline circulation
A simplified summary of the path of the Thermohaline Circulation. Darker arrows represent deep-water currents, while lighter arrows represent surface currents
A simplified summary of the path of the Thermohaline Circulation. Darker arrows represent deep-water currents, while lighter arrows represent surface currents

The thermohaline circulation (THC) is the global density-driven circulation of the oceans. Derivation is from thermo- for heat and -haline for salt, which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) head polewards from the equatorial Atlantic Ocean, cooling all the while and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1600 years) upwell in the North Pacific (Primeau, 2005). Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's ocean a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the climate of the Earth. Image File history File links Please see the file description page for further information. ... Image File history File links Please see the file description page for further information. ... In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is... Thermo may refer to: Thermodynamics, the physics of energy, heat, work, and entropy, which is also discussed in the article Heat The Thermo Electron Corporation A prefix referring to heat or temperature, , thermochemistry, thermophile, thermonuclear, thermoelectricity, thermionic. ... Annual mean sea surface salinity for the World Ocean. ... This article describes water from a scientific and technical perspective. ... For the album by Ocean Colour Scene, see North Atlantic Drift (album) The Gulf Stream is orange and yellow in this representation of water temperatures of the Atlantic. ... North Atlantic Deep Water The North Atlantic Deep Water is a water mass, buit in the Atlantic Ocean. ... This article is about Earth as a planet. ...


The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyer, the global conveyor belt, or, most commonly, the meridional overturning circulation (often abbreviated as MOC).

Contents

Overview

The movement of surface currents pushed by the wind is intuitive: we have all seen wind ripples on the surface of a pond. Thus the deep ocean — devoid of wind — was assumed to be perfectly static by early oceanographers. However, modern instrumentation shows that current velocities in deep water masses can be significant (although much less than surface speeds).


In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature (the more saline the denser, and the colder the denser). There is often confusion over the components of the circulation that are wind and density driven[1]. Note that ocean currents due to tides are also significant in many places; most prominent in relatively shallow coastal areas, tidal currents can also be significant in the deep ocean. This article is about tides in the ocean. ...


The density of ocean water is not globally homogeneous, but varies significantly and discretely. Sharply defined boundaries exist between water masses which form at the surface, and subsequently maintain their own identity within the ocean. They position themselves one above or below each other according to their density, which depends on both temperature and salinity. An oceanographic water mass is an identifiable body of water which has physical properties distinct from surrounding water. ... In physics, density is mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg V is... This article includes a list of works cited or a list of external links, but its sources remain unclear because it lacks in-text citations. ... Annual mean sea surface salinity for the World Ocean. ...


Warm seawater expands and is thus less dense than cooler seawater. Saltier water is more dense than fresher water because the dissolved salts fill interstices between water molecules, resulting in more mass per unit volume. Lighter water masses float over denser ones (just as a piece of wood or ice will float on water, see buoyancy). This is known as "stable stratification". When dense water masses are first formed, they are not stably stratified. In order to take up their most stable positions, water masses of different densities must flow, providing a driving force for deep currents. An oceanographic water mass is an identifiable body of water which has physical properties distinct from surrounding water. ... In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i. ...


The thermohaline circulation is mainly triggered by the formation of deep water masses in the North Atlantic and the Southern Ocean and Haline forcing caused by differences in temperature and salinity of the water. For other uses, see Atlantic (disambiguation) The Atlantic Ocean is Earths second-largest ocean, covering approximately one-fifth of its surface. ...


Formation of deep water masses

The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean. In these polar regions, seawater at the surface of the ocean is intensively cooled by the wind. Wind moving over the water also produces a great deal of evaporation, leading to a decrease in temperature, called evaporative cooling. Evaporation removes only molecules of pure water, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass. For other uses, see Atlantic (disambiguation) The Atlantic Ocean is Earths second-largest ocean, covering approximately one-fifth of its surface. ... Evaporative cooling is a system in which latent heat of evaporation is used to carry heat away from an object to cool it. ...


In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Great Britain. It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait. The Norwegian Sea (Norwegian: Norskehavet) is part of the North Atlantic Ocean northwest of Norway, located between the North Sea (i. ... North Atlantic Deep Water The North Atlantic Deep Water is a water mass, buit in the Atlantic Ocean. ... In geology, a sill is a tabular, often horizontal mass of igneous rock that has been intruded laterally between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. ... Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor. ... Satellite photo of the Bering Strait Photo across the Bering Strait Nautical chart of the Bering Strait The Bering Strait (Russian: ) is a sea strait between Cape Dezhnev, Russia, the easternmost point (169°43 W) of the Asian continent and Cape Prince of Wales, Alaska, the westernmost point (168°05...


The formation of sea ice also contributes to an increase in seawater salinity; saltier brine is left behind as the sea ice forms around it (pure water preferentially being frozen). Increasing salinity depresses the freezing temperature of seawater, so cold liquid brine is formed in inclusions within a honeycomb of ice. The brine progressively melts the ice just beneath it, eventually dripping out of the ice matrix and sinking. This process is known as brine exclusion. By contrast in the Weddell Sea off the coast of Antarctica near the edge of the ice pack, the effect of wind cooling is intensified by brine exclusion. An icebreaker navigates through young (1 year old) sea ice Sea ice is formed from ocean water that freezes. ... The Weddell Sea is part of the Southern Ocean. ...


The resulting Antarctic Bottom Water (AABW) sinks and flows north into the Atlantic Basin, but is so dense it actually underflows the NADW. Again, flow into the Pacific is blocked, this time by the Drake Passage between the Antarctic Peninsula and the southernmost tip of South America. Categories: Possible copyright violations ... Drake Passage between South America and Antarctica. ... Antarctic Peninsula map Booth Island and Mount Scott flank the narrow Lemaire Channel on the west side of the Antarctic Peninsula. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ...


The dense water masses formed by these processes flow downhill at the bottom of the ocean, like a stream within the surrounding less dense fluid, and fill up the basins of the polar seas. Just as river valleys direct streams and rivers on the continents, the bottom topography steers the deep and bottom water masses.


Note that, unlike fresh water, saline water does not have a density maximum at 4 °C but gets denser as it cools all the way to its freezing point of approximately -1.8°C.


Movement of thermohaline circulation

Formation and movement of the deep water masses at North Atlantic Ocean, creates sinking water masses that fills the basin and flows very slowly into the deep abyssal plains of the Atlantic. This high latitude cooling and the low latitude heating drives the movement of the deep water a polar southward flow. The deep water flows through Arctic Ocean Basin around South Africa where it is split into two routes: one into the Indian Ocean and one past Australia into the Pacific. Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor. ...


At the Indian Ocean, some of the cold and salty water from Atlantic -- drawn by the flow of warmer and fresher upper ocean water from the tropical Pacific -- causes a vertical exchange of dense, sinking water with lighter water below. It is known as overturning. In the Pacific Ocean, the rest of the cold and salty water from the Atlantic undergoes Haline forcing and slowly becomes warmer and fresher.


The out-flowing undersea of cold and salty water makes the sea level of the Atlantic slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher than the Pacific. This generates a large but slow flow of warmer and fresher upper ocean water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago to replace the cold and salty Antarctic Bottom Water. This is also known as Haline forcing (net high latitude freshwater gain and low latitude evaporation). This warmer, fresher water from the Pacific flows up through the South Atlantic to Greenland, where it cooled off and undergoes evaporative cooling and sinks to the ocean floor, providing a continuous thermohaline circulation[2]. World map depicting Malay Archipelago The Malay Archipelago is a vast archipelago located between mainland Southeastern Asia (Indochina) and Australia. ... Categories: Possible copyright violations ... For other uses, see Atlantic (disambiguation) The Atlantic Ocean is Earths second-largest ocean, covering approximately one-fifth of its surface. ... Evaporative cooling is a system in which latent heat of evaporation is used to carry heat away from an object to cool it. ...


Hence, a recent and popular name for the thermohaline circulation, emphasizing the vertical nature and pole-to-pole character of this kind of ocean circulation, is the meridional overturning circulation.


The deep water masses that participate in the MOC have chemical, temperature and isotopic ratio signatures and can be traced, their flow rate calculated, and their age determined.>


Gulf Stream

Main article: Gulf Stream

It is also known that North Atlantic Current, warm ocean current that continues the Gulf Stream northeast, is largely driven by the global Thermohaline Circulation to further east and north from the North American coast, across the Atlantic and into the Arctic Ocean. For the album by Ocean Colour Scene, see North Atlantic Drift (album) The Gulf Stream is orange and yellow in this representation of water temperatures of the Atlantic. ... The North Atlantic Current (North Atlantic Drift and the North Atlantic Sea Movement) is a powerful warm ocean current that continues the Gulf Stream northeast. ...


Upwelling

Main article: Upwelling

All these dense water masses sinking into the ocean basins displace the water above them, so that elsewhere water must be rising in order to maintain a balance. However, because this thermohaline upwelling is so widespread and diffuse, its speeds are very slow even compared to the movement of the bottom water masses. It is therefore fiendishly difficult to measure where upwelling occurs using current speeds, given all the other wind-driven processes going on in the surface ocean. Deep waters do however have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth- and a number of authors have tried to use these tracers to infer where the upwelling occurs. Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-deplete surface water. ...


Wallace Broecker, using box models, has asserted that the bulk of deep upwelling occurs in the North Pacific, using as evidence the high values of silicon found in these waters. However, other investigators have not found such clear evidence. Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean, associated with the strong winds in the open latitudes between South America and Antarctica. While this picture is consistent with the global observational synthesis of William Schmitz at Woods Hole and with low observed values of diffusion, not all observational syntheses agree. Recent papers by Lynne Talley at the Scripps Institution of Oceanography and Bernadette Sloyan and Steven Rintoul in Australia suggest that a significant amount of dense deep water must be transformed to light water somewhere north of the Southern Ocean. Wallace S. Broecker is the Newberry Professor of Geochemistry at Columbia University. ... Scripps Institution of Oceanography pier Scripps Institution of Oceanography (sometimes referred to as SIO, Scripps Oceanography or just Scripps) in La Jolla, California, is one of the oldest and largest centers for ocean and earth science research, graduate training, and public service in the world. ...


Effects on global climate

The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions. Changes in the thermohaline circulation are thought to have significant impacts on the earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters are exposed to the surface, it may also play an important role in determining the concentration of carbon dioxide in the atmosphere. While it is often stated that the thermohaline circulation is the primary reason that Western Europe is so temperate, it has been suggested that this is largely incorrect, and that Europe is warm mostly because it lies downwind of an ocean basin[3]. However, the underlying assumptions of this particular analysis are not generally supported[4], and much research supports the role of the THC in transporting heat to Europe[5]. The radiation budget represents the balance between incoming energy from the Sun and outgoing thermal (longwave) and reflected (shortwave) energy from the Earth. ...


Large influxes of low density meltwater from the Greenland ice sheet is thought to have led to a disruption of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas. An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² (19,305 mile²). The only current ice sheets are Antarctic and Greenland; during the last ice age at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of Canada... Three temperature records, the GRIP one clearly showing the Younger Dryas event at around 11 kyr BP The Younger Dryas stadial, named after the alpine / tundra wildflower Dryas octopetala, and also referred to as the Big Freeze [1], was a brief (approximately 1300 ± 70 years [1]) cold climate period following...


For a discussion of the possibilities of changes to the thermohaline circulation under global warming, see shutdown of thermohaline circulation. Shutdown or slowdown of the thermohaline circulation is a possible effect of global warming. ...


References

  1. ^ Schmidt, G., 2005, Gulf Stream slowdown?, RealClimate
  2. ^ United Nations Environment Programme / GRID-Arendal, 2006, [1]. Potential Impact of Climate Change
  3. ^ Seager,R., 2005, The Source of Europe's Mild Climate, American Scientist
  4. ^ Rhines and Hakkinen, 2003, Is the Oceanic Heat Transport in the North Atlantic Irrelevant to the Climate in Europe?, ASOF Newsletter
  5. ^ Marsh, R. et al. (2004) Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model. Climate Dynamics, 23: 761-777
  • Apel, J. R., 1987, Principles of Ocean Physics, Academic Press, (ISBN 0-12-058866-8)
  • Gnanadesikan, A., R. D. Slater, P. S. Swathi, and G. K. Vallis, 2005: The energetics of ocean heat transport. Journal of Climate, 18, 2604-2616.
  • Knauss, J. A., 1996, Introduction to Physical Oceanography, Prentice Hall (ISBN 0-13-238155-9)
  • Primeau, F., 2005, Characterizing transport between the surface mixed layer and the ocean interior with a forward and adjoint global ocean transport model, Journal of Physical Oceanography,35, 545-564.
  • Rahmstorf, S., 2006, Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier, Amsterdam.
  • Rahmstorf, S., 2003, The concept of the thermohaline circulation. Nature, 421, 699.
  • United Nations Environment Programme / GRID-Arendal, 2006, [2]. Potential Impact of Climate Change

RealClimate is a commentary site (blog) on climate science by working climate scientists for the interested public and journalists. ...

See also

Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. ... Hydrothermal circulation in the oceans is the passage of the water through mid-ocean Ridge (MOR) systems. ... An ocean current is any more or less continuous, directed movement of ocean water that flows in one of the Earths oceans. ... Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-deplete surface water. ... For the album by Ocean Colour Scene, see North Atlantic Drift (album) The Gulf Stream is orange and yellow in this representation of water temperatures of the Atlantic. ... The North Atlantic Current (North Atlantic Drift and the North Atlantic Sea Movement) is a powerful warm ocean current that continues the Gulf Stream northeast. ...

External links


  Results from FactBites:
 
CRU Information Sheet no. 7: Thermohaline circulation (792 words)
The density of sea water is controlled by its temperature (thermo) and its salinity (haline), and the circulation driven by density differences is thus called the thermohaline circulation.
The critical part of the thermohaline circulation (THC) is the sinking in the North Atlantic Ocean.
It is warmer in the North Atlantic because warm water is brought by the thermohaline circulation from the tropical and South Atlantic.
Thermohaline circulation - Wikipedia, the free encyclopedia (1432 words)
Changes in the thermohaline circulation are thought to have significant impacts on the earth's radiation budget.
Insofar as the thermohaline circulation governs the rate at which deep waters are exposed to the surface, it may also play an important role in determining the concentration of carbon dioxide in the atmosphere.
Large influxes of low density meltwater from the Greenland ice sheet is thought to have led to a disruption of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas.
  More results at FactBites »

 
 

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