Thermohaline circulation
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The thermohaline circulation is a term for 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 downhill into the deep water basins, only resurfacing in the northeast Pacific Ocean 1200 years later. 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 piggyback 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 our planet.
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The thermohaline circulation is sometimes called the ocean conveyor belt or, less commonly, the global conveyor belt.
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The system
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, however, the predominant driving force is differences in density and temperature. There is often confusion over the components of the circulation that are wind and density driven; this is partially addressed [1] (http://www.realclimate.org/index.php?p=159).
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. Hot water expands and is thus less dense than colder water. Fresh water is less dense than salt water because it has less mass than salty water per unit volume. Lighter water masses float over denser ones (just as a piece of wood or ice will float on water). In order to take up their most stable positions, water masses of different densities must flow, providing a driving force for deep currents.
Formation of the 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. Here the water is intensively cooled by the wind, then becomes salty as sea ice forms and excludes the salt fraction of the water. The increasing salinity pushes the freezing temperature of the brine down, so cold liquid brine is formed in inclusions within a honeycomb of ice. Being extremely dense, it slowly drips out of the ice matrix and sinks to the sea bottom. These deep water masses are so dense they flow downhill, 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 bottom water masses.
Note that, unlike fresh water, saline water does not have a density maximum at 4 oC but gets denser as it cools all the way to its freezing point of approximately -1.8 oC.
In the Norwegian Sea wind 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 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.
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. 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.
Movement of deep water masses
The North Atlantic Deep Water which is the result of density subsidence in the North Atlantic is not a static mass of water, but rather a slowly southward flowing current. The route of the deep water flow is through the Atlantic Basin around South Africa and into the Indian Ocean and on past Australia into the Pacific Ocean Basin.
The deep water masses have chemical and isotopic ratio signatures and can be traced, their flow rate calculated, and their age determined.
Upwelling
See 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. This signature can be found in surface waters in the North Pacific, indicating that this is where most upwelling happens.
Impacts on global climate
Thermohaline circulation is a major part of the oceanic convective system which distributes heat energy from the equatorial oceans to the polar regions. It is the return flow from the surface North Atlantic Drift and Gulf Stream currents.
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.
References
- Apel, John R., 1987, Principles of Ocean Physics, Academic Press, (ISBN 0120588668)
- Knauss, John A., 1996, Introduction to Physical Oceanography, Prentice Hall (ISBN 0132381559)
See also
fr:Circulation thermohaline ja:熱塩循環 nl:Thermohaliene circulatie