Thermohaline circulation

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Overview of the global thermohaline oceanic circulation
Red: Warm; Blue: cool
Volume 11 of the 4th edition of Meyers Konversations-Lexikon , 1885–1890, article "Sea", map "Sea currents and recent deep sea currents"
Ocean currents (1943)

The thermohaline circulation , also colloquially global conveyor belt ( English ocean conveyor belt ), is an oceanographic term for a combination of ocean currents that connect four of the five oceans with each other and thereby unite to form a global cycle.

The drive for this extensive mass and heat exchange is thermohaline in nature. That means: It is caused by temperature and salt concentration differences within the oceans , both of which are responsible for the different density of the water . The temperature difference is caused in turn by the dependence of the extent of solar radiation on the geographical latitude .

history

In the early 1970s it was possible for the first time to evaluate oceanographic data from the entire earth. This synopsis showed the interlinking of the wind and density-driven currents and it was postulated that the well-known Gulf Stream is only a partial current of a global circulation. Based on a mechanical conveyor belt, this earth-encompassing current was called the “great marine conveyor belt” or simply “global conveyor belt” or, in scientific usage, also “global thermohaline circulation”.

Circulation pattern

Deep current blue, surface current red ( circumpolar current not included)

The circulating currents occur both on the surface and in the depths. The cold water moves at a depth of 1.5 to 3.0 km mostly parallel to the continental slope on the western side of the ocean basins, due to the earth's rotation.

The global heat exchange band is essentially initiated by the winter sinking of the salty and cold sea water in the North Atlantic to a depth of 1 to 4 km, which is why these regions of the global cycle are suitable for observing the circulation pattern from here. The sinking is triggered by cooling and an increase in the salt content through evaporation and ice formation. On or near the seabed, the sunken water flows as a cold deep current ( North Atlantic deep water ) to the exit of the South Atlantic and is then transported with the circumpolar current into the Indian Ocean and the Pacific . The circumpolar current of the southern ocean flows around the entire globe and mixes the water masses of the three adjacent oceans. It is probably the area where most of the cold water rises and is heated by wind-driven mixing. From there, the water masses modified by mixing move back to the surface (surface water, Pacific) or a few hundred meters below it (intermediate water, Ind). As a result, the water heats up, especially in the equatorial areas, and flows as a warm surface current first past Indonesia , then around the southern tip of Africa into the Gulf region of Central America and finally as a Gulf Stream into the North Atlantic, where it sinks again and closes the cycle.

Surface temperature in the western North Atlantic. North America appears black and dark blue (cold) the Gulf Stream red (warm). Source: NASA

In addition to purely thermohaline effects, the distribution of the continents , the Coriolis force (as a result, the currents occur mainly on the west coast) and the wind-related effect of the corkscrew current play a decisive role. Together, these lead to a regionally very complex formation of different ocean currents, for example in the form of large current eddies on the southeast coast of South America (see below). To a small extent, water masses from the Arctic Ocean also flow into the Atlantic, which is why this also partly takes part in the global conveyor belt. Since many of these factors depend on the local intensity of solar radiation , these ocean currents can also experience different characteristics over the course of the year, for example in the Indian Ocean due to the monsoons . Significant effects are upwelling and downwelling .

Due to the long-term effects of the continental drift on the land-sea distribution, the main currents are also variable over time. As a comparatively short-term influencing factor, caused by the melting of the polar ice caps, a more or less strong weakening of the North Atlantic Current is considered possible. Examples of this can also be found in climate history .

Deep edge currents

The deep marginal currents represent a further contribution to the global seawater circulation . This is understood to mean near-shore deep-sea eddies, as they are e.g. B. occur off the coast of Brazil in the Brasilstrom (so-called Brasilstromringe). These eddies occur periodically in terms of time and space, so a so-called vortex street is created . An exact explanation for this phenomenon, which was discovered in 2004 , is still pending, but according to computer models the Brazilian current breaks up into a turbulent current at the level of the Brazilian city of Recife because of the receding coast and the resulting reduced friction , compare eddy (fluid mechanics) .

Risks

Due to the increasing ice melt on the polar ice caps associated with global warming , the salt content of the seawater changes with the additional fresh water input . This also changes the thermohaline dynamics there: the formation of Antarctic bottom water z. B. is one of the "engines" of thermohaline circulation.

When a “ tipping point ” is reached, a “ domino effect ” with the collapse of the relevant systems is feared. The thermohaline circulation is influenced by a strong melting of the Greenland ice , which is possible even with a global warming between 1 and 3 degrees . Its collapse is in turn fed back to the El Niño Southern Oscillation , the partial death of the Amazon rainforest and the melting of Antarctic sea - and later land ice .

See also

literature

Web links

Individual evidence

  1. ↑ http://www.meereisportal.de/meereiswissen/die-globale-bedeutung-von-meereis/wechselhaben-von-meereis-mit-anderen-verbindungen-des-klimasystems/meereis-und-ozeanische-kreisulation/
  2. ↑ deutschlandfunk.de , Forschungs aktuell , August 31, 2016, Dagmar Röhrlich : Antarctica: The drive for global ocean circulation is weak (September 3, 2016)
  3. ↑ On the way to the "hot time"? Planet could exceed critical threshold. Potsdam Institute for Climate Impact Research , August 6, 2018, accessed on September 13, 2018 .