Ocean circulation can be envisaged simply as a system of conveyer belts for large-scale oceanic currents. These are driven by tidal forces, the wind and by changes in the density of the water masses, associated with changes in temperature and salt content.

This so-called "thermohaline circulation" describes how, for example, the gulf stream transports warm surface waters, high in salt content due to the high levels of evaporation in the subtropics, to the North Atlantic. The water cools down in the North Atlantic and its high salt content is further increased by the formation of sea ice, which itself is practically free of salt. This renders the remaining water masses so dense that massive quantities sink down to depths of two to three kilometres. These sinking processes create an undertow and thus pull more new warm waters that are high in salt content from the Caribbean to the North. An important driving mechanism for the thermohaline circulation is therefore located in the North Atlantic. The Gulf Stream example also serves to illustrate how substantial quantities of heat transported with the water masses from the Caribbean to the North make possible a relatively mild climate in Europe. Changes in ocean circulation such as that of the Gulf Stream will therefore have direct effects on the climate. In this case, the ocean affects both the atmosphere and the continents, causing, for example, changes in the monsoon and trade wind system, the El Niño Southern Oscillation, the global distribution of CO₂ in the oceans or global patterns of precipitation and evaporation.

The ocean circulation is also important for the transport and distribution of carbon, nutrients and trace elements. Due to the solubility of CO₂ in surface water, the ocean has presently taken up roughly 25 % of anthropogenic CO₂ emmissions, which are stored in the ocean’s interior after subduction of these water masses. In the sun-lit surface layers, marine micro algae fix carbon and nutrients on the order of about 50 % of the global annual biomass production. In areas of intense upwelling, for example the Peru-Chile current in the eastern tropical Pacific, high phytoplankton productivity serves as a food source for fish and other marine organisms making this region traditionally important as an area of food supply for large parts of the worls population.

Predictions based on existing climate models suggest a reduction in the formation of deep waters will take place over the next one hundred years, if temperatures on the Earth's surface continue to rise. Initial signs of this process can already be measured – for example, the seas of the Northern Hemisphere and West Pacific have warmed up over the past 20 years, while those of the Southern Hemisphere and East Pacific have cooled down. Such temperature changes in the ocean will affect the climate on land in the long term.

 

Contact

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• Prof. Birgit Schneider

  Institution:

  Institut für Geowissenschaften

  Phone:

  +49 431 880-3254

  E-mail:

  bschneider@gpi.uni-kiel.de

• Prof. Ralph R. Schneider

  Institution:

  Institut für Geowissenschaften

  Phone:

  +49 431 880-1457

  E-mail:

  schneider@gpi.uni-kiel.de

• Prof. Mojib Latif

  Institution:

  Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR)

  Phone:

  +49 431 600-4050

  E-mail:

  mlatif@ifm-geomar.de