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THERMOHALINE CIRCULATION AND
THE OCEANIC CONVEYOR BELT

 

The ocean is not a still body of water. There is constant motion in the ocean caused by a combination of currents produced by temperature and salinity changes in the deeper parts of the ocean on the one hand (called the thermohaline currents), and wind driven currents at the surface.  The deep ocean currents are sustained because cold, dense salty water sinks to the bottom of the ocean while warm water of lesser density remains at the surface. Thermohaline circulation begins in the Earth's polar regions. When ocean water in these areas gets very cold, sea ice forms. The surrounding seawater gets saltier, increases in density and sinks.

Thermohaline Circulation

 

 Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes.   The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content – factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) travel towards the poles from the equatorial Atlantic Ocean, cooling en route, 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 1000 years) upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans 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.

The ocean conveyor begins in the Norwegian Sea, where warm water from the Gulf Stream heats the atmosphere in the cold northern latitudes. This loss of heat to the atmosphere makes the water cooler and denser, causing it to sink to the bottom of the ocean. As more warm water is transported north, the cooler water sinks and moves south to make room for the incoming warm water. This cold bottom water flows south of the equator all the way down to Antarctica. Eventually, the cold bottom waters returns to the surface through mixing and wind-driven upwelling, continuing the conveyor belt that encircles the globe.

The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt.

 

Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.

In the Earth's polar regions ocean water gets very cold, forming sea ice. As a consequence the surrounding seawater gets saltier, because when sea ice forms, the salt is left behind. As the seawater gets saltier, its density increases, and it starts to sink. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. This initiates the deep-ocean currents driving the global conveyer belt.

This picture shows the path of the global conveyer belt. The blue arrows indicate the path of deep, cold, dense water currents. The red arrows indicate the path of warmer, less dense surface waters. It is estimated that it can take 1,000 years for a "parcel" of water to complete the journey along the global conveyor belt.

 

Thermohaline circulation drives a global-scale system of currents called the “global conveyor belt.” The conveyor belt begins on the surface of the ocean near the pole in the North Atlantic. Here, the water is chilled by arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense, due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a current.

Description: global conveyer belt - stage 1

Cold, salty, dense water sinks at the Earth's northern polar region and heads south along the western Atlantic basin.

This deep water moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets "recharged." As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean.

Description: global conveyer belt - stage 2

The current is "recharged" as it travels along the coast of Antarctica and picks up more cold, salty, dense water.

These two sections that split off warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling). They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again.

 

 

Description: global conveyer belt - stage 3

The main current splits into two sections, one traveling northward into the Indian Ocean, while the other heads up into the western Pacific.

The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-driven or tidal currents (tens to hundreds of centimeters per second). It is estimated that any given cubic meter of water takes about 1,000 years to complete the journey along the global conveyor belt. In addition, the conveyor moves an immense volume of water – more than 100 times the flow of the Amazon River.

Description: global conveyer belt - stage 4

The two branches of the current warm and rise as they travel northward, then loop back around southward and westward.

The conveyor belt is also a vital component of the global ocean nutrient and carbon dioxide cycles. Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as they travel through the conveyor belt as deep or bottom layers. The base of the world’s food chain depends on the cool, nutrient-rich waters that support the growth of algae and seaweed.

 

Description: global conveyer belt - stage 5

The now-warmed surface waters continue circulating around the globe. They eventually return to the North Atlantic where the cycle begins again.

The global conveyor belt is a strong, but easily disrupted process. Research suggests that the conveyor belt may be affected by climate change. If global warming results in increased rainfall in the North Atlantic, and the melting of glaciers and sea ice, the influx of warm freshwater onto the sea surface could block the formation of sea ice, disrupting the sinking of cold, salty water. This sequence of events could slow or even stop the conveyor belt, which could result in potentially drastic temperature changes worldwide, particularly in Europe.

 

 

 

 

 

Reference: This article is reformatted from NOAA Ocean Service Education at http://oceanservice.noaa.gov/education/kits/currents/06conveyor.html

 

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