Cold water discovered in the subtropical Atlantic was brought there by a current which had originated in the polar region; temperature measurements in the real ocean and computer models show there is a southward outflow of cold deep water from the Arctic throughout the Atlantic. This cold water is replaced by warm surface waters, which gradually give off their heat to the atmosphere as they flow northward towards Europe. This acts as a massive heating system for all the land downwind.

The heat released by this system is enormous: it measures around 1015 W, equivalent to the output of a million large power stations. If we compare places in Europe with locations at similar latitudes on the North American continent, its effect becomes obvious. Bodö in Norway has average temperatures of -2°C in January and 14°C in July; Nome, on the Pacific Coast of Alaska at the same latitude, has a much colder -15°C in January and only 10°C in July. And satellite images show how the warm current keeps much of the Greenland-Norwegian Sea free of ice even in winter, despite the rest of the Arctic Ocean, even much further south, being frozen.

Creating the Belt

What drives this remarkable circulation? Why does it occur only in the Atlantic, why don't the Pacific and Indian Oceans have similar heating systems?

In general, ocean currents are driven either by winds or by density differences. Density in the ocean depends on temperature and salinity, and the Atlantic Conveyor is a thermohaline circulation (from the Greek words for 'heat' and 'salt'). When surface waters become dense enough - through cooling or becoming saltier (or a combination of these two factors), a mixing process takes place in which they sink and form deep water.

The deep mixing takes place because of convection; sea water from near the surface sinks down 2km or more and then spreads at this depth. This is called 'deep water formation'.

Because this convection only takes place at specific sites, the image of a plug-hole in a bathtub is suggestive. At certain locations on the broad expanse of ocean, the surface waters descend into the deep. They don't exactly gurgle down the plug-hole, rather they subside so slowly that it is hard to measure directly. In the present climate, deep water forms in the Greenland-Norwegian Seas and in the Labrador Sea. There are no deep convection sites in the North Pacific. The only other place where the global ocean's deep water is formed is near the Antarctic continent in the Southern Hemisphere.

At these convection sites, the water has become dense enough to push away the underlying deep water and sink because it has cooled - cold water is denser than warm. In the Indian Ocean, the water is too warm to sink; the ocean's basin does not extend very far north of the Equator.

But cold is not the only factor. The waters of the North Pacific are cold, but even as they approach freezing point they still lack the density which would enable them to sink down and thus drive a conveyor flow. This is because the North Pacific is less salty than the North Atlantic (around 32%, compared to 35% in the northern North Atlantic) , and salt is the second crucial factor in the density of ocean water.

Bibliography

Big Melt May Bring Big Freeze. http://www.junkscience.com/jan99/icemelt.htm; May 10, 2000.

Broecker, W. Chaotic Climate. Scientific American, 44-50 (1995).

De Villiers, Marq. Water. Stoddart Publishing Co. Limited: Toronto. (1999).

Dunn, Margery. Exploring Your World: The Adventure of Geography. National Geographic

Press: New York. (1990).

Gottmann, Jean. A Geography of Europe. Holt, Rinehart and Winston: New York. (1970).

Great Conveyor Belt. http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm; May 10, 2000.

Rahmstorf, S. Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature 378, 145-149 (1995).

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