The Gulf Stream moves huge amounts of water, salt, and heat around the Atlantic Ocean. Many scientists have argued that the position of the Gulf Stream has shifted at times in the geologic past, causing significant changes in the climate of Greenland and northern Europe.

The Gulf Stream has a peak flow of more than a hundred times that of all the rivers and streams on Earth, combined. Much of this is warm water from the Gulf of Mexico or the Caribbean, being transported to the North Atlantic, transferring a great amount of heat. A strong belief has developed that this ocean current is responsible for the mild climate of Europe. Recent data and global climate models indicate, however, that it is only one of several factors, as the heat transported by winds is several times greater.

The Sun warms the surface waters of the oceans, expanding them and reducing their density. The prevailing wind patterns in the subtropics (near 20œ latitude) pile up these surface waters into hills, which are actually lens-shaped water masses floating on denser, cooler water, and bounded in part by continents. These hills have "peaks" no more than two meters higher than their edges, thousands of kilometers away. As the water at the surface tries to flow down this slope, the Coriolis effect, which is a consequence of the Earth's rotation, causes it to be deflected. In the Northern Hemisphere this deflection is to the right, so the circulation actually follows the hill contours, forming a rotating gyre. In the North Atlantic and North Pacific, subtropical gyres rotate in a clockwise sense. Constraints of flow on a rotating planet require that gyres are asymmetric, with a very steep slope on the western side. The rate at which these geostrophic currents flow is proportional to the slope. The Gulf Stream is a result of this steep slope, produced by a combination of solar heating, basin geometry, prevailing wind directions, and the rotation of the Earth.

The magnitude of this flow is prodigious. Peak flows are about 150 x 106 cubic meters per second. Put another way, a mass of ocean water, equivalent to the mass of the entire human population, flows by in less than four seconds. In a year the Gulf Stream moves 4.71 million cubic kilometers of water, which is huge compared to the annual runoff from all the continents (40,000 cubic kilometers) or even the total water moving between the surface of the Earth and the atmosphere (489,000 cubic kilometers).

Along with all the water, the Gulf Stream moves a great deal of heat to the north. At the northern end of its range (near 50œ north latitude), the North Atlantic Drift, with a flow of 14 x 106 cubic meters per second, diverges from it, moving this water farther north, where it cools and sinks. If it loses 20œ Celsius in the process, or 20 calories per gram, about 1 x 1015 watts are released. This is fifty times the total human energy use of 0.018 x 1015 watts, and about half of the total poleward ocean heat transport on the planet. However, the heat transported by the atmosphere is about four times greater.

The cause of this disparity lies in the heat of vaporization of water. Every gram that evaporates moves at least 540 calories of heat from the ocean to the atmosphere. Of the 413,000 cubic kilometers of water evaporated from the sea every year, perhaps 56,000 cubic kilometers, evaporate near the equator in the Atlantic and precipitate farther north. That would release 4 x 1015 watts.

The Gulf Stream may not be directly responsible for the mild climate of Europe, but it probably is responsible for the warm temperatures of the North Atlantic. If these temperatures fall, the ocean surface may become covered with ice, the wind will not be warmed, and Europe will surely chill. Furthermore, the strength of the North Atlantic Oscillation (NAO) may well depend on these warm temperatures, and hence on the Gulf Stream.

Intuitively, it would seem that a current that brings about 1 x 1015 watts of heat into the North Atlantic should be a dominant player in the climate of Europe. Since its discovery, many people have thought so and have written about it. Recent studies indicate, however, that the heat brought by the Gulf Stream is only a tenth of the heat transported, it is responsible for only a few degrees of warming, and it does nothing to explain why Europe is 15œ-20œ Celsius warmer, in winter, than places at similar latitudes on the East Coast of North America. Most of the heat, and all of the heat producing the contrast between Europe and North America, comes from atmospheric transport and the heating of the surface waters during the summer. Because the myth is so entrenched in the collective understanding of climate science, changing the way people think about it has proven to be extremely difficult. The strength of the NAO may be more important to Earth's climate than the strength of the Gulf Stream; however, the Gulf Stream may control the NAO.

Bibliography:

1) Fagan, Brian M. The Little Ice Age: How Climate Made History, 1300-1850. New York: Basic Books, 2000.

2) Gaskell, Thomas Frohock. The Gulf Stream. New York: John Day, 1973.

3) MacLeish, William H. The Gulf Stream: Encounters with the Blue God. Boston: Houghton Mifflin, 1989.

4) Seager, R. "The Source of Europe's Mild Climate: The Notion That the Gulf Stream Is Responsible for Keeping Europe Anonymously Warm Turns out to Be a Myth." American Scientist 94 (2002): 340-31.

5) Seager, R., et al. "Is the Gulf Stream Responsible for Europe's Mild Winters?" Quarterly Journal of the Royal Meteorological Society 128, no. 586 (2002): 2563-2586.

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