Gulf Stream

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The Gulf Stream is orange and yellow in this representation of water temperatures of the Atlantic. Source: NASA.

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. At about 30°W, 40°N, it splits in two, with the northern stream crossing to northern Europe and the southern stream recirculating off West Africa. The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe.

1 The Gulf Stream proper and the North Atlantic Drift
2 Normal behaviour of the Gulf Stream
3 Localized effects
- 3.1 Europe
- 3.2 North America
4 The effect of global warming
5 See also
6 References
- 6.1 Footnotes

[edit] The Gulf Stream proper and the North Atlantic Drift

The Gulf Stream proper is a western-intensified current, largely driven by wind stress.^[1] The North Atlantic Drift, in contrast, is largely thermohaline circulation driven. By carrying warm water northeast across the Atlantic, it makes Western Europe (and especially Northern European winters) warmer than they otherwise would be. However, the extent of its contribution to the actual temperature differential between North America and Europe is a matter of dispute.^[2] There is speculation that global warming might affect the thermohaline circulation, diminish the NAD, and perhaps lead to relative cooling in Western Europe. (Discussion of this topic often refers, erroneously, to the "potential shutdown" of the Gulf Stream proper.)

[edit] Normal behaviour of the Gulf Stream

A river of sea water, called the Atlantic North Equatorial Current, flows westward off the coast of northern Africa. When this current interacts with the northeastern coast of South America, the current forks into two branches. One passes into the Caribbean Sea, while a second, the Antilles Current, flows north and east of the West Indies. These two branches rejoin north of the Straits of Florida, as shown on the accompanying map.

Consequently, the resulting Gulf Stream is a strong ocean current, transporting about 1.4 petawatts of heat, equivalent to the output of 1 million power stations and 100 times the world energy demand.^[3] It transports water at a rate of 30 million cubic meters per second (30 sverdrups) through the Florida Straits. After it passes Cape Hatteras, this rate increases to 80 million cubic meters per second. The volume of the Gulf Stream dwarfs all rivers that empty into the Atlantic combined, which barely total 0.6 million cubic meters per second. It is weaker, however, than the Antarctic Circumpolar Current.

Typically, the Gulf Stream is 80–150 km wide and 800–1200 m deep. The current velocity is fastest near the surface, with the maximum speed typically about 2 m/s.^[4]

As it travels north, the warm water transported by the Gulf Stream undergoes evaporative cooling and brine exclusion. The cooling is wind driven: wind moving over the water cools it and also causes evaporation, leaving a saltier brine. In this process, the water increases in salinity and density, and decreases in temperature. The second process involves the formation of sea ice, which likewise increases the salinity of the brine solution, thereby decreasing its freezing point. These two processes produce water that is denser and colder (or, more exactly, water that is still liquid at a lower temperature). In the North Atlantic Ocean, the water becomes so cold and dense that it begins to sink down through warmer, less salty and less dense water. (The convective action is not unlike that of a lava lamp.) This downdraft of heavy, cold and dense water becomes a part of the North Atlantic Deep Water, a southgoing stream.

Schematic of the world's ocean currents. Click for larger image.

[edit] Localized effects

[edit] Europe

The Lofoten islands, Norway, enjoy much higher temperatures than would be normal for their latitude.

The North Atlantic Drift is one of the reasons why certain parts of the west of Ireland, Great Britain and France are an average of several degrees warmer than most other parts of those countries. Indeed, in Cornwall, and particularly the Isles of Scilly, as well as coastal areas of southwest Ireland, its effects are such that plants associated with much warmer climates, such as varieties of palm trees not requiring high summer temperatures, are able to survive the rigours of northern winters. Logan Botanic Garden in Scotland benefits strongly from the Gulf Stream, allowing their specimens of Gunnera manicata to grow to over 3 metres tall. It is estimated that the processes associated with the Gulf Stream keep all of Great Britain at least 5 °C (9 °F) warmer than it would otherwise be.^[3] However, the significance of the warming effect compared to that caused by atmospheric transfer of heat has been shown to be much less than commonly assumed.^[5]

The effects of the North Atlantic Drift are also important farther north. Average monthly temperatures in Røst^[6] and Værøy^[7] (both in Lofoten, Norway, inside the arctic circle) never drop below freezing in winter; this represents the world's largest positive temperature anomaly relative to latitude.

While the Gulf Stream certainly impacts the climate of Western Europe, especially in the coastal areas in the northern parts of the continent, it is sometimes erroneously stated that the Gulf Stream is the primary force responsible for the (significant) temperature difference between the east coast of North America and the west coast of Europe.^[8]^[9] This difference is largely due to the pattern of prevailing winds that results in continental climate in eastern United States and Canada.^[2] In fact, the climate of Western Europe is very similar to that of equal-latitude coastal regions of the west coast of North America, even though there is no equivalent of the Gulf Stream in the Pacific Ocean: Plymouth is only 3 °C warmer in winter than Vancouver; average temperatures in Bordeaux are virtually identical to averages in Portland. Further north the differences grow larger; coastal areas at 63–71 °N in Europe are 10–15 °C warmer in winter than corresponding latitudes in coastal Alaska. But this is also an effect of the geography of the coast itself; while Alaska protrudes west and blocks the maritime air from the pacific, the Norwegian coast slopes northeast, allowing the westerlies to enter the Arctic.

[edit] North America

The Gulf Stream is influential on the climate of the east coast of Florida, especially southeast Florida, helping to keep temperatures warmer than in the rest of the southeastern United States during the winter.^{[citation needed]} During the summer, the effect is opposite but small.^{[citation needed]} The Gulf Stream makes the climate of offshore islands of Massachusetts, Martha's Vineyard, and Nantucket milder than that of Massachusetts Bay, which is isolated from Gulf Stream effects by Cape Cod.

[edit] The effect of global warming

Main article: Shutdown of thermohaline circulation

There is some speculation that global warming could decrease or shutdown thermohaline circulation and therefore reduce the North Atlantic Drift. This could trigger localised cooling in the North Atlantic and lead to cooling (or lesser warming) in that region, particularly affecting areas that are warmed by the North Atlantic Drift, such as Scandinavia and Great Britain. The chances of this occurring are unclear.

At present, most available data show that Gulf Stream flow was stable over the past 40 years.^[10] One report, based on a snapshot survey, suggested that the deep return flow has weakened^[11] by 30% since 1957, which would imply a weakening in the North Atlantic Deep Water production.^[12] However, this should have caused a temperature drop of several degrees in northwest Europe, which has not been observed. It was later discovered, using the first cross-Atlantic array of moored current meters, that variations within one year were just as large.^[13] At least part of the apparent weakening of the Gulf Stream (if real) may be cyclical and connected to recent positive values of North Atlantic Oscillation.^[14] Recent research ^[15] shows that Gulf Stream Little Ice Age volume transport was ten per cent weaker than today’s, implying that diminished oceanic heat transport may have contributed to the 16th- to the mid-19th-century cooling in the North Atlantic.