Gulf Stream
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The Gulf Stream is orange and yellow in this representation of water temperatures of the Atlantic. Source: NASA. |
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Gulf Stream currents off North America. |
The
Gulf Stream, together with its northern extension, 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.
Its extension toward
Europe, called the
North Atlantic Drift, makes
Western Europe (and especially
Northern European winters) considerably warmer than they otherwise would be. For example, in January, the temperature difference between coastal
Norway and northern parts of continental
Canada is approximately 30 °C on average, even though they are the same
latitude.
The Gulf Stream is a
western-intensified current, largely driven by the wind stress
[Carl Wunsch, What Is the Thermohaline Circulation?, Science 8 November 2002: 1179-1181, DOI: 10.1126/science.1079329 (Official Science site; Freely available pdf); see also Rahmstorf]; its extension, the North Atlantic Drift, is largely
thermohaline circulation driven. Speculation that
global warming might
affect the thermohaline circulation, perhaps leading to relative cooling in Western Europe, often erroneously refers to the Gulf Stream, whereas it is the North Atlantic Drift which might be diminished by shutdown of the thermohaline circulation.
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
[The Australian: Scientists probing a dying current bring worst climate fears to the surface]. 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, Gulf Stream is 80â€"150 km wide and 800â€"1200 m deep. The current velocity is fastest near the surface, with the maximum speed typically about 2 m/sec.
[USNA/John's Hopkins]As it travels
north, the warm
water transported by the Gulf Stream undergoes two processes, evaporative cooling and brine exclusion. The first of these processes 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 and, thereby, decreases its freezing point. These two processes produce water that is denser and colder (or, more exactly, water that is still liquid at a colder 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.
Europe
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The Lofoten islands, Norway, enjoy much higher temperatures than what 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, its effects are such that plants associated with much warmer climates, such as
palm trees, 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 five degrees warmer than it would otherwise be
.
The effects of the North Atlantic Drift are also important further north. Average monthly temperatures in
Røst[WorldClimate: Rost II, Norway] and
Værøy[ Meteorologisk Institutt: Væroy] in
Lofoten,
Norway, inside the
arctic circle, never drops below freezing in winter; this represents the world's largest positive temperature anomaly relative to
latitude.
North America
It 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 in the winter. In summer the effect is opposite but small. 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 its effects by
Cape Cod.
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, affecting in particular areas like
Scandinavia and Great Britain that are warmed by the North Atlantic Drift. The chances of this occurring are unclear.
At present, the Gulf Stream itself shows stability over the past 40 years
[Gavin Schmidt and Michael Mann, Real Climate: Decrease in Atlantic circulation? (30 November 2005)], although there is evidence that the deep return flow is weakening, which would imply a weakening in the North Atlantic Drift
[BBC News: Ocean changes 'will cool Europe' (30 November 2005)]. However, this should cause a temperature drop of a few degrees in North West Europe, which has not been observed.
*
Corona Magazine Issue 124: Science (German, Transported amount of power)
*
Hycom Consortium*
Atlantic Ocean overturning that maintains Europe's moderate climate has slowed by 30 per cent*
Increased temperature and salinity in the Nordic SeasFootnotes
