Gulf Stream

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 stressCarl 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.

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 annual output of 1 million power stationsThe 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 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/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.

Localised effects

Europe

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 RøstWorldClimate: 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.

The effect of global warming

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 yearsGavin 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 DriftBBC 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. The main article referred to above gives further information.

References

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 Seas

Footnotes

Ocean currents | Atlantic Ocean