The Gulf Stream is the most important ocean-current system in the northern hemisphere, which stretches from Florida to north-western Europe. It incorporates several currents: the Florida current, the Gulf Stream itself, and an eastern extension, the North Atlantic Drift. The Florida Current is fast, deep, and narrow, but after passing Cape Hatteras the Gulf Stream becomes less effective at depth and develops a series of large meanders which form, detach, and re-form in a complicated manner. After passing the Grand Banks (off Newfoundland), the flow forms the diffuse, shallow, broad slow-moving North Atlantic Drift.
The relatively warm waters of the North Atlantic Drift are responsible for moderating the climate of western Europe, so that winters are less cold than would otherwise be expected at its latitude. Without the warm North Atlantic Drift, the UK and other places in Europe would be as cold as Canada, at the same latitude. For example, without this steady stream of warmth the British Isles winters are estimated to be more than 5 °C cooler, bringing the average December temperature in London to about 2°C.
Within the Gulf of Mexico, the Gulf Stream is very narrow, only 50 miles wide, and travels very fast at 3 mph, carrying water at about 25°C. The North Atlantic Drift widens considerably to several hundred miles, slows to less than 1 mph and splits into several sub-currents. Off the British Isles it splits into two branches, one going south (the Canary Current) and the other going north along the coast of W and N Europe, where it exerts considerable influence upon the climate as far as northwestern Europe. For example, the Drift is particularly important because it keeps many Norwegian ports free of ice throughout the year.
The two main driving forces behind it are the prevailing southwesterly trade winds and the circulation of the water far below the oceans surface, the North Atlantic Deep Water (NADW) circulation. Water in the north Atlantic sinks because it is dense. Density of water is increased by both salinity and temperature - the colder and saltier the water is the denser it is. This deep water flows to the Gulf of Mexico until it warms enough to resurface and flow back north as the Gulf Stream.
Some 11,000 years ago the NADW shut down in response to subtle shifts in global climate. This slowed and diverted the course of the Gulf Stream to such an extent that the regional climate of the Northeast Atlantic became considerably cooler. As a result Northwestern Europe dropped back to ice age conditions within tens of years. It is now suspected that global warming may trigger a shutdown in the NADW, and a slowing or diversion of the Gulf Stream, which would ironically lead to colder climates throughout the UK and Northwest Europe.