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Trevor Day
1686

Gulf Stream

The Gulf Stream is the most famous of the ocean currents of the North Atlantic. It begins as a recognizable current within the Florida Straits and disperses at around 45 Degrees N, 45 Degrees W, off the Grand Banks of Newfoundland. Off Cape Hatteras, North Carolina, the Gulf Stream’s flow averages some 3 kilometers (1.9 miles) per hour, but elsewhere its flow reaches 9 kilometers (5.6 miles) per hour. It is one of the world’s fastest major surface currents and has a volume flow several times that of all the world’s rivers.

The Gulf Stream, like most major surface currents, is primarily wind-driven, but modified by the Earth’s rotation. It is also influenced by so-called thermohaline circulation, which is driven by salinity and temperature differences in seawater.

The Gulf Stream is the western boundary current of the North Atlantic gyre. This gyre turns clockwise, as do other gyres in the Northern Hemisphere. The eastward rotation of the Earth deflects wind-driven surface currents to the right in the Northern Hemisphere, thus creating the tendency towards a clockwise circulation.

The Gulf Stream is fed primarily by warm water emerging from the Gulf of Mexico. It is supplemented by water deflected northward from the westward-flowing North Equatorial Current and from the Sargasso Sea. Much of the water entering the Caribbean Sea from the North Equatorial Current reaches the Gulf of Mexico through the Yucatán Channel where it circulates and is further warmed before leaving through the Florida Strait to join the Gulf Stream.

From about 40^oN, the Gulf Stream’s eastward flow of water is assisted by westerly winds. Beyond this, after meeting the Newfoundland Rise, the current becomes the Gulf Stream Extension. It splits into the northeasterly North Atlantic Drift and the southeasterly Azores Current that joins the Canary Current to the south. The Canary Current feeds the North Equatorial Current, which itself is driven westwards by trade winds, so completing the gyre.

Today, the Gulf Stream is perhaps the most heavily investigated of any of the ocean’s major currents. The behavior of the Gulf Stream has climatic impacts that resonate across eastern North America, Western Europe and deep into the Arctic Circle.

A River in the Sea

The Gulf Stream is commonly depicted as a well-defined surface current, “a river in the sea.” In fact, the Gulf Stream extends to the ocean floor. From its westerly extent, near 79^oW to 65^oW, the Gulf Stream shifts north-south by as much as 40 kilometers (25 miles) from one year to the next. The limit of the Gulf Stream to the east is less clearly defined because it breaks up into meandering water masses. A substantial part of the Gulf Stream’s volume has disappeared by this point, in part, through the formation of eddies that break off from the main flow. Using infrared satellite imagery and satellite-tracked, free-drifting buoys, these eddies have been the subject of intensive study since the 1970s. In fact, the flow within the Gulf Stream system, particularly at deep levels, is more complex than the forgoing description suggests.

Eddies

One of the most striking features of the Gulf Stream is its interaction with the cold, relatively low-salinity Labrador Current. When the Labrador Current meets the Gulf Stream the interaction of the two currents creates eddies, which spawn rings of water that detach from the main current. These rings may be 50 to 500 kilometers (31 to 310 miles) across. North of the Gulf Stream these rings tend to be at the smaller end of this range and they rotate clockwise and enclose warm water. South of the main current, the rings tend to be larger and rotate counterclockwise and enclose cold water. The northern, so-called warm core rings, contain captured Gulf Stream or Sargasso seawater. The southern, or cold core rings, enclose Labrador Current water. Both kinds of rings may survive for many months and often retain a flora and fauna distinct from that of the surrounding water. For example, in warm core rings, phytoplankton blooms may be characteristic of Sargasso Sea flora. The water within rings is sometimes associated with increased productivity.

Influence of the Gulf Stream

The climatic impact of the Gulf Stream on the U.S. East Coast and northwestern Europe is profound. In both regions, the Gulf Stream warms coastal areas and moderates winter temperatures, raising air temperatures by as much as 5^oC (41^oF).

The Gulf Stream’s great influence on the climate of Europe is due, in large part, to the presence of two oceanic pumps, one in the southern Labrador Sea and the other, more particularly, east of Greenland. These pumps, which draw down warm surface water to the cool depths, help pull Gulf Stream water northeastward across the Atlantic. The Gulf Stream and North Atlantic Drift waters release heat into the cold northern atmosphere at an estimated rate of a trillion kilowatts (1015 W) – equivalent to about a hundred times humanity’s current energy consumption.

The Gulf Stream acts as a wall that prevents the cold, nutrient-rich waters of the Labrador Current from reaching the central Atlantic Ocean. The interaction between the two currents causes nutrient-rich water to remain off the coast of Nova Scotia and Newfoundland where it is warmed, fueling phytoplankton blooms, which are harvested by zooplankton that in turn support populations of larger predators. Important fisheries for cod, haddock and other demersal (bottom-living) fish were located here. Overfishing has heavily depleted these stocks.

Icebergs calved off Greenland’s glaciers drift into Baffin Bay and the Labrador Sea and many are carried south on the Labrador Current. When the Labrador Current interacts with the Gulf Stream, remnant icebergs normally melt rapidly – the temperature differential between the two currents is sometimes in excess of 20^oC (68^oF). The Gulf Stream undoubtedly provides a great service in keeping the central Atlantic Ocean free of most traffic-endangering icebergs. However, not all icebergs melt before reaching major shipping lanes. In April 1912, one such iceberg sank the RMS Titanic.

Many marine species take advantage of the Gulf Stream’s powerful flow and its warming influence. The larvae of the European freshwater eel, Anguilla anguilla, hatch in the Sargasso Sea. They drift on the Gulf Stream and North Atlantic Drift to reach the rivers of northwest Europe. As far north as the Barents Sea, the interaction of warm Gulf Stream water and cold Arctic water creates comparatively warm, nutrient-rich conditions to support phytoplankton production. Here, bowhead whales feed and more than 60 large seabird colonies thrive on Franz Josef Land.

The Future

The North Atlantic pumps, and the Gulf Stream, are parts of the global ocean circulation system (dubbed the “conveyer belt” by oceanographers). Warm surface water is drawn northward throughout the Atlantic. On reaching the Greenland and Labrador seas, the water sinks. This water, the North Atlantic Deep Water, then travels southward at a depth of about 2 kilometers (1.24 miles) and eventually reaches the Southern Ocean.

The pumps are driven by water density differences, which are determined primarily by salinity and temperature. Cold temperatures and high salinities increase density and cause water to sink relative to warmer or less saline water. At North Atlantic pumps, warm surface water arriving from the south is cooled by sub-Arctic air and sinks, so driving the pumps. The temperature of the arriving water, and its salinity, is crucial in maintaining the pumps’ activity. If the arriving surface water is too warm, and it is diluted excessively by rain, rivers and melting snow, then its density might fall sufficiently to stop the pumps working.

Since the early 1980s, concern has been expressed that global warming – the result of increased levels of carbon dioxide and other greenhouse gases in the atmosphere - might affect the pump’s efficiencies. Meteorologists and oceanographers are studying this possibility using climate-modeling experiments. Paleoclimatologists are also searching the Greenland Ice Sheet and accumulated Atlantic seafloor sediment for evidence of climatic shifts in the last few tens of thousands of years. These might be linked to the shutdown of the global conveyor belt system in the North Atlantic. Climate models are being refined, and the evidence for past climatic shifts is still equivocal. The possibility of a southward shift in the Gulf Stream’s influence, plunging northwest Europe into temperatures 5 to 10^oC (41 to 50^oF) lower than those at present, is a very worrying prospect. Such a shift would also have a major impact on marine life in the North Atlantic and parts of the Arctic.