Gulf Stream

The Gulf Stream is a geostrophic surface current that constitutes the northwestern part of the North Atlantic Gyre. It moves huge quantities of water at remarkably fast velocities across vast distances, with many geological, physical, and biological repercussions. However, it is not solely responsible for Western Europe's mild climate.

Water Circulation

It is convenient to consider any systematic movement of water at sea as being part of either a wind-driven circulation system or a thermohaline circulation system. In the former, the linkage with atmospheric movement is direct, the currents are usually at or near the sea surface, and the velocities of the flows are often in the range of several centimeters per second (or several knots). In thermohaline circulation, the driving force is gravity, which causes denser water to sink and flow to the deepest parts of the sea, and the currents are usually much slower. In the North Atlantic Ocean, thermohaline circulation occurs on a vast scale as saline surface water gives up its heat, sinks, and eventually flows across the bottom of both the North and South Atlantic basins.

Wind-driven circulation develops a gigantic clockwise circular motion called the North Atlantic Gyre. The most intense flow of this gyre is along its northwestern boundary and is called the Gulf Stream. Because the flow is circular, it does not actually have a beginning or end. As it is usually geographically defined, however, the Gulf Stream begins in the straits of Florida, where water leaving the Gulf of Mexico and the Caribbean joins with water continuing to go around the gyre. There, the Gulf Stream moves about 30 million cubic meters per second past any point. The volume of water entrained in this flow continues to increase, and by the time it reaches Cape Hatteras, North Carolina, 85 million cubic meters are moving by any point every second. When the Gulf Stream reaches longitude 65 degrees west, off Newfoundland's Grand Banks, it moves 150 million cubic meters per second.

To put this in perspective, consider that 1 cubic meter of water has a mass of 1,000 kilograms. A 150-pound person has a mass of 68 kilograms. Therefore, 1 cubic meter of water has the mass of about fifteen people. The flow off Cape Hatteras would be equivalent to 1.2 billion people, only slightly less than the population of China, streaming by every second. When it gets to the Grand Banks, the flow would be larger by 65 million cubic meters, or 975 million more people. The Gulf Stream truly dwarfs most of nature’s other wonders. The total hydrologic cycle, which includes all the rain, snow, sleet, and hail that falls on Earth (oceans plus continents), moves an average of only 10 million cubic meters of water per second.

Coriolis Effect

The secret to maintaining such huge flows of water lies in their circular nature. A “hill” of warm, low-density water sits over the core of the North Atlantic Gyre. Water on or within this hill is driven by gravity or a pressure gradient to move downslope or away from the center of this hill. This moving water is deflected by the Coriolis effect, a consequence of the planet’s rotation. In the Northern Hemisphere, the Coriolis effect causes things moving with a horizontal velocity to move to their right. Therefore, water continuously trying to move out from the center of this hill is deflected to move around the gyre instead. Eventually, a balance is achieved between the Coriolis effect and the pressure gradient forces. As a result, the water does not move away from the center of the hill but instead circles clockwise. This kind of current is called a “geostrophic” current.

Earth’s spherical shape means that the Coriolis effect varies with latitude. One result is that the flow within the North Atlantic Gyre varies with location. The hill is not symmetrical but has its steepest slopes on its northwest edge. Because currents must balance slopes, the gyre is most intense there. The center of the gyre is in the western Atlantic, not far from Bermuda. The hill also slopes away to the east, but very gradually, so that the southern flows of the gyre are slow and spread out over a very large region.

Driving the gyre and maintaining the Bermuda High are the winds over the Atlantic Ocean. This subtropical atmospheric feature is a region of descending, dry air near the center of the North Atlantic Ocean. After descending, this air pushes out across the ocean. It, too, is deflected by the Coriolis effect, developing into somewhat circular, clockwise winds. Near the center of this system, winds are weak, and precipitation is uncommon. Early sailors, faced with long, dry periods of calm, sometimes made their thirsty horses walk the plank. This is the origin of the term “horse latitudes,” sometimes used to describe this region.

With little cloud cover and a subtropical latitude, this region receives intense solar radiation that warms the surface water and causes intense evaporation. This causes the water of this area, the Sargasso Sea, to become extraordinarily saline and very warm. Just as oil floats on vinegar in a salad dressing, this warm and correspondingly less dense, saline water floats on top of the cooler, denser water below.

It is this warm water that forms the hill driving the Gulf Stream. The warm water tries to spread out and flow over cooler surface waters far from the center of the gyre. The Coriolis effect makes it move around the hill, not down it, and the boundary between this warm, rotating mass of water and the cooler water it is trying to flow over is where the currents are most obvious. This is the Gulf Stream, a distinct boundary between the productive coastal waters, green and teeming with life, and the dark blue, nearly lifeless Sargasso Sea waters.

Gulf Stream as a Boundary Current

For decades, schoolchildren have been taught that the mild climate of the British Isles and Western Europe gets its heat from the Gulf Stream. The Gulf Stream is often presented as a river of warm water moving north and then east, eventually delivering its heat to the European continent. The significance of the Gulf Stream as a source of heat changes when it is recognized for what it is: just a boundary current. It separates the huge hill of warm, saline Sargasso Sea water from the colder surrounding waters.

The Gulf Stream is a very active boundary region with motions driven by the wind and geostrophic currents surrounding the vast quantity of surface water, heated by the sun in subtropical high-pressure zones and made especially salty by accompanying evaporation. The Gulf Stream gets all the press, but this vast quantity of water conveys heat to Europe.

In the North Atlantic, cold winds remove heat from the surface waters beyond the northern extent of the circulating gyre. These winds are warmed in the process and bring pleasant temperatures to Europe. However, by removing heat, these winds cool the saline surface waters until they become dense enough to sink to the bottom of the sea. This is called thermohaline circulation. A gradual northward flow of surface water replaces the sinking waters. Most of this surface water likely spent time in the North Atlantic Gyre, but its transport northward was independent of that circular motion. If the gyre were to stop tomorrow, the thermohaline circulation would continue, and Europe would stay just as warm as it is today. Some researchers have suggested that the strength of the Gulf Stream actually reduces the warming effects of this thermohaline circulation. If they are correct, were the Gulf Stream to stop, Europe might grow warmer.

Although its role as a heat-delivery system may have been overstated, the Gulf Stream is still an incredibly powerful element of oceanic circulation and dramatically influences the surface ocean's biology and chemistry. This significance is easily seen where meanders develop on the Gulf Stream, typically beyond Cape Hatteras. Just as meanders can grow and develop on a slow-moving river, they also develop on the Gulf Stream. Whereas a stream meander may form an oxbow lake if its course closes in upon itself, a meander in the Gulf Stream produces a circulating eddy separated from the rest of the stream when it closes upon itself. These eddies will have a warmer, less fertile water core and rotate clockwise if they close off on the southeastern side of a meander. They will have a cooler, more fertile water core and rotate counterclockwise if they close off on the northwestern side of a meander. These rings persist for months to a year or more and may establish their own ecosystems during their lifetimes.

The Gulf Stream disperses eggs, seeds, and juvenile and adult organisms. As a chemical agent, it stirs up the surface waters, keeping its warm waters well mixed. As a physical agent, it moves enormous quantities of water. It is a remarkable current and an essential part of the global ecosystem.

Study of the Gulf Stream

The Gulf Stream is studied by directly measuring the strength of its currents at different depths and locations and by examining their effects by monitoring the position of floats released within it and designed to stay at particular depths.

Floating current meters can be moored to anchors at the bottom of the sea. Their depth is controlled by the length of the tether, which keeps them attached to the anchor. They can record data electronically, storing it in computer memory. When a vessel is at the surface, ready to retrieve the meter and its data, it transmits a special coded sound pulse. This instructs the meter to release itself from the tether and rise to the surface, where it transmits a radio signal, allowing the vessel to home in on it for recovery. The data are incorporated in complex computer models that combine the results obtained from hundreds of current meters deployed during overlapping periods. Snapshots of the current system can then be obtained, and sequences of these snapshots reveal the behavior of the currents over time.

Floating objects can be released at sea and tracked by satellite. To ensure that ocean currents rather than surface winds are moving these objects, they usually have a large parachute or sail deployed in the water beneath them. Because the density of seawater increases with depth, floats are often designed to have neutral buoyancy at a particular depth (neither sinking below nor floating above that depth). A layer of the ocean (the SOFAR channel) acts as a wave guide for sound waves. Floats in this layer can transmit sounds over tremendous distances, permitting them to be tracked efficiently by a small number of surface ships with sonar receivers suspended into this layer.

Because geostrophic currents are driven by the slopes of the ocean’s surface, any technique that can measure those slopes can provide valuable insight into the driving forces behind the Gulf Stream. These slopes are very gradual—total dynamic relief over all the world’s oceans is about 2 meters—consequently, direct measurement is difficult. Satellite techniques, coupled with computer models to filter out waves, tides, and dozens of other confounding effects, are approaching where they can measure this topography. Yet this dynamic topography is generally determined indirectly by measuring temperature and salinity as a function of depth and position. These data are used to determine the density of the seawater as a function of depth and position. By assuming that the horizontal pressure gradients have disappeared at some depth, it is possible to reconstruct the differences in the height of the water column needed to accommodate these variations in density. Then, the velocities and directions of the resulting currents can be calculated. The results agree when the theoretical models are compared with currents measured by moored meters or revealed by the paths of floating objects. This strongly supports the theoretical concepts underlying the study of ocean currents.

Many of the approaches used to study the Gulf Stream are exercises in applied mathematics. That computed and measured results agree so well is a triumph of geophysical fluid dynamics.

Significance

The North Atlantic Gyre, with the Gulf Stream as its northwest boundary current, dominates surface flow in the Atlantic Ocean. This system and its winds affected voyages of discovery, exploration, conquest, and exploitation to some extent.

The Gulf Stream, studied since the eighteenth century, has provided the basis for much of what scientists know of ocean currents. Elaborate mathematical constructs, including the entire concept of geostrophic currents, have been developed to describe, analyze, and comprehend this powerful system.

As scientists have learned more about the Gulf Stream, they have discovered many new areas of study, including branches in the stream, countercurrents at the surface and at depth, and fluctuations in flow and velocity with time and place. Some researchers devote their entire careers to studying just the rings of the Gulf Stream, which contain entire ecosystems. Others investigate the location and strength of the Gulf Stream in the distant past, during and even before the ice ages. There is evidence that, at times, the current has taken different paths across the continental shelf, perhaps scouring out valleys in the ocean floor in the process.

Scientists’ understanding of the planet's dynamics relies on comprehending the transfer of energy from the equator to the poles. The Gulf Stream is an important component of this transfer. As people have become more aware of the fragility of the environment and become more concerned about global climate change, the role of the Gulf Stream and thermohaline circulation in influencing temperatures in Europe and elsewhere on the planet has taken on a new importance.

The Gulf Stream's critical role in global climate change has become increasingly apparent as the twenty-first century progressed. Studies indicate that the currents in the Gulf Stream are weakening, slowing down 4 percent over the previous four decades. This weakening could slow the flow of warm water to Europe and drastically affect its climate, making it substantially cooler. Although dwarfed by the rise in sea levels due to melting glaciers, the weakening Gulf Stream current could also lower sea levels while increasing the intensity and frequency of hurricanes. Although anthropogenic climate change plays a clear role in changes in the Gulf Stream, some scientists suggest the changes could also be partly due to natural climate variations. 

Principal Terms

Coriolis effect: an apparent force acting on a rotating coordinate system; on Earth, this causes things moving in the Northern Hemisphere to be deflected toward the right and things moving in the Southern Hemisphere to be deflected toward the left

geostrophic current: a current resulting from the balance between a pressure gradient force and the Coriolis effect; the current moves horizontally and is perpendicular in direction to both the pressure gradient force and the Coriolis effect

gyre: the major rotating current system at the surface of an ocean, generally produced by a combination of wind-generated currents and geostrophic currents

pressure gradient: a difference in pressure that causes fluids (both liquids and gases) to move from regions of high pressure to regions of low pressure

thermohaline circulation: a mode of oceanic circulation that is driven by the sinking of denser water and its replacement at the surface with less dense water

wind-driven circulation: the surface currents on the ocean that result from winds and geostrophic currents

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