One of the most powerful and influential of the world’s ocean currents, the Gulf Stream received its name early-on due to its apparent origin in the Gulf of Mexico. The stream, however, is only part of a huge oceanic circulation system in the North Atlantic that is driven by surface wind and the spin of Earth.
Signs and indicators of the stream are available to the offshore sailorsuch as a row of moist cumulus clouds that can appear over its northern or western edge, or the change in water temperature. But the entirety of the stream itself, at least before the age of high-flying jets and satellites with infrared cameras, has rarely been visible to the human eye.
John Elliot Pillsbury, who in the late 1800’s may have been the first seagoing scientist ever to anchor a research ship in the depths of the Gulf Stream, wrote that man cannot truly appreciate the incredible power of this current without being fixed to the bottom within it.
“In a vessel floating on the Gulf Stream, one sees nothing of the current and knows nothing but what experience tells him,” Pillsbury wrote. “But to be anchored in its depths, far out of sight of land, and to see the mighty torrent rushing past at a speed of three to four miles per hour, day after day, one begins to think that all the wonders of the world combined cannot equal this one river in the ocean.”
Early American scientists and observers of the Gulf Stream Benjamin Franklin, Alexander Bache, J. Bartlett, Matthew Maury, Pillsbury, and others soon came to realize that the Gulf Stream had been misnamed. To Pillsbury, aboard his research vessel Blake anchored in thousands of feet of water in the Yucatan Channel, it was immediately obvious that the driving forces of this current had origins stemming far beyond the Gulf of Mexico. In fact, thanks to revolutionary work done earlier this century by Henry Stommel, a scientist at the Woods Hole Oceanographic Institute (WHOI), the Gulf Stream is well known today to originate in the southernmost regions of the North Atlantic as an offshoot or continuation of what is known as the North Equatorial Current. The streamtransports about 3.99 billion cubic feet of water per second. The Gulf Stream transports more water than all the rivers of the world combined, with a flow equal to 1,000times that of the Mississippi River. In the Straits of Florida the stream extends all the way to the bottom, sweeping it clean; while out in the Atlantic, the stream may extend as deep as 1,500 fathoms.
With speeds varying from two to five knots, the currents that are the source of the stream flow northward from the equatorialregions, through the Yucatan Channel and into the Gulf of Mexico. The stream exits the gulf and squeezes through the Straits of Florida before flowing along the southeast coast of the United States until it turns offshore around Cape Hatteras and begins to meander and form circular eddies.
The Gulf Stream is driven via a combination of prevailing winds in the North Atlantic and density differences. It is shaped by the presence of continents in the west and the tremendous body of water known as the Sargasso Sea in the east. It is a piece of the great North Atlantic Gyre, a series of currents that circle the North Atlantic Ocean. And it is influenced by apparent forces first defined by great scientists such as Coriolis and Ekman.
Driving forces
“A good estimate of the forces driving the stream would put the wind at 65 percent and what is known as thermohaline circulation at 35 percent,” explained George Maul, a Miami-based oceanographer for NOAA. “Even if there were no winds, there would be a Gulf Stream in some form. For the most part, though, we think of it as a wind-driven current.” (Thermohaline circulation involves vertical movement of water due to temperature/density differences. More on that below.)
Take a moment to picture prevailing winds in the North Atlantic. It’s easy to see that winds essentially blow in a circle around the North Atlantic. The Northeasterly trade winds drive water to the west and, to a lesser extent, south between latitudes 10° N and 30° N. From 30°N, winds take a turn, blowing to the north and east in the belt known as the prevailing westerlies. At the center of this circulation, above the Sargasso Sea, is a dominant high pressure area.
“Surface friction will cause winds to move water at a speed of roughly one tenth of the wind’s speed,” said Steve Baig, a scientist at the National Hurricane Center in Florida. “On the simplest level, there is a balance set up between wind, friction of water, and Coriolis force that requires an intensified flow on the western boundary of the North Atlantic Gyre.”
“Think of it this way,” said Maul, “the westerlies are trying to pull water away from North America in the north, while the easterlies are trying to shove it back in the south.”
“The winds are responsible for making the ocean spin,” added Nelson Hogg, a scientist at WHOI. “They don’t, however, have anything like the Gulf Stream in them. There are other influencing factors.”
The other primary contributor to the stream is the large-scale density differences between water in the tropics and water off the coast of Nova Scotia. Waters of different latitudes have different pressures due to variations in temperature. Consequently, a body of water will act in much the same manner as a body of air. Areas of warm, high pressure tend to move to fill an area of colder, lower pressure. Circulation is very similar to that in the atmosphere.
“Water gathered in the south can be as much as 30° or 40° Fahrenheit warmer than that farther north; and warmer water, being less dense, tends to stand higher,” explains oceanographer Maul. “Sea level is as much as a foot higher down south than up north, where dense water cools and sinks. The result is a pressure gradient, and warm water flows northward downhill.” Maul pointed out that one could think of water pressure as correlating directly to height (dense water = low pressure), with minor influencing factors. The resulting circulation of water that contributes to the Gulf Stream is known as thermohaline circulation. Water in the tropics warms and rises while water farther north cools and sinks, leaving a void to be filled. Warm water flows north on the surface. The colder water flows south at a speed of around a knot at great depth, crossing under the Gulf Stream. It slowly warms again and rises, sometimes rejoining the stream after travelling hundreds of miles throughout the oceans of the world.
“You can actually think of a water particle that sinks at the top of the Gulf Stream as circumnavigating the globe,” explained Maul. “Eventually, after visiting the Pacific and Indian Oceans it rises to the point that it feeds back into the Gulf Stream around the equator.” The result is a vertical circulation system in the western North Atlantic or, as Hogg likes to label it, an “overturning cell.”
“In an open ocean, thermohaline differences have to be accounted for,” said Baig. “The ocean always tends toward uniform thermohaline conditions.”
Coriolis effect
There are other influences that make the Gulf Stream the fastest and most defined current of the entire North Atlantic Gyre. One cannot discount the importance of the Coriolis effect, which, among other things, is responsible for causing the prevailing winds which push the gyre. The Coriolis effect is viewed by many as a “force,” but it is, in fact, an observed result of Earth’s spinning on its axis. The best way to conceptualize this is to use an example.
Imagine two locomotives traveling due east on parallel tracks that are 100 feet apart. One engine moves at five miles an hour and the other locomotiveon which is perched a man with a baseballis moving at 20 miles an hour. Imagine, also, that both engines are momentarily abreast. If the man with the baseball attempts to toss the ball through the engineer’s window on the slower engine, he will miss. The ball will appear to curve to the right and pass ahead of the slower locomotive. Seen from an airplane overhead, the ball will move in a straight line, but to observers on both engines, the ball will appear to follow a curved path.
Now imagine throwing a ball from Bogotá, Colombia, directly north to New York City. Because the earth is rotating counterclockwise (when viewed from above the North Pole), both Bogotá and New York are moving east. Bogotá, being closer to the equator, moves faster than New York, since it has a longer distance to cover in 24 hours.
The ball thrown from Bogotá to New York will travel through the air in a straight line, while the earth rotates beneath it. The ball will have an acceleration northward equal to that at which the thrower released it and an acceleration eastward equal to that velocity at which Bogotá is moving. Therefore, the ball will speed north in relation to the earth and east in relation to New York, which is also moving east, but at a slower rate. The ball curves to the right.
A northerly flowing current of water (one which meets negligible resistance) will arc to the right as it gets to higher latitudes. Furthermore, the current will arc to the right more rapidly the farther north it gets because the earth’s surface is moving more slowly since it has much less ground to cover every 24 hours. Thus, the Coriolis effect, or the effect of the earth’s rotation, becomes more pronounced with an increase in latitude.
“Water does what it does because it is a moving fluid on a rotating sphere,” said Baig. “One does not need the thermohaline circulation for the stream. The thermohaline contribution just gives it a third dimension.” Hogg confirmed this, “If the earth were not spherical, there would be no Gulf Stream.”
Spiraling with depth
Just like wind moving north curves to the right, so does water. The Gulf Stream picks up energy from this “push” by the earth’s rotation. The Sargasso Sea is a body of water within the North Atlantic Gyre that sailors have often cursed for its characteristic lack of wind. It maintains this feature predominantly because of a principle called the Ekman Spiral, after the physicist who first pointed out its existence. This principle maintains that the net flow of water is 90° to the direction of the wind moving it. Again, an analogy offers a better understanding.
Imagine that the ocean 100 miles east of New York is a still body of water: no currents. Then, someone turns on a prevailing southwesterly wind. After about 12 hours, developing waves will cause the water to move. Moving water will be affected by the Coriolis effect in the same way a ball wouldit will tend to the right in the northern hemisphere.
On the surface of an ideal ocean (one which has no other forces acting upon it), water tending to the right will flow roughly 45° to the wind direction. Over time, lower levels of water will begin to move as well because of internal friction between molecules. Each layer of water will move in a direction 45° to the one above it (just like the surface layer flows at 45° to the wind). Of course, with an increase of depth, each layer moves more slowlyjust like a surface current does not flow at the same speed its influencing wind blows, but instead it flows at about one tenth that speed, according to oceanographer Baig. At a certain depth (about 100 meters), the direction of flow, now very slow, is opposite to the wind direction. Thus, the net flow of water is 90° to the wind direction.
“Although the Ekman Spiral is represented in a simple equation dependent on ideal conditionsno outside wind or currentwe believe it applies . . . although it is probably complicated by real life out there,” said WHOI scientist Hogg.
While the prevailing winds are circling the North Atlantic, the Ekman Spiral is making currents flow into the center. That gives rise to the “hill” of water called the Sargasso Sea. The sea level of the Sargasso Sea is almost three feet above that along the North American east coast.
Density differences arising from the temperature differences between water in the middle of the ocean and water along the edges keeps water within the Sargasso Sea. “That water piled up by the wind could just as easily flow out underneath but for the need to maintain a pressure balance,” explained Steve Baig. “It is like putting lighter, less dense water opposing colder, dense water in a U-tube. More warmer water will be needed to maintain a pressure balance and its level will be higher.”
“A pressure gradient develops between the hill and its edges, and that sets the water into motion.” explained George Maul. “The Coriolis effect coupled with the wind patterns sets the whole of the Sargasso Sea into a lazy clockwise motion.” Maul added that the waters of the Sargasso Sea could be likened to those within an estuary which is affected by a wind blowing up it. “The water level at its top would be higher than that at the river mouth. It is water that wants to run out, but cannot.” The result is stored energythat causes the water to move in a clockwise direction.
“In the mean, the Sargasso Sea is just at the center of a slowly spinning wheel,” said Steve Baig.
While Maul feels that this clockwise rotation is a re-enforcement to an existing gyre, Baig points out that perhaps its influence is negligible. “We know the Sargasso Sea to be the eastern boundary of the stream where the flow of water becomes uniform,” said Baig. “But, some physical models I have seen have created the Gulf Stream where there is no Sargasso Sea. They use a fixed boundary, or wall, as its inner limit.”
Entrained water
Nelson Hogg points out that much of the Gulf Stream, especially farther north, is fueled by horizontally recirculating water. “There is a component that we call Ekman pumping which is the movement of water from the center of the Sargasso Sea to the southwest at a slight downward angle,” said Hogg. “Water recirculates because the Sargasso Sea moves it to the southwest and the Gulf Stream picks it up and takes it north again. The Sargasso Sea is not a uniform spin.” In fact, as Maul points out, the Gulf Stream carries about 900 million cubic feet of water per second off Miami, but carries as much as four billion cubic feet of water per second between Bermuda and New York. The stream is narrow and well defined in southern areas, while it spreads out and sheds many eddies farther north.
Hogg disagrees with Maul and thinks that perhaps the clockwise circulation of the Sargasso Sea is not as contained. “There is a pressure difference in the Sargasso Sea,” explained Hogg, “but it is not great enough to cause that kind of circulation. The water tries to flow out toward uniform temperate conditions, but it gets caught in the stream and turns to the right with the Coriolis effect.”
Perhaps the least mentioned influence to the stream is the effect of the continents. The Gulf Stream essentially flows along the edge of North America from Florida to Nova Scotia, although it spreads offshore after Cape Hatteras. North America and, to a lesser extent, the Bahamas, dictates the stream’s path. For example, the stream is constricted in the Straits of Florida, and the east coast of North America contains the stream’s flow. “Without the continents, there would be no Gulf Stream,” explained Maul. “The fact that the ocean has boundaries is a critical thing shaping the Gulf Stream. Continents create weather and affect wind. Without them, due to prevalent wind conditions, I suppose you would only have a westward flow in the south and a similar flow eastward in the north.”
It is typically understood that the Gulf Stream terminates where the water begins to cool and sink. This occurs approximately where the stream meets the Labrador Current. From there, according to Maul, oceanographers like to believe that a new system begins. This new system, known as the North Atlantic drift, is very similar to the Gulf Stream since it is fueled by winds and density differences in water. “There are temperature differences in water between New England and Greenland, but you do not see the same vertical circulation as in the Gulf Stream,” explained Maul. “It is really a one-way system.”
Steve Baig was quick to point out that ocean circulation is very complicated. “If you looked at satellite imagery, you would see currents of all different kinds,” said Baig. “We are just talking about the general forces which contribute to and shape the path of the stream.”
Thus, the ocean currents exist because of prevailing winds and thermohaline effects.
