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What causes the Gulf Stream?

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Benjamin Franklin published this map of the Gulf Stream in 1769.
Benjamin Franklin published this map of the Gulf Stream in 1769.

By Bob Baldwin

Benjamin Franklin created the chart above to show the mysterious river of water flowing across the Atlantic from North America to Europe. Believed to originate in the Gulf of Mexico, this river became known as the Gulf Stream, and Franklin’s depiction was intended as a guide for American sea merchants to reduce travel times to and from Europe. Unknown to him at the time, the Gulf Stream is part of a larger, year-round clockwise current system that flows around the perimeter of the Atlantic Ocean.[1]

Surface currents like the Gulf Stream are created by wind and molded by earth’s rotation. As strong winds blow across any body of water, friction with the ocean’s surface causes the water to move in the same direction as the prevailing wind.

As the wind becomes stronger and more sustained, this surface friction becomes greater and the water moves faster. If the wind remains constant over long periods of time, a surface current is generated that flows until it meets an obstruction, like the shoreline in an enclosed lake, where it is deflected in both directions parallel to the shore. If this wind-driven surface current were blowing across the south end of a lake and a similar wind blew in the opposite direction across the north end, a circulation of surface water called a gyre would be created around its perimeter. This gyre would consist of the north and south currents moving in opposite directions across the lake, deflected on opposite shores and running parallel to the shoreline as boundary currents pulled by the water displaced by each opposing wind. Further, as the gyre currents circulate and move water from different areas of the lake, they can take on different temperatures and densities than the water around them, becoming easily distinguished in the lake by their flow, color, and temperature characteristics.[2]

Similarly, the Gulf Stream is a boundary current in the North Atlantic clockwise gyre, in part driven by the year-round northeast trade winds that blow from Africa to North America at the equator (the North Equatorial Current between 5o and 25o N) and the prevailing westerlies that blow from North America to Europe (the North Atlantic Current) in the higher latitudes[3] (Incidentally, winds are always described for the direction from which they come, so a “westerly” wind comes from the west and a northeast trade wind comes from the northeast). The North Equatorial Current flows west into the Caribbean and Gulf of Mexico from Africa and exits the Gulf between Florida and Cuba. From here, it wraps around the Florida panhandle and flows northeast along the east coast of the United States as the Gulf Stream boundary current. Because the Gulf Stream carries water from equatorial regions, it is warmer and has more saline than the Atlantic Ocean water within which it flows, having a deep blue color that is easily distinguished from the surrounding green of the Atlantic.[4] It varies from 49-93 miles (80-150km) wide and 2,600-4000 feet (800-1200m) deep, with a surface speed between 2-5 knots, and moves incredible volumes of water, as much as 150 Sverdrup at 65W longitude.[5] (What is a Sverdrup?) To gain an appreciation of its volume, consider that about 4 billion tons of water flows past Miami every minute in the Gulf Stream, which is more than 100 times the volume of flow of the Mississippi River.[4]

The Gulf Stream current flows northeast and eventually joins the North Atlantic current which is driven by prevailing westerly winds. The North Atlantic current flows eastward until reaching Europe, where part of it is deflected south and forms the Canary Current. This eastern Atlantic boundary current flows south along the coasts of Europe and Africa until it reaches the equator and rejoins the North Equatorial Current, thus closing the North Atlantic gyre. Interestingly, the Gulf Stream’s transport of warm, equatorial water to the North Atlantic has a warming influence on the climate of Northern Europe, where temperatures are warmer than would be expected at such high latitudes. Similar gyres form in every ocean basin in the world,[7] providing knowledgeable sailors throughout history oceanic highways to get to where they want to go.

 

SOURCES:
[1] Dale E. Ingmanson and William J. Wallace, Oceanography: An Introduction, 5th ed. (Belmont: Wadsworth Publishing, 1995). 156.

[2] Ibid., 150-55.

[3] Ibid., 154.

[4] Ibid., 156.

[5] Ibid., 154.

[6] Ibid., 156.

[7] Ibid., 152.

Briney, Amanda. “The Gulf Stream: The Warm Ocean Current Flows from The Gulf of Mexico into the Atlantic Ocean.” Accessed October 12, 2015. http://geography.about.com/od/physicalgeography/a/gulfstream.htm.

NOAA. “The Seaward Deflection of the Gulf Stream at the Charleston Bump.” Last modified December 12,2006. http://celebrating200years.noaa.gov/magazine/charleston_bump/#drift.

 
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