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Internal Tides and the Continental Slope A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Internal Tides and the Continental Slope Curious waves coursing beneath the surface of the sea may shape the margins of the world’s landmasses David A. Cacchione and Lincoln F. Pratson or many of us, a drive to the posing the continental shelf to the open on them, sediments piled underwater Focean is, at the very least, a yearly air (truly). Here and there on this large- can easily maintain stable slopes of 15 pilgrimage. Imagine for a moment ly flat plain, you could happen on degrees or more. Yet some 80 percent what it would be like if, during one fields of ripples or maybe even dunes, of the continental slopes around the of these trips, you arrived at the which were created by the motion of world dip downward at less than 8 de- beach to discover that the water was waves and current in the now-missing grees, and their average incline is only gone. After getting over your initial ocean. about 3 degrees. Acting alone, the con- disappointment, you would probably Driving farther, you eventually ar- stant influx of sediment eroded from decide that exploring what had been rive at the edge of the continental shelf. the land would tend over time to steep- hidden territory would be just as Depending on where you are in the en the grade, so, clearly, one or more much fun as playing in the surf. So world, this change could come just 5 natural forces are ensuring that these you’d drive on cautiously, wondering kilometers out from shore (as it does, slopes remain low. We believe that a what was in store. for example, off central Chile) or more primary factor—one not recognized be- At first it’s easy going: You are tra- than 100 kilometers (the situation for fore—is something called the internal versing the continental shelf—a nearly much of the East Coast). Here, where tides: submerged waves largely hidden flat plateau made of sediments eroded the ocean used to be more than 100 me- from view that pulse through the body from the land. To the left and right, you ters deep, you stop your car and get of the ocean with the same twice-daily might spy relict river channels, which out to take in the magnificent view. Be- period as the normal lunar tides. were formed during former ice ages fore you is the continental slope, which when the buildup of glaciers lowered descends some 3 kilometers vertically Waves Beneath the Waves sea level by more than 100 meters, ex- as it ramps downward toward the Waves moving along the surface of the abyssal ocean floor. This incline repre- sea are easy enough to understand, but David A. Cacchione received a Ph.D. in oceanog- sents the seaward face of the vast pile how can there be waves within the raphy (jointly) from the Massachusetts Institute of of sediments eroded from the interior ocean? The simplest way to grasp this Technology and Woods Hole Oceanographic Insti- of the continent and deposited along strange concept is first to remember tution. At the time (1970), he was serving as an its watery edge by rivers, waves, cur- that surface waves are, in fact, travel- officer in the U.S. Navy. He continued to work for rents and, in some regions, glaciers. ing along the interface between two the Navy until 1975, becoming director of the As with the continental shelf, the sur- fluids: air and water. Now imagine that Boston branch of the Office of Naval Research. face of the continental slope varies from the upper fluid were a liquid instead Cacchione then switched coasts and took up a posi- place to place. There are spots where the of a gas; what would happen? There tion as a research oceanographer for the U.S. Geo- shelf drops off precipitously and where could still be waves at the interface, but logical Survey in Menlo Park, Calif., where he is currently a Senior Scientist Emeritus. Since retir- the descent to the base of the slope they would act somewhat strangely. If, ing from government service in 1998, Cacchione is quite rugged. This is generally the for example, the upper liquid is only a has remained busy, conducting research and con- case where submarine canyons—some little bit less dense than the lower one, sulting on problems affecting the marine realm of which are larger than the Grand the waves at the interface would seem through a company he founded: CME (Coastal & Canyon—cut into the continental slope. Marine Environments). Lincoln F. Pratson re- But between these chasms, the ground Figure 1. Oceanic internal waves rarely mani- ceived a Ph.D. in geology from Columbia Univer- falls away much more gradually: If you fest themselves to the naked eye, but one sity in 1993. He continued his research in marine were to climb back in your car and dri- place where they can be seen under suitable conditions is the Strait of Gibraltar. This sun- geology at Columbia’s Lamont-Doherty Earth Ob- ve down such a slope, it would be like glint photograph, taken from the Space Shut- servatory and then in 1996 moved to the Universi- descending a steep highway pass out of ty of Colorado. Two years later Pratson joined the tle in 1984, shows alternating bands of light Nicholas School of the Environment and Earth the Rockies or the Appalachians. and dark, which arise because internal waves Sciences at Duke University, where he remains on What is perplexing (and thus in- forming here influence the roughness of the the faculty. Address for Cacchione: CME, 844 triguing) for marine geologists is the surface above. This view is from the north, Newport Circle, Redwood City, CA 94065. question of why the continental slope is with Spain at the lower right and Morocco at Internet: [email protected] not any steeper. When only gravity acts the upper left. (Courtesy of NASA.) 130 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 March–April 131 with permission only. Contact [email protected]. flow sometimes encounters submarine continental shelf continental slope mountains and ridges. Deflected by such topographic obstacles, tidal cur- rents can easily generate up-and-down continental rise motions, which propagate away from the source in great waves that share the same 12-hour period as the familiar twice-daily ocean tides. These waves constitute what oceanographers call the semi-diurnal internal tides. The pioneering Norwegian ocean- ographer (and noted humanitarian) Fridtjof Nansen discovered the first ex- amples of internal waves in the Arctic basin at the end of the 19th century. But not until the 1960s did oceanographers become aware that internal waves (and tides) are ubiquitous, that they contain huge amounts of energy and that they regularly mix ocean water as they abyssal plain slosh along the continental slopes. It was during that lively decade—a phenomenally influential one for Earth science—that one of us (Cacchione) and his Ph.D. advisors at the Massachusetts Figure 2. Continental margins, like that of the northeastern United States, can be divided into Institute of Technology, Carl Wunsch the continental shelf, the continental slope, the continental rise and the abyssal plain. Sub- and John Southard, first postulated that stantial vertical exaggeration is, however, required to recognize these zones in this shaded- internal waves could, over geologic relief perspective (top). Seeing the same area without such distortion (bottom) reveals that the time, act to shape the seafloor. Wunsch, actual topography of the seafloor is, in fact, quite modest. The steepest inclines, found on the a prominent physical oceanographer, continental slope, are typically just a few degrees. had done calculations predicting that internal waves moving up undersea to have bizarrely large amplitudes, To help understand how internal slopes must cause strong currents and, intriguingly, they would appear waves can arise in such situations, you along the bottom. But at the time to move in slow motion. (These same might imagine for a moment tagging a Wunsch’s theory needed to be tested effects can be clearly seen in the little small parcel of seawater in some way, under controlled laboratory conditions, wave tanks sold at novelty stores, usu- perhaps by filling a balloon with water something Cacchione did as part of his ally next to the lava lamps). at some depth in the sea and then al- thesis work. Similar interfacial waves exist within lowing it to float freely at that level. That exercise required a rectangular the ocean. They often form at the base What would happen then if you were tank that simulated the real ocean. of what oceanographers call the mixed to lift the balloon upward where the Wunsch and Cacchione thus made layer, the topmost stratum of the sea, water is less dense? Because the water sure that the water in the tank had an which, being churned around con- that fills the balloon is denser than its increasing concentration of salt (giving stantly by the wind and waves, is new surroundings, gravity would pull the water increased density) with rather homogenous in its physical the balloon back down.
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