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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 and the Continental Slope

Curious waves coursing beneath the surface of the 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 to the open on them, 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 , 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 , 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- to discover that the water was waves and in the now-missing grees, and their average incline is only gone. After getting over your initial . about 3 degrees. Acting alone, the con- disappointment, you would probably Driving farther, you eventually ar- stant influx of 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 , 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 (as it does, slopes remain low. We believe that a what was in store. for example, off central ) 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 ). Here, where tides: submerged waves largely hidden flat 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 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 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 Institute of of sediments eroded from the interior ocean? The simplest way to grasp this Technology and Woods Hole Oceanographic Insti- of the and deposited along strange concept is first to remember tution. At the time (1970), he was serving as an its watery edge by , 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 instead Cacchione then switched and took up a posi- place to place. There are spots where the of a ; what would happen? There tion as a research oceanographer for the U.S. - 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 —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 & —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 of Gibraltar. This sun- geology at Columbia’s Lamont-Doherty 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 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 and ridges. Deflected by such topographic obstacles, tidal cur- rents can easily generate up-and-down 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) discovered the first ex- amples of internal waves in the 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 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 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 ) with rather homogenous in its physical the balloon back down. But the mo- depth. They then generated a series of properties. Beneath the mixed layer the mentum it gains during descent would internal waves using a mechanical temperature (and hence the density) of make it overshoot its equilibrium po- paddle that oscillated back and forth seawater changes abruptly. (Scuba sition, and the descending balloon about a horizontal pivot centered in the divers are sometimes surprised as they would soon find itself surrounded by water. This geometry produced simple pass from the warmer sunlit surface water that is denser than the water it internal waves in which horizontal wa- zone through the base of the mixed contains. Up then it would go, continu- ter velocities were maximum at the top layer and into to the much colder wa- ing this vertical oscillation until friction and bottom of the tank and zero in the ters below.) The base of the mixed layer brought things eventually to rest. middle. These waves traveled from is usually less than 100 meters deep, With a mental picture of this bob- one end of the tank to the other, where and the interfacial waves that form at bing balloon in mind, it is not hard to a tilting plastic bottom mimicked the such modest depths are not relevant to see how such up-and-down motions continental slope. shaping the continental slope, which could be generated within the ocean. In these experiments, dramatic lies deeper down. The internal waves One source of energy for driving these changes to the internal waves arose that influence the continental slope are oscillations is the daily tides. Although over the simulated slope, just as rather different in that they require no people normally think of the tides as Wunsch’s theory had predicted. When distinct density interface. Rather, they being responsible for changing sea the waves encountered the plastic in- form simply because the water gets level, they also cause current to move cline, their amplitudes grew, their hori- gradually denser with depth. over the ocean bottom, where the zontal wavelengths shortened, and wa-

132 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. mented the universality and power of the internal tides both within subma- rine canyons and over other, more reg- ular areas, where the strong bottom currents presumably have important effects. For example, investigators at the University of Washington moni- tored the influence of internal waves and tides using an instrumented moor- ing that they maintained in about 450 meters of water on the upper continen- tal slope off northern California from 1995 to 1999. Throughout the five years of study, internal tides were consistent- ly responsible for the strongest flows near the bottom. The fastest bottom currents reached 40 centimeters per second—pulses of colder water, which moved upslope as an advancing , mixing the bottom waters with vigor. Oceanographers have found similar goings-on at many other places Figure 3. Novelty wave tank containing two (one clear, one blue) of slightly different such as off Oahu, Hawaii, (at about 460 density demonstrates internal waves forming at the interface. Such waves appear to have meters depth), off Virginia (at about strangely large amplitudes (compared with surface undulations of similar wavelength), and 1,100 meters depth), and off southwest they seem to move slowly compared with water waves. Internal waves can also form even where Ireland (at about 3,000 meters depth, at no distinct interface exists: All that is needed is a vertical gradient in the density of the liquid. the base of the continental slope). The wealth of recent evidence for the ter velocities increased—similar in Proof in the (Mud) Pudding internal creating bottom currents many ways to what one sees when sur- Although Cacchione had studied inter- strong enough to prevent deposition face water waves hit the beach. In some nal waves extensively in the laboratory and sometimes even to erode accumu- instances, the internal waves formed as a graduate student, his first personal lated sediments prompted us to recon- tidal bores (large, turbulent, wall-like experience with their effects took waves with abrupt fronts), which trav- place years later within Hydrographer eled upslope and then broke up. These Canyon, which lies just south of breaking internal waves often caused a Cod, during a dive in NR-1, a nuclear- Moon great deal of and backwash, powered research submarine of the which in the real ocean would scour the U.S. Navy. While Cacchione and noted bottom. marine Bruce Heezen were With their test tank, Cacchione and busy mapping the seafloor and taking Wunsch explored a special “critical” photographs within this canyon (one of condition in which the energy of the many that cut into the continental slope internal waves essentially became in this region), strong currents buffeted trapped in a narrow zone along the in- the vessel. The maximum speed of clined floor of the tank. At the time, these currents was about a half-meter this phenomenon had not been de- per second near the seafloor, and they scribed theoretically, but with their ex- reversed direction every 12 hours or so. Earth perimental investigation, Cacchione Unlike other nuclear submarines, NR-1 and Wunsch discovered that under this has small windows, which allowed the critical condition, water velocities and scientists aboard to witness the period- internal breaking were greatly intensi- ic pulses of muddy water the strong fied along the faux slope, well beyond currents raised. These undersea mud- Figure 4. Tides arise in large part from the what they had observed for other storms obscured the otherwise clearly gravitational influence of the Moon on the experimental runs. In subsequent labo- visible for an hour or so, just as Earth, which forms two broad bulges in the ratory experiments, Cacchione and the tidal currents reached their highest level of the ocean (shown here much exagger- Southard demonstrated that internal speeds. This experience convinced Cac- ated for clarity). These tidal bulges track the waves could resuspend bottom sedi- chione that internal waves could in- Moon as the Earth rotates beneath, account- ing for the approximately 12-hour period of ment and create sediment ripples. See- deed move sediment along the seafloor the tides, which vary in amplitude from place ing these effects, Cacchione, Wunsch and motivated his continued explo- to place—often dramatically—as a result of and Southard proposed that if the criti- ration of this phenomenon. resonant effects. Because they, too, largely re- cal condition were satisfied along conti- Research dives elsewhere, along sult from the influence of the Moon’s gravity, nental slopes in the real ocean, internal with numerous measurements from the internal tides also contain considerable waves might well control sedimentation. moored instruments, have since docu- energy with a roughly 12-hour period. www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 March–April 133 with permission only. Contact [email protected]. sider the idea that this process might way, sending a dense mixture of water have done so, in some parts of the indeed control the geologic evolution and sediment tumbling down subma- globe, for more than 100 million years. of the continental slope. Of course, rine slopes. The rates, frequency and Internal tides have an omnipresence many other natural phenomena are spatial extent of these processes vary that approaches the tug of gravity, known to affect this portion of the sea from one to the making them a prime candidate in our bottom—everything from the tectonic next, which makes it rather difficult to minds for the cause of the low overall movements of the lithospheric plates evaluate their overall influence in gradient of the continental slope. to the episodic scouring action of tur- shaping the continental slope. The in- Two years ago we set out to evaluate bidity currents, essentially underwater ternal tides are different. They pass this notion. Understanding our strategy mudslides, which arise, for example, over continental slopes all around the is not difficult but requires learning a lit- when a portion of the seafloor gives planet, every day of every year and tle more about how internal tides work.

A Critical Evaluation Unlike surface waves, the energy of the internal waves can propagate not only horizontally but also vertically and any direction in between. The an- gle of propagation—”the characteris- tic angle”—depends only on frequen- cy (in this case, two cycles per day), on geographic and on how the water changes density with depth. If Hawaii the characteristic angle is greater than the dip of the seafloor, the energy of the propagates into shal- low water as it bounces between the base of the mixed layer and the seafloor. If the characteristic angle is shallower than the dip of the seafloor, the energy of the internal tide caroms off the base of the mixed layer, then down to the bottom and finally back out to the open sea. Between these two extremes lies the interesting case where the characteris- tic angle is just equal to the slope of the seafloor—the critical condition that Cacchione and Wunsch had studied in the laboratory. Under this special cir- 1,000 watts per meter cumstance, the energy of the internal tides is trapped along the bottom, and current velocities reach a maximum, 15 which tends to keep any sediment that 10 the water holds in suspension. Thus 1 when the angle of propagation match- 5 2 es the dip of the sea bottom, internal 0 tides could prevent sedimentation. 3 The density structure of the is –5 such that the characteristic angle of the 4 internal tides is typically between 2 depth (kilometers) Hawaiian –10

centimeters per second and 4 degrees. Is it merely a coinci- ridge –15 dence that continental slopes around 200100 0 100 200 the world are generally inclined at southdistance (kilometers) north about 3 degrees—that is, at just the an- gle that causes the bottom currents Figure 5. Generation of the internal tides takes place where horizontal currents impinge on un- from internal tides to be at or near their dersea mountains or ridges. One well-studied example of this phenomenon takes place around strongest? We think not. Rather, we the of Hawaii and their submerged extensions to the northwest. Through study of sub- suspect that the internal tides create tle surface effects, investigators have been able to document how internal tidal energy flows away from the Hawaiian ridge (arrows in upper panel). Numerical models reveal that the ener- bottom currents that are sufficient to gy of these internal waves flows along arching beams, indicated by the yellows and reds in the prevent the sediments swept over the lower panel, a north-south cross section for the position indicated with a red line in the upper edge of the shelf from accumulating on panel. (Map at top derived from Ray and Cartwright 2001; diagram at bottom derived from Hol- the continental slope and steepening it loway and Merrifield 2003. Courtesy of the American Geophysical Union.) beyond the local characteristic angle.

134 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. abc surface

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Figure 6. When beams of internal tidal energy strike the continental margins, the result varies. If the characteristic angle of the beam is steep- er than the slope of the seafloor (a), internal tidal energy is transmitted landward as it bounces between the seafloor and the base of the mixed layer. If the characteristic angle is shallower than the slope (b), the energy is reflected back toward the deep ocean. When these two angles are equal (c), the energy is trapped near the seafloor, where it may be sufficient to stir up the sediments or at least to prevent suspended sediments from settling out.

To test this idea, we (along with A. S. The oceanographic conditions in The key question at the outset of our Ogston, a colleague at the University the two areas differ as well. Historical study was whether the characteristic of Washington) examined the corre- measurements show that the seasonal angles in these two places match the spondence between the characteristic differences in the way density varies local dip of the continental slope. Us- angle of the internal tides and the dip with depth over the continental slope ing water temperature and salinity of the continental slope at two places are larger in the Atlantic off New Jer- measurements compiled by the Na- where the ocean bottom has been sey than they are in the Pacific off tional Oceanic and Atmospheric Ad- mapped in excellent detail: off the northern California. As a result, the ministration, we worked out the aver- of northern California and off geometry of the internal tides changes age density structure of the ocean in southern New Jersey. The comparison more over the course of the year off the two study areas and then derived of these two locales is telling because the Jersey shore than it does off north- the characteristic angles for the inter- they differ in several important ways. ern California. nal tides. Remarkably, the mean char- For one, the slope near northern Cali- fornia is part of a narrow continental water surface margin, which is located along the boundary between two tectonic plates that are colliding into each other. These motions continuously deform pivot paddle the seabed and cause frequent earth- wavemaker quakes, some of which trigger subma- internal waves rine avalanches. By contrast, the New Jersey slope is part of a wide continen- tal margin, one located within the rela- bottom slope tively quiet interior of a tectonic plate. here are far weaker and less frequent, and tectonic deformation largely ceased more than 100 million years ago. Another marked difference be- tween the two regions involves the source of their seafloor sediments. Northern California’s Mad and Eel rivers have among the highest sedi- ment yields of all rivers in the Unit- ed States, and a significant fraction of this copious supply makes it onto the nearby continental slope. In con- trast, little sediment accumulates off- Figure 7. Wave-tank experiments clearly show the importance of geometry. Canting the in- shore from New Jersey, where the clined portion of the tank to something less than the characteristic angle of the internal continental slope is covered with waves results in a borelike disturbance, which runs up the slope and then breaks apart very fine sediments eroded from the (lower left). Steepening the angle of the plastic incline to match the characteristic angle of the continental shelf and swept over the internal waves causes a great deal of energy to be dissipated in turbulent movements along shelf edge. the bottom (lower right). www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 March–April 135 with permission only. Contact [email protected]. acteristic angle in each study area perature and salinity in these areas, it imal and deposition dominates. In equaled or nearly equaled the mean was easy enough for us to compute the such settings internal tides would be gradient of the local continental slope: characteristic angles of the internal tides expected to have their greatest influ- Both quantities are about 2 degrees in for a dense array of geographic grid ence, and the close correspondence be- northern California and about 4 de- points and then compare these angles tween the dip of the Hudson Apron grees in southern New Jersey. with the slope of the seafloor at those and the local characteristic angle sug- We went on to examine the corre- same spots. gests that internal tides do indeed con- spondence between the characteristic The results turned out to be more trol the amount of incline one finds on angle of the internal tides and the in- complicated than with our general the sea bottom there. cline angle of the continental slope in analysis, but they nevertheless sup- The continental slope off northern more detail using highly resolved mea- ported our suspicion that internal tides California also provided some impor- surements of the seafloor, which are help to shape the continental slope. In tant evidence. Because of the tectonic available for these two study areas. the New Jersey study area, large sec- deformation going on along the West Members of the U.S. Geological Survey tions of the slope between the depths Coast, portions of the slope in the cen- and academic investigators participat- of 200 and 2,000 meters dip at an angle tral and southern sections of our west- ing in a program of the Office of Naval that is at or nearly equal to the charac- ern study area have large relief, reflect- Research called STRATAFORM (an abbre- teristic angle of the internal tides at one ing the ongoing action of tectonic uplift viation for “STRATA FORmation on time of the year or another. In winter, and folding as well as a history of un- continental Margins”) collected this de- this correspondence even holds up dersea landslides. Despite the presence tailed topographic information using a in the interiors of many submarine of these confounding processes, seg- technique called swath mapping. That canyons, such as the central portion of ments of the slope between 200 and 450 method uses a research vessel equipped , where internal tides meters depth dip at or near the charac- with an array of acoustic transducers are known to cause high-velocity cur- teristic angle for the internal tides. The mounted on the bottom of the hull. rents. Open-slope regions in deeper strength of the bottom currents mea- These devices broadcast energy sections also lie close to the characteris- sured at this site suggests that internal down to the seafloor in a fanlike series tic angle of the internal tides, but at tides can control sedimentation and of beams that extend both to the left and other times of the year. For example, hence the shape of at least some of the right. In this way, technicians on board during summer the characteristic an- continental slope. Admittedly, here can map the depth of the water over a gle matches the slope of the seafloor (and elsewhere on the seafloor) other wide swath centered under the path of over much of the Hudson Apron, a processes are at work too. In particular, the ship. And using both historical and large expanse immediately south of marine geologists have long been well more recent soundings of water tem- Hudson Canyon where erosion is min- aware of the fact that earthquakes trig-

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April 10 April 11 April 12 Figure 8. Portion of the seafloor off the coast of northern California has been mapped in considerable detail (left). Oceanographic instruments moored to the bottom in this region recorded current velocity and water temperature (right), revealing pulses of colder water moving up the slope and then sloshing back down with a 12-hour period (blue arrows). The detailed shape of the traces suggests that the cold current is prob- ably attributable to internal waves forming a tidal bore, which moves rapidly up the slope and then breaks apart. (Seafloor image courtesy of Peter Dartnell, USGS.)

136 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 50 kilometers

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bottom slope/characteristic angle Figure 9. Comparison of the bottom slope and the characteristic angle of internal waves allows the seafloor to be partitioned into three zones: where the bottom is inclined appreciably less than the characteristic angle of the internal tides (blue), where the bottom tilts at roughly this an- gle (green) and where bottom slopes at more than this angle (red). During both winter (top) and summer (bottom), only a portion of the continental slope in the western study area (off northern California, left) satisfies the “critical” condition, for which the incline of the seafloor matches the characteristic angle of the internal tides. For the eastern study area (off New Jersey), oceanographic conditions in winter are such that the relatively steep walls of many submarine canyons are critical (top right); in summer, areas between canyons where the tilt of the bottom is relatively low be- come critical (bottom right). Thus at one time of the year or another, most the continental slope here must experience the critical condition. ger currents from time to Bibliography cific. Geophysical Research Letters 28:1259–1262 Southard, J. B., and D. A. Cacchione. 1973. Ex- time, and these muddy flows can move Cacchione, D. A., and D. E. Drake. 1986. Neph- a lot of sediment into deeper parts of periments on bottom sediment movement eloid layers and internal waves over conti- by breaking internal waves. In Shelf Sedi- the ocean. However, based on the uni- nental shelves and slopes. Geo-Marine Let- ment Transport: Process and Pattern, ed. D. J. versality of the phenomenon and its ters 6:147–152. Swift, D. B. Duane and O. H. Pilkey. persistence over geologic time, we be- Cacchione, D. A., L. F. Pratson and A. S. Stroudsburg, Pa.:Dowden, Hutchinson & lieve that the internal tides deserve to Ogston. 2002. The shaping of continental Ross, pp. 83–97. slopes by internal tides. Science 296:724–727. be credited as a primary influence Thorpe, S. A. 1992. The generation of internal Cacchione, D., and C. Wunsch. 1974. Experi- waves by flow over rough topography of a shaping the overall configuration of the mental study of internal waves over a slope. continental slope. Proceedings of the Royal So- continental slope. Journal of Fluid Mechanics 66:223–239. ciety of London, Series A 439:115–130. We will soon be mounting investiga- Eriksen, C. C. 1982. Observations of internal Wunsch, C. 1969. Progressive internal waves tions of the continental margins in wave reflection off sloping bottoms. Journal on slopes. Journal of Fluid Mechanics 35: other parts of the world to test the cor- of Geophysical Research 87:525–538. 131–145. respondence we have found so far be- Gilbert, D. 1993. A search for evidence of criti- cal reflection on the continen- tween the dip of the slope and the char- tal rise and slope off Nova Scotia. Atmos- acteristic angle of the internal tides. We phere-Ocean 31: 99–122. believe that the results will confirm our Holloway, P. E., and M. A. Merrifield. 2003. On expectations, but, of course, we have to the spring-neap variability and age of the For relevant Web links, consult this be prepared for some surprises. Per- internal tide at the Hawaiian Ridge. Journal issue of American Scientist Online: haps we will find evidence that the of Geophysical Research 108(C4/3126):1–9 simple picture we derived from look- Legg, S., and A. J. Adcroft. 2003. Internal wave breaking at concave and convex continental http://www.americanscientist.org/ ing at California and New Jersey is, in slopes. Journal of Physical IssueTOC/issue/561 fact, not so tidy after all. Or maybe the 33:2224–2246. suggestive correspondence we found Ray, R. D., and D. E. Cartwright. 2001. Esti- will indeed prove to be universal. Only mates of internal tide energy fluxes from time (and, well, tide) will tell. Topex/Pseidon altimetry: Central North Pa- www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 March–April 137 with permission only. Contact [email protected].