FEATURE ARTICLE
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 Cacchione, Lincoln Pratson
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 angle of propagation—"the characteristic angle"—depends only on frequency (in this case, two cycles per day), on geographic latitude and on how the water changes density with depth. If the characteristic angle is greater than the dip of the seafloor, the energy of the internal tide propagates into shallow 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 characteristic 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 circumstance, the energy of the internal tides is trapped along the bottom, and current velocities reach a maximum, which tends to keep any sediment that the water holds in suspension. Thus when the angle of propagation matches the dip of the sea bottom, internal tides could prevent sedimentation.
The density structure of the oceans is such that the characteristic angle of the internal tides is typically between 2 and 4 degrees. Is it merely a coincidence that continental slopes around the world are generally inclined at about 3 degrees—that is, at just the angle that causes the bottom currents from internal tides to be at or near their strongest? We think not. Rather, we suspect that the internal tides create bottom currents that are sufficient to prevent the sediments swept over the edge of the shelf from accumulating on the continental slope and steepening it beyond the local characteristic angle.
To test this idea, we (along with A. S. Ogston, a colleague at the University of Washington) examined the correspondence between the characteristic angle of the internal tides and the dip of the continental slope at two places where the ocean bottom has been mapped in excellent detail: off the shores of northern California and off southern New Jersey. The comparison of these two locales is telling because they differ in several important ways. For one, the slope near northern California is part of a narrow continental margin, which is located along the boundary between two tectonic plates that are colliding into each other. These motions continuously deform the seabed and cause frequent earthquakes, some of which trigger submarine avalanches. By contrast, the New Jersey slope is part of a wide continental margin, one located within the relatively quiet interior of a tectonic plate. Earthquakes here are far weaker and less frequent, and tectonic deformation largely ceased more than 100 million years ago.
Another marked difference between the two regions involves the source of their seafloor sediments. Northern California's Mad and Eel rivers have among the highest sediment yields of all rivers in the United States, and a significant fraction of this copious supply makes it onto the nearby continental slope. In contrast, little sediment accumulates offshore from New Jersey, where the continental slope is covered with very fine sediments eroded from the continental shelf and swept over the shelf edge.
The oceanographic conditions in the two areas differ as well. Historical measurements show that the seasonal differences in the way density varies with depth over the continental slope are larger in the Atlantic off New Jersey than they are in the Pacific off northern California. As a result, the geometry of the internal tides changes more over the course of the year off the Jersey shore than it does off northern California.
The key question at the outset of our study was whether the characteristic angles in these two places match the local dip of the continental slope. Using water temperature and salinity measurements compiled by the National Oceanic and Atmospheric Administration, we worked out the average density structure of the ocean in the two study areas and then derived the characteristic angles for the internal tides. Remarkably, the mean characteristic angle in each study area equaled or nearly equaled the mean gradient of the local continental slope: Both quantities are about 2 degrees in northern California and about 4 degrees in southern New Jersey.
We went on to examine the correspondence between the characteristic angle of the internal tides and the incline angle of the continental slope in more detail using highly resolved measurements of the seafloor, which are available for these two study areas. Members of the U.S. Geological Survey and academic investigators participating in a program of the Office of Naval Research called strataform (an abbreviation for "STRATA FORmation on continental Margins") collected this detailed topographic information using a technique called swath mapping. That method uses a research vessel equipped with an array of acoustic transducers mounted on the bottom of the hull. These devices broadcast sound energy down to the seafloor in a fanlike series of beams that extend both to the left and right. In this way, technicians on board can map the depth of the water over a wide swath centered under the path of the ship. And using both historical and more recent soundings of water temperature and salinity in these areas, it was easy enough for us to compute the characteristic angles of the internal tides for a dense array of geographic grid points and then compare these angles with the slope of the seafloor at those same spots.
The results turned out to be more complicated than with our general analysis, but they nevertheless supported our suspicion that internal tides help to shape the continental slope. In the New Jersey study area, large sections of the slope between the depths of 200 and 2,000 meters dip at an angle that is at or nearly equal to the characteristic angle of the internal tides at one time of the year or another. In winter, this correspondence even holds up in the interiors of many submarine canyons, such as the central portion of Hudson Canyon, where internal tides are known to cause high-velocity currents. Open-slope regions in deeper sections also lie close to the characteristic angle of the internal tides, but at other times of the year. For example, during summer the characteristic angle matches the slope of the seafloor over much of the Hudson Apron, a large expanse immediately south of Hudson Canyon where erosion is minimal and deposition dominates. In such settings internal tides would be expected to have their greatest influence, and the close correspondence between the dip of the Hudson Apron and the local characteristic angle suggests that internal tides do indeed control the amount of incline one finds on the sea bottom there.
The continental slope off northern California also provided some important evidence. Because of the tectonic deformation going on along the West Coast, portions of the slope in the central and southern sections of our western study area have large relief, reflecting the ongoing action of tectonic uplift and folding as well as a history of undersea landslides. Despite the presence of these confounding processes, segments of the slope between 200 and 450 meters depth dip at or near the characteristic angle for the internal tides. The strength of the bottom currents measured at this site suggests that internal tides can control sedimentation and hence the shape of at least some of the continental slope. Admittedly, here (and elsewhere on the seafloor) other processes are at work too. In particular, marine geologists have long been well aware of the fact that earthquakes trigger turbidity currents from time to time, and these muddy flows can move a lot of sediment into deeper parts of the ocean. However, based on the universality of the phenomenon and its persistence over geologic time, we believe that the internal tides deserve to be credited as a primary influence shaping the overall configuration of the continental slope.
We will soon be mounting investigations of the continental margins in other parts of the world to test the correspondence we have found so far between the dip of the slope and the characteristic angle of the internal tides. We believe that the results will confirm our expectations, but, of course, we have to be prepared for some surprises. Perhaps we will find evidence that the simple picture we derived from looking at California and New Jersey is, in fact, not so tidy after all. Or maybe the suggestive correspondence we found will indeed prove to be universal. Only time (and, well, tide) will tell.
Bibliography
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