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15 :_::NA~11J=RE::....V.:..:O:::;L::.;;.3;::.12...:.1....:.N:;.:O~VE=:;M;::.B:::::ER::....:..:I984::::______NEWS AND VIEWS ------

Oceanography dissipated. On the other hand 6 x 10 7 J dissipated on a lO km wide shelf in half a day provides about 50 m W m -z towards the , solitons and nutrients vertical mixing of the water and the erosion of the base of the mixed layer. This is at from Melbourne G. Briscoe least ten times the energy supply estimated from internal dissipation in the deep TIDAL energy mixes the coastal waters, pro­ is dominant. One might also expect a and several times larger than estimates viding nutrients for biological processes. spring-neap cycle in the intensities of the of the energy available from typical winds. The mechanism is indirect; the topography process, but I am aware of no good data A.R. Osborne and T.L. Burch 3 have of the shelf-break, the stratification of the that confirm this; since the dynamics of the described solitons in the Andaman Sea coastal waters and the tides interact to pro­ generation of the solitons and of their near Sumatra that are 15 times more duce groups of solitons - strong, short, eventual propagation depend on the energetic than the examples of Sandstrom high frequency internal - and it is strength and depth of the , the and Elliott. Even if all of the coastlines of the breaking of these solitons as they inevitable changes in the with the world were covered with such solitons, propagate inshore to even shallower water space and time probably obscure spring­ and all these solitons were dissipated in half that is responsible for the mixing. That is neap effects. a day, this would still only account for 0.3 the suggestion offered by H. Sandstrom Surprisingly, solitons are often easy to x lO 12 W, just 20 percent of what needs to and J .A. Elliott of the Bedford Institute of observe in the . The interaction of be dissipated from the surface in 1 in Canada • Their idea, wave currents in the with short sur­ shallow according to G. Miller's 4 based on their work on the Scotian Shelf, face waves causes a modulation of the calculations • (These take into account seems sound but perhaps does not go far roughness of the sea surface that is visible surface tide dissipation due to conversion enough. to the eye and to radar. Landsat satellite to internal tides or solitons on the shelves Solitons are so-called because of their images described by C. Sawyer2 show that for some areas of the world, but not for the similarity to the mathematically precise the entire eastern seaboard of North North American east coast where so many idea of a non-linear wave that retains its America is covered with a pattern of sur­ solitons have been observed.) identity and form as it propagates along as face streaks characteristic of solitons. This So even this extreme estimate of the a singular bump on the sea surface. How­ is especially so in the 50-150m depth range energetics and dissipation of solitons, we ever, in most situations an ac­ of the shelf, and is most marked in summer are not much closer to understanding tually travels as a singular wave of depres­ and early autumn, when the is where tidal energy goes, but Sandstrom sion, since there is usually a deeper layer of well developed. (Georges Bank and its sur­ and Elliott's suggestion 1 of the importance water underneath the pycnocline (the layer roundings perhaps have the most solitons of solitons for shelf-mixing and for the sup­ where the water density changes most in the western North Atlantic; does this ac­ ply of nutrients to the euphotoic zone quickly with increasing depth) than above count for the nutrient supply and the seems plausible on the Scotian Shelf. It it. As the internal depression on the pyc­ fishing industry on Georges Bank?) may even be applicable to thousands of nocline propagates, it tends to disperse into Sandstrom and Elliott propose that the kilometres of other soliton-covered the free-wave components that compose it. surface tides at the shelf edge are converted shelves. D This tendency is balanced by the non­ to internal tides and then to solitons that I. Sandstrom, H. & Elliott, J.A. J. geophy. Res. 89, 6415 (1984). linearities that arise from the finite dissipate on the shelf'. They calculate a 2. Sawyer, C. NOAA Tech. Memo. Envir. Res. Lab. Pacific amplitude of the depression and allow the figure of 6 x lO 7 J per metre of crest length mar. envir. Lab. Publ. 46 (1983). 3. Osborne, A.R. & Burch, T.L. Science 208,451 (1980). wave to retain its soliton form. This fragile for their solitons. If extrapolated to the 4. Miller, G. J. geophys. Res. 71, 2485 (1966). balance of forces is easily realized in equa­ coastlines of the world this yields I x 10 15 J, tions, can sometimes be simulated in the less than a third of a percent of the total in­ Melbourne G. Briscoe is at the Woods Hole laboratory, but probably never really oc­ ternal tidal energy, a negligible contribu­ Oceanographic Institution, Woods Hole, curs in the ocean; I therefore use the word tion to the problem of how the energy is Massachusetts 02543, USA. soliton with some reservations. Solitons are thought to be produced by the surface tide running up against the edge of the . The pycnocline overlying the shelf is disturbed vertically by the tidal flows and a depression in the pyc­ nocline begins to travel shoreward. There is some uncertainty about the mechanism that forms the depression: Sandstrom and Elliott consider that the surface tide forms an essentially linear, at the shelf edge, and that the leading edge of the propagating internal tide is the depression 1 that finally resolves into the soliton • The scales of these processes differ greatly. The surface and internal tides (called baro­ tropic and baroclinic tides, respectively, in the oceanographic literature) occur once or twice a day, depending on location, but the wave sequence in the soliton is of just a few minutes period. The horizontal scales of these three processes decrease from A masked image of the star (J Pictoris taken by R.J. Terrile (Jet Propulsion Laboratory) thousands of kilometres, to tens of kilo­ and B.A. Smith (University of Arizona) using a charge coupled device on the 2.5 m metres, to hundreds of metres. telescope at Las Campanas Observatory, Chile. The bright extended emission is thought Because of the tidal generation of soli­ to come from a circumstellar disk of silicates, ices and carbonaceous organic material. The tons they can be recorded every 12.4 h in disk, seen edge-on, is 40 billion miles across; the star is 50 light years away. This disk is the those locations where the semi-diurnal tide first of its kind to be seen clearly in astronomical photographs. (JPL and U. Arizona). © 1984 Nature Publishing Group