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Internal Solitary Waves in the Sulu Sea

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Internal Solitary Waves in the Sulu Sea ________________________________________________________R&DUPDATES JOHN R. APEL and JAMES R. HOLBROOK INTERNAL SOLITARY WAVES IN THE SULU SEA Large internal oscillations of a density-stratified body of water often occur in the form of nonlinear solitary waves, or solitons. Recent field and theoretical investigations have delineated much of the dynamics of these waves, including the processes of generation, propagation, and atten­ uation. Some groups of solitons measured in the Sulu Sea near the Philippines have amplitudes of up to 100 meters, wavelengths in excess of 10 kilometers, and lifetimes of 2 Y2 days. Their major proper­ ties may be reproduced theoretically in a satisfactory way. INTERNAL WAVES tive gravity, g e.ff' and in many geophysical fluids is as small as l/lOOOth of the actual value of g. The radian Gravity waves on the surface of the sea are perhaps frequency, w, of an internal oscillation is thereby the most visible example of a class of fluid motion decreased by the square root of this ratio, and the falling under the rubric of geophysical fluid dynam­ period of an internal wave is correspondingly much ics. Ocean surface waves have long occupied the at­ longer than its surface wave equivalent, all else being tention of intellectuals ranging from natural philoso­ equal. phers through Freudian psychologists to poets. A Internal waves also travel more slowly than surface less-well-appreciated (and less-visible) type of motion waves. To appreciate this, recall that a small-ampli­ called an internal gravity wave also exists and has tude, long surface wave in a shallow channel filled been known to oceanographers for over 75 years. with fluid to a depth, h, will travel with a phase speed Such waves -may occur within the body of a fluid C = (gh) Y' . If such a channel is instead filled to the whose density varies in the vertical, whenever that same depth with two layers of fluid that differ slight­ fluid is disturbed by some means. The vertical density ly in density, as above, with the two layer depths be­ gradient may come about through temperature varia­ ing h I and h2' an internal wave excited on the inter­ tions (largely resulting from solar heating of the up­ face between the layers will now move with a speed per layers of the ocean), salinity variations (usually Co = (ge!!he!! ) I/} , where the effective gravity is as de­ resulting from lighter, fresher water from rivers or fined above, but where the effective depth, h e!!' is the fjords flowing across colder, saltier water below), or, harmonic mean of the layer depths: he!! = hI h2/ (hI least important, fluid compressibility (resulting from + h 2 ). This effective depth is always smaller than the the weight of the overlying water). depths of either the upper or the lower layer. Thus, Internal gravity waves are most familiar in devices with both a reduced gravity and a small effective sold in novelty stores wherein two immiscible fluids depth, internal waves propagate very slowly indeed. differing only slightly in density are confined to Owen M. Phillips discusses both surface and internal layers in a closed, clear plastic box. By quickly tilting waves in his excellent book. I the box, one may excite a large, slow, low-frequency A surface gravity wave mode still exists in the two­ wave propagating along the fluid interface, which fluid channel, of course, as long as the upper surface then reflects off the ends to oscillate back and forth is free, and it has essentially the same properties as in as a kind of internal seiche. The amplitude of the the single-layer case. In the parlance of geophysical wave, which is zero at the top and bottom of the box fluid dynamics, the surface wave is known as the because of the rigid boundaries there, has its max­ barotropic mode, while the internal wave is known as imum at the interface between the fluids. This inter­ the baroclinic mode, the latter because surfaces of nal wave is of low frequency because of the small constant pressure (baro) are inclined with respect to density change, !:lp, across the interface; the buoyan­ surfaces of constant density in such modes. cy provided to an upwardly displaced parcel of water Strictly speaking, an internal wave in the two-fluid by the surrounding lighter fluid reduces the box is an interfacial wave, but analogous internal downward acceleration of gravity, g, to an amount waves exist in the continuously stratified fluids of the equal to the actual gravity times the fractional densi­ earth's atmosphere and oceans, and indeed on other ty change. This quantity, g!:lp/ p, is termed the effec- planets having fluid envelopes. The combination of Volume 4, Number 4, 1983 267 J. R. Apel and J. R. Holbrook - Internal Solitary Waves in the Sulu Sea solar heating and gravity results in a stratification of PHYSICS OF a planetary fluid such that its density generally de­ SOLITARY INTERNAL WAVES creases with height. The atmosphere is usually most stable at night, when solar heating of the surface and Strictly speaking, a solitary wave in or on a fluid is the resultant unstable upward convection of air have an isolated, propagating disturbance that moves over ceased and the atmosphere has settled down to gravi­ large distances without change of shape. Regarded as tational equilibrium. On the other hand, the ocean is something of a hydrodynamic curiosity until relative­ most stable during the daytime when solar heating of ly recently, the existence of a surface soliton was first the upper layers has rendered them lighter and hence noted during the last century by John Scott Russell,2 more buoyant than the deeper waters. Under stably who observed the generation of a "mound of water" stratified conditions, any forces that excite vertical caused by a tow barge brought to an abrupt stop in a excursions of either of these fluids will generally shallow canal in England. The bow wave launched by result in the propagation of internal waves away the halted barge was described by Russell as an iso­ from the source. Such vertical forces may arise from lated pulse of elevation that propagated for several the flow of currents over topographic features (e.g., miles along the canal as he followed it on horseback. mountain ranges, the edge of the continental shelf, Struck by its solitary character and its relatively small underwater sills, or sea mounts) and result in signifi­ change of shape, he dubbed the phenomenon a "soli­ cant vertical motion of fluid at depth. They may also tary wave" and subsequently carried out controlled occur in currents where the flow velocity varies experiments to determine its properties. The essential sharply with height; such shear flow can become element necessary to support this surface wave was unstable if intense enough and result in vertical ex­ found to be shallow water, with YJ required to be com­ cursions of the fluid that take on wavelike character­ parable with h in order for the solitary character to istics. Indeed, any forces that move the stratified be established and maintained. Under a balance of fluid vertically may be regarded as candidate sources body forces that comes into play with these condi­ for internal waves. tions, as is explained below, the wave relatively If the stratified fluid is disturbed by some means, quickly develops a characteristic shape and thereafter the disturbance propagates away from the point of maintains its form indefinitely in the absence of origin as a train of waves characterized by polariza­ spreading or dissipation. Subsequently, Korteweg tion largely perpendicular to the direction of propa­ and deVries3 developed a theoretical explanation of gation and by a well-defined relationship among the solitary wave phenomenon. wavelengths, frequencies, and angles of propagation. Twentieth century workers in hydrodynamics came That is, the waves obey a dispersion relation that re­ to the realization that a stratified fluid could equally lates w to the wave vector, k, via an equation of the well support solitary internal waves but of a some­ form w = w (k), which also implicitly involves the what more varied and complicated form than the sur­ vertical density gradient and the water depth in addi­ face solitons of the English barge canal. It turned out tion to the wave number and direction. For small­ that in a density-stratified fluid, the depth need not amplitude linear waves, the dispersion relation does be shallow to support solitary waves-indeed the not depend on the wave amplitude, YJ, just as the fre­ fluid could be arbitrarily deep-as long as the vertical quency of a clock pendulum does not depend on the scale of the depth region containing the significant amplitude of its swing, to a first approximation. density gradient was small enough.4 The gradient­ However, if the wave amplitude is large, either sur­ containing region acts as a kind of waveguide, con­ face or internal waves may become nonlinear and the fining the major portion of the wave energy to a rela­ propagation characteristics thereby become depen­ tively narrow vertical range while allowing propaga­ dent on amplitude in addition to the other dependen­ tion in the horizontal, and thus rendering the total cies mentioned. water depth of lesser importance. If the wave ampli­ Although a wide variety of nonlinear waves is tude is comparable to the scale of the gradient and known, a particular type called the solitary wave has also is steep enough, it may then move as a solitary come under very active investigation recently and wave; on the other hand, if the wave height is small, considerable understanding of its unusual properties it will still propagate, but now as a linear wave.
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