Internal solitary waves are a common and important feature of shelves, straits and estuaries and have distinct remote sensing signatures. New theoretical insights suggest that, in addition, they they may draw energy from the larger scale circulation and produce substantial mixing. The Knight Inlet Experiment was designed to investigate these ideas.
Large-amplitude, small-scale, wave-like disturbances are common in shallow water regions and are often called "solitary internal waves". The passage of these features is marked by dramatic changes in the stratification and rapid current fluctuations capable of disrupting offshore oil drilling operations. Similar considerations apply to a wide variety of naval operations, such as mine laying, mine hunting, and reconniassance. They also affect acoustic propagation in shallow water. Fortunately, these features are potentially highly predictable, being generally produced by tidal currents flowing over rough topography and propagating many tens of kilometers from their source. The waves strongly modulate the surface wave field above them, leading to a zone of enhanced wave steepness and breaking which is easily detected by radar and visual remote sensing. Such images show these features are ubiquitous in coastal regions, straits, and estuaries.
The extensive literature on these features is, unfortunately, mostly often qualitative and the observations not complete enough to develop realistic models of propagation over long distances, such as across a continental shelf. Existing analysis almost universally apply the Korteweg-de Vries (KdV) equation, which assumes that the features are inviscid and localized. This approach has lead to the common practice of calling these features "solitons" . Our analysis, described below, shows that many of the observations are inconsistent with the KdV equation and that these features are often more like nonlinear bores than solitons. Although they are localized, they draw energy from the surrounding flow; although they are wavelike, they have high rates of mixing and dissipation and may form the leading edge of a large increase in transport. We coin the term 'solibore' to indicate this combination of both wave-like and bore-like properties.
There are a number of distinctions between a more bore-like wave and a more soliton-like wave. One such distinction is the relationship between the state of the water before and after the passage of the wave. The pure soliton leaves the stratification and shear the same after its passage as they were before. The bore leaves them quite different; it is the leading edge of a change in the state of the fluid. The normal situation is that the upper layer, which is thin in comparison to the lower layer, thickens considerably. This is seen clearly in the Figure, taken in the Strait of Gibraltar (Wesson and Gregg,1980). As a result, the upper layer, which may have been going in either direction relative to the lower layer before the wave came along, follows the wave after its passage. Thus, when the bore description is appropriate, there is greatly enhanced transport in the upper layer. As a result of the asymmetry of the initial and final states, there is a net energy flux into (or possibly, but not likely, out of) the wave. The energy flux is expected to result in a high dissipation rather than causing the wave to grow in amplitude. In the pure bore case, the energy supply should balance the dissipation, and the amplitude should remain constant. By way of contrast, dissipation in the soliton should cause the amplitude to decay. The term "solibore" is coined to indicate that these waves can have properties anywhere between the extremes of a soliton and a pure internal bore. A major theme of our work is to investigate where within this continuum they lie.
We have examined solibores from a variety of environments. In all of the examples, the solibores are asymmetric and thus have some bore-like qualities, however, they are never purely one case or the other. We expect that they start out as mostly bores, but become more like solitons as they decay. This change will be accompanied by a decrease in their dissipation, transport and mixing.