*University of Colorado, CIRES; NOAA ETL*
*Boulder, CO, USA.*

**Abstract**

Model evolution equations for strongly nonlinear internal waves in the ocean are discussed in application to recent field experiments including COPE. For the cases considered, a commonly used Korteweg-de Vries (KdV) equation is inapplicable, and modified models with adding higher-order terms are needed (CombKdV equation). A most adequate and physically substantiated model uses an ``exact'' (for very long waves) nonlinearity and nonlinear dispersion. Theoretical results are compared with observational data. Some problems are discussed in conclusion.

**Introduction**

There exists a relatively large body of literature concerning observations and theoretical models of nonlinear internal waves (IW) in shelf areas (see, e.g., the review paper [1]). A common tool for description of these processes is the Korteweg-de Vries (KdV) equation that is applicable to relatively small-amplitude waves. However, in some cases really strong nonlinear waves are observed. A recent example is the Coastal Ocean Probe Experiment (COPE), which has been carried out in September 1995 in the Oregon Bay. Its data clearly indicated the presence of extremely strong trains of tidally-generated internal waves [2],[3]. The isotherm depressions consisted of solitary pulses (we shall call them solitons independently of how strict this definition could be) with amplitudes of thermocline displacements typically of order 10-20 m, and sometimes reaching 30 m. Even more important is the fact that they propagate into the background of a very shallow, 5-7m depth, pycnocline. This testifies for an extremely strong nonlinearity of the process (a high ``Mach number,'' using the gas dynamic terminology). Thus, the problem of creating an adequate theoretical model of these processes is evidently actual.

**KdV model**

As mentioned, the models of shallow-sea solitons are commonly based
on the classical Korteweg-de Vries equation

(1) |

where
is the displacement of an isopycnal surface, and *c* is the phase
velocity of a long wave corresponding to a given mode. This parameter is
defined as an eigenfunction of the corresponding linear boundary problem
for long-wave limit. Here we shall consider a simplified model of stratification
in which the fluid consists of two layers: upper, of thickness *h*_{1}
and density
and lower, with *h*_{2} and
respectively. In this case, the parameters of
are

(2) |

(all these expressions suggest that which is always true in the ocean). This equation has a family of well known solitary solutions. Such a model works well for a number of IW observations but is inapplicable for the cases considered here.

**CombKdV model**

The next-level model which may be tried takes into account the higher
degree on nonlinearity; it is so-called CombKdV equation [1]:

(3) |

The parameters *c*,
and
are the same as for the KdV equation. for
The cubic nonlinearity parameter
for a two-layer model, is

(4) |

Note that always
whereas the sign of
depends on the position of the interface (pycnocline). In our case *h*_{1}<*h*_{2},
and
as well, so that in the solitary wave, (
a depression). Moreover, there exist a critical value of displacement
for which the nonlinearity tends to zero, namely: .

The equation is known to be completely integrable. It also has a solution in the form of a soliton. Its amplitude is limited by the value If is sufficiently small, the wave is close to that defined by the KdV equation, but for larger solitons, the wave is broadening and when approaches , it acquires almost rectangular shape.

Comparison with the experimental data shows that the CombKdV model gives a much better agreement than the usual KdV model.

**An equation with ``exact'' nonlinearity and nonlinear
dispersion.**

A more consistent approach can be developed based on the expression
for ``exact'' nonlinear term obtained as an analog of the shallow-water
theory for the surface waves. We suggest to use the equation

(5) |

with the same
and
depending on the local pycnocline displacement: in particular,

(6) |

In case of a thin upper layer most of wave energy is concentrated in
this layer, and the local long-wave velocity is

(7) |

For arbitrary relationship between *h*_{1} and *h*_{2},
the local wave velocity must be found from differential equations (D. Pelinovsky):

(8) |

(9) |

The equation also has a family of solitons which fit the COPE data quite well. It is notable that there exist a rather wide intermediate range where the soliton duration is almost independent of its amplitude; this agrees well with observational data [2].

**Conclusions**

These results show that even for strongly nonlinear waves, one-dimensional evolution equations, albeit not strictly derived from the basic hydrodynamic equations, can provide a satisfactory description of real processes in a broad range of their parameters. Now we are continuing to actively study evolution equations with nonlinear dispersion.

It appears that strongly nonlinear internal waves are a rather typical
phenomenon for some shelf areas during spring tides; note that the PRIMER
experiment performed by WHOI near the East coast, also demonstrated the
presence of strong IWs. These areas deserve a special attention of oceanographers
as those of especially active mixing, transport of biomasses and pollutants,
etc. Regarding the theory, it is very desirable, along with further studies
of model equations, to supplement these studies with direct numerical modeling
of hydrodynamic equations.