The degeneration of large-scale interfacial gravity waves in lakes

Mechanisms for the degeneration of large-scale interfacial gravity waves ar e identified for lakes in which the effects of the Earth's rotation can be neglected. By assuming a simple two-layer model and comparing the timescale s over which each of these degeneration mechanisms act, regimes are defined in which particular processes are expected to dominate. The boundaries of these regimes are expressed in terms of two lengthscale ratios: the ratio o f the amplitude of the initial wave to the depth of the thermocline, and th e ratio of the depth of the thermocline to the overall depth of the lake. C omparison of the predictions of this timescale analysis with the results fr om both laboratory experiments and field observations confirms its applicab ility. The results suggest that, for small to medium sized lakes subject to a relatively uniform windstress, an important mechanism for the degenerati on of large-scale internal waves is the generation of solitons by nonlinear steepening. Since solitons are likely to break at the sloping boundaries, leading to localized turbulent mixing and enhanced dissipation, the transfe r of energy from an initial basin-scale seiche to shorter solitons has impo rtant implications for the lake ecology.