High-frequency internal waves in large stratified lakes

Observations are presented from Lake Biwa and Lake Kinneret showing the ubiquitous and often periodic nature of high-frequency internal waves in large stratified lakes. In both lakes, high-frequency wave events were observed within two distinct categories: (1) Vertical mode 1 solitary waves near a steepened Kelvin wave front and vertical mode 2 solitary waves at the head of an intrusive thermocline jet were found to have wavelengths ;64–670 m and ;13–65 m, respectively, and were observed to excite a spectral energy peak near 1023 Hz. (2) Sinusoidal vertical mode 1 waves on the crests of Kelvin waves (vertically coherent in both phase and frequency) and bordering the thermocline jets in the high shear region trailing the vertical mode 2 solitary waves (vertically incoherent in both phase and frequency) were found to have wavelengths between 28–37 and 9–35 m, respectively, and excited a spectral energy peak just below the local maximum buoyancy frequency near 1022 Hz. The waves in wave event categories 1 and 2 were reasonably described by nonlinear wave and linear stability models, respectively. Analysis of the energetics of these waves suggests that the waves associated with shear instability will dissipate their energy rapidly within the lake interior and are thus responsible for patchy turbulent events that have been observed within the metalimnion. Conversely, the finite-amplitude solitary waves, which each contain as much as 1% of the basinscale Kelvin wave energy, will propagate to the lake perimeter where they can shoal, thus contributing to the maintenance of the benthic boundary layer. The degree of ambient stratification in lakes is significant in regulating the vertical transport of nutrients, plankton, and oxygen. Accordingly, stratification can instigate the occurrence of hypolimnetic anoxia (e.g., Mortimer 1987). Simple energy models (Imberger 1998) and microstructure (e.g., Wüest et al. 2000; Saggio and Imberger 2001) and tracer (e.g., Goudsmit et al. 1997) observations suggest that turbulent buoyancy flux, which erodes the ambient stratification, is negligible within the lake interior and occurs primarily at rates an order of magnitude greater than in the interior along the lake boundaries. The physical processes responsible for this spatial heterogeneity of mixing within the lacustrine environment are also believed to occur within larger scale oceanic flows. Observed vertical mixing rates within the vast ocean interior (e.g., Ledwell et al. 1993) are an order of magnitude smaller than classical global estimates (Munk 1966; Gregg 1987). Observations show this difference is accounted for by enhanced mixing within topographic boundary layers (e.g., Polzin et al. 1997; Ledwell et al. 2000). Our present knowledge suggests that the augmented mixing and dissipation within the lacustrine benthic boundary layer results from shear-driven turbulence as baroclinic 1 Corresponding author ([email protected]).