Spatial Variability of Turbulent Mixing From an Underwater Glider in a Large, Deep, Stratified Lake

Abstract

AbstractRecent efforts using microstructure turbulence measurements have contributed to our understanding of the overall energy budget in lakes and linkages to vertical fluxes. A paucity of lake‐wide turbulence measurements hinders our ability to assess how representative such budgets are at the basin scale. Using an autonomous underwater glider equipped with a microstructure payload, we explored the spatial variability of turbulence in pelagic and near‐shore regions of Lake Geneva. Dissipation rates of kinetic energy and thermal variance were estimated by fitting temperature gradient fluctuations spectra to the Batchelor spectrum. In deep waters, turbulent dissipation rates in the surface and thermocline were mild (∼10−8 W kg−1) and weakened toward the hypolimnion (∼10−11 to 10−10 W kg−1). The seasonal thermocline exhibited inhibited interior mixing, with extremely low values of mixing efficiency (Rif ≪ 0.1). In contrast, in the slope zone, a band of significantly enhanced energy dissipation (∼5 × 10−8 W kg−1) extended well above the bottom boundary layer and was associated with strong, efficient mixing (Rif > 0.17). The resulting contribution of the slope region to basin‐scale mixing was large, with 90% of the basin‐wide mixing—and only 30% energy dissipation—occurring within 4 km of the shoreline. This boundary mixing will modify overturning circulation and the transport pathways of dissolved compounds exchanged with the sediments. The dynamics responsible for the shift in the mixing regime, which appears crucial for the mixing budget of lakes, could not be fully unraveled with the collected observations. Additional model data analyses hint at the role of submesoscale instabilities.