Spatial variability of turbulent mixing from an underwater glider in a
large, deep stratified lake
Abstract
Spatial variability of physical properties induced by circulation and
stirring remains unaccounted for in the energy pathway of inland waters.
Recent efforts in microstructure turbulence measurements have unraveled
the overall energy budget in lakes. Yet, 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 Lake Geneva. Microstructure analyses
allowed turbulent dissipation rates and thermal variances estimations by
fitting temperature gradient fluctuations spectra to the Batchelor
spectrum. In open waters, results indicate mild turbulent dissipation
rates in the surface and thermocline (~10⁻⁸ W kg⁻¹),
which weaken towards the deep hypolimnion (~10⁻¹¹ –
10⁻¹⁰ W kg⁻¹). The strong thermal stratification inhibited interior
mixing in the thermocline. In contrast, measurements along the coastal
slope reveal a notorious enhancement of turbulent dissipation
(~5×10⁻⁸ W kg⁻¹) above the sloping topography way above
the known extent of the bottom boundary layer. These distinct turbulence
patterns result from differing large-scale dynamics in the interior and
coastal environments. Current measurements in open waters show dominant
internal Poincaré waves. On the coast, three-dimensional numerical
results from meteolakes.ch suggest that enhanced bottom dissipations
arise from the development of centrifugal instabilities. A process
driven by coastal cyclonic circulation interacting with the sloping
bottom reported for the ocean but so far overlooked in large lakes. The
spatially-distributed turbulence measurements we report here highlight
the potential of underwater glider deployments for further lake
exploration.