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Spatial variability of turbulent mixing from an underwater glider in a large, deep stratified lake
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  • Oscar Sepúlveda Steiner,
  • Alexander LeBaron Forrest,
  • Jasmin Mcinerney,
  • Bieito Fernández Castro,
  • Sébastien Lavanchy,
  • Alfred Wuest,
  • Damien Bouffard
Oscar Sepúlveda Steiner
Eawag

Corresponding Author:[email protected]

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Alexander LeBaron Forrest
University of California - Davis
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Jasmin Mcinerney
University of California - Davis
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Bieito Fernández Castro
University of Southampton
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Sébastien Lavanchy
École Polytechnique Fédérale de Lausanne
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Alfred Wuest
Swiss Federal Institute for Environmental Science andTechnology (EAWAG)
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Damien Bouffard
EAWAG
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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.