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A generalized marginal stability criterion for shear-induced ocean interior diapycnal turbulent mixing
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  • Ali Mashayek,
  • Colm Caulfield,
  • Lois Baker,
  • B. B. Cael
Ali Mashayek
Imperial College London, Imperial College London

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Colm Caulfield
University of Cambridge, University of Cambridge
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Lois Baker
Imperial College London, Imperial College London
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B. B. Cael
National Oceanography Centre, National Oceanography Centre
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Turbulent mixing induced by breaking internal waves is key to the ocean circulation and global tracer budgets. While the classic marginal shear instability of Richardson number ∼ 1/4 has been considered as potentially relevant to turbulence wave breaking, its relevance to energetic zones where tides, winds, and buoyancy gradients excite non-linearly interacting processes has been suspect. We show that shear instability is indeed relevant in the ocean interior and propose an alternative generalized marginal stability criterion, based on the ratio of Ozmidov and Thorpe turbulence scales, which not only applies to the ocean interior, but remains relevant within turbulent boundary layers. This allows for accurate quantification of the transition from downwelling to upwelling zones in a recently emerged paradigm of ocean circulation. Our results help climate models more accurately calculate the mixing-driven deep ocean circulation and fluxes of tracers in the ocean interior.