A generalized marginal stability criterion for shear-induced ocean
interior diapycnal turbulent mixing
Abstract
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.