Turbulence in sheared, salt-fingering favorable environment .

摘要

Instability and turbulence in sheared, salt-fingering favorable stratification are studied using three-dimensional direct numerical simulations (DNS). Salt-fingering favorable stratification is gravitationally stable, because the unstable vertical gradient of salinity is stabilized by temperature (warm, salty over cool, fresh water-masses). Salt-fingering instability can occur at the interface of these different water-masses. Salt-fingering instability generates cells of rising and sinking fluid because of the difference in diffusivity of heat and salt. In the presence of a vertically varying horizontal current (shear), salt-fingering instability is supplanted by salt-sheet instability. Salt-sheet instability generates alternating planar regions of rising and sinking fluid, aligned parallel to the direction of the sheared current.As the salt sheet reaches the finite amplitude, secondary instability appears at the edges of salt sheets and introduces quasi-periodic dependence along the direction of the sheared current. The secondary instability disrupts the growth of salt sheets and brings the flow into the turbulent regime. Secondary instability can be treated approximately as linear normal mode of the finite-amplitude salt sheets. The secondary instability is shown to be an oscillatory instability, driven primarily by buoyancy.In the turbulent regime, it is shown that thermal and saline buoyancy gradients become more isotropic than the velocity gradients in the dissipation-range scale. In the velocity field, the geometry of the primary instability is embedded in the dissipation-range scale geometry even in the turbulent regime therefore, the flow geometry from primary instability biases the estimation of the turbulent kinetic energy dissipation rate. Estimation of the turbulent kinetic energy dissipation rate by assuming isotropy, a common method in the interpretations of observations, can underestimate its true value by a factor of 2 to 3.Of primary interest of the oceanographic community is the turbulent transport of momentum, heat, and salt associated with salt-sheet instability, which can modify water-masses and lower the potential energy of the ocean. The effective diffusivites of momentum, heat, and salt are used to describe the turbulent state. The effective diffusivity of momentum is an order of magnitude smaller than that of salt turbulence associated with salt-sheet instability is therefore relatively inefficient in transferring momentum. These effective diffusivities are compared to observational estimates.