Evolution of layered structures in the final stage of turbulent decay in a stably stratified fluid

Abstract

The dynamics of layered structures in the final stage of turbulent decay in a stably stratified fluid is considered. Such buoyancy-driven low Froude and Reynolds number motions are distinguished together with the internal waves and quasi-horizontal vortices. A linear analysis of the velocity, temperature and salinity perturbations was carried out by using multi-scale methods. It was shown that although relations for most long-lived layered structures at large and small Prandtl number are similar [J. Fluid Mech. 134 (1983) 195], the different underlying mechanisms are responsible for formation of these structures. The stable salinity stratification causes enhancement of the decay rate of temperature inhomogeneities. However, the lifetime of the salinity inhomogeneities increases when the stable temperature stratification is involved. In the presence of shear, the decay rate of the scalar (temperature, salinity) perturbations is less than that of the vertical velocity. It was shown that the finite amplitude layered structures exist at small internal Froude and Reynolds numbers and large Prandtl numbers. The interaction between structures as a possible mechanism for the formation of microstructure layers in the ocean was studied numerically. The rearrangement of the density field between mixed regions is important in the process of merging of structures.