TURBULENCE DECAY IN STRATIFIED AND HOMOGENEOUS MARINE LAYERS

A spectral approach is applied to shear-induced turbulence in stratifi ed layers. A system of spectral equations for stationary balance of tu rbulent energy and temperature variances was deduced in the vicinity o f the local shear scale L(U)=(epsilon/UZ(3))(1/2). At wavenumbers betw een the inertial-convective (k(-5/3)) and weak turbulence (k(-3)) subr anges, additional narrow spectral intervals-'production' subranges-may appear (E approximate to k(-1), E(T)=k(-2)). The upper boundary of th ese subranges is determined as L(U), and the lower boundaries as L(R) approximate to(epsilon/UZN(2))(1/2) and L(T) approximate to(epsilon/U- Z)(-1/2)(chi/T-Z(2)). It is shown that the scale L(U) is a unique spec tral scale that is uniform up to a constant value for every hydrophysi cal field. It appears that the spectral scale L(U) is equivalent to th e Thorpe scale L(Th) for the active turbulence model. Therefore, if tu rbulent patches are generated in a background of permanent mean shear, a linear relation between temperature and mass diffusivities exists. In spectral terms, the fossil turbulence model corresponds to the regi me of the Boldgiano-Obukhov buoyancy subrange (E approximate to k(-11/ 5), E(T) approximate to k(-7/5)). During decay the buoyancy subrange i s expanded to lower and higher wavenumbers. At lower wavenumbers the b uoyancy subrange is bounded by L*=3(chi(1/2)/(NTZ)-T-1/2), which is e quivalent to the Thorpe scale L(Th). In such a transition regime only, when the viscous dissipation rate is removed from the set of main tur bulence parameters, the Thorpe scale does not correlate with the buoya ncy scale L(N) approximate to epsilon(1/2)/N-3/2 and fossil turbulence is realized. Oceanic turbulence measurements in the equatorial Pacifi c near Baker Island confirm the main ideas of the active and fossil tu rbulence models.