Stratified Turbulence near a Critical Dissipation Rate

Abstract

Turbulence in the thermocline was investigated using data observed by the submarine Dolphin. Various length scales, the Reynolds and the Froude numbers of the Dolphin data were compared to those values of a laboratory experiment presented in Itsweire et al. In addition to these datasets, the data obtained by the sub-mersible Pisces IV provide the range of ε/νN2 above the thermocline values. Our thermocline data demonstrate that the dynamical condition of the laboratory experiment is confirmed as identical to the turbulence in the thermocline when ε/νN2<100. Therefore, the critical dissipation rate, εC = 16νN2, is applicable to the thermocline. However, the applicability of laboratory experiments for high values of ε/νN2, such as the generating stage of turbulence, requires a cautious interpretation for turbulence in the thermocline. The power spectrum of velocity closely follows the Nasmyth's empirical universal spectrum in the high wavenumbers even when the dissipation rate is as low as εC. Additional data, not used in this study, suggest that two modes of “weak” turbulence exist when the dissipation rate is below εC: one follows the universal spectrum in the high wavenumbers (locally isotopic turbulence); and the other strongly exhibits two-dimensionally oriented flows (anisoitropic turbulence).