FINESCALE PARAMETERIZATIONS OF TURBULENT DISSIPATION

Fine- and microstructure data from a free fall profiler are analyzed t o test models that relate the turbulent dissipation rate (epsilon) to characteristics of the internal wave field. The data were obtained fro m several distinct internal wave environments, yielding considerably m ore range in stratification and wave properties than has been previous ly available. Observations from the ocean interior with negligible lar ge-scale flow were examined to address the buoyancy scaling of e. Thes e data exhibited a factor of 140 range in squared buoyancy frequency ( N-2) with depth and uniform internal wave characteristics, consistent with the Garrett-Munk spectrum. The magnitude of epsilon and its varia tion with N(epsilon similar to N-2) was best described by the dynamica l model of Henyey et al. A second dynamical model, by McComas and Mull er, predicted an appropriate buoyancy scaling but overestimated the ob served dissipation rates. Two kinematical dissipation parameterization s predicted buoyancy scalings of N-3/2; these are shown to be inconsis tent with the observations. Data from wave fields that depart from the canonical GM description are also examined and interpreted with refer ence to the dynamical models. The measurements came from a warm core r ing dominated by strong near-inertial shears, a region of steep topogr aphy exhibiting high-frequency internal wave characteristics, and a mi docean regime dominated at large wavelengths by an internal tide. Of t he dissipation predictions examined, those of the Henyey et al. model in which epsilon N-2 scales as E(2), where E is the nondimensional spe ctral shear level, were most consistent with observations. Nevertheles s, the predictions for these cases exhibited departures from the obser vations by more than an order of magnitude. For the present data, thes e discrepancies appeared most sensitive to the distribution of interna l wave frequency, inferred here from the ratio of shear spectral level to that for strain. Application of a frequency-based correction to th e Henyey et al. model returned dissipation values consistent with obse rved estimates to within a factor of 2. These results indicate that th e kinetic energy dissipation rate (and attendant turbulent mixing) is small for the background Garrett and Munk internal wave conditions (0. 25 epsilon N-2 similar to 0.7 X 10(-5) m(2) s(-1)). Dissipation and mi xing become large when wave shear spectral levels are elevated, partic ularly by high-frequency waves. Thus, internal wave reflection/generat ion at steep topographic features appear promising candidates for achi eving enhanced dissipation and strong diapycnal mixing in the deep oce an that appears required by box models and advection-diffusion balance s.