Biases in Thorpe-Scale Estimates of Turbulence Dissipation. Part I: Assessments from Large-Scale Overturns in Oceanographic Data

Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale L-T and the Ozmidov length scale L-O. Historic evidence supporting L-T similar to L-O has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of L-T and L-O, using vertical profiles of temperature and microstructure shear collected in the Luzon Straita site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that L-T/L-O increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin-Helmholtz (K-H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting L-T similar to L-O are revisited, and conceptual regimes explaining the relationship between L-T/L-O and a nondimensional overturn size (L-T) over cap are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.