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

Abstract

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 ; 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 Strait-a 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 ; L O are revisited, and conceptual regimes explaining the relationship between L T /L O and a nondimensional overturn size c L T are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…Turbulent quantities are important to the energy balance of the global ocean and are prevalently parameterized in largescale circulation models. Results from previous work using finestructure methods to interpret mixing rates have recently been shown to be biased high (Frants et al (2013), Mater et al (2015), Waterman et al (2014)), often by up to two orders of magnitude. This work has revisted the values and patterns of mid-depth mixing in this region to inform modeling efforts and large-scale estimates of vertical diffusivity.…”

“…However, recent work has shown the potential for biases between mixing rates inferred from both of those methods and those from direct measurements. Mater et al (2015) found Thorpe scale methods overestimate mixing rates when compared to direct measurements especially in the presence of large overturns. In the Southern Ocean, mixing rates inferred from finescale parameterizations are shown to overestimate those from microstructure measurements by as much as 1-2 orders of magnitude (Frants et al, 2013).…”

Observations of Small-Scale Secondary Instabilities during the Shoaling of Internal Bores on a Deep-Ocean Slope

“…The measured stratification is thus representative of the near-bottom region, which is also small compared to the full water depth (about 100 m compared with a total depth of 920 m). This approach is more or less similar to some direct numerical simulation (DNS) studies using Thorpe reordering on a fixed domain rather than on single overturns (e.g., Mater et al 2013). By differencing the potential energy contained in the raw and reordered density profiles, it is also possible to (Dillon 1984;Crawford 1986;Dillon and Park 1987) …”

Abyssal plain hills and internal wave turbulence

“…This reflects passive scalar turbulence dominated by shear (Tennekes and Lumley, 1972). After sufficient averaging this passive scalar turbulence is efficient (Mater et al, 2015). At intermediate depth levels and in short frequency ranges of the spectral data, slopes vary between −2 and −1.…”