Quantifying Diapycnal Mixing in an Energetic Ocean

Abstract

Turbulent diapycnal mixing controls global circulation and the distribution of tracers in the ocean. For turbulence in stratified shear flows, we introduce a new turbulent length scale Lρ dependent on χ. We show the flux Richardson number Rif is determined by the dimensionless ratio of three length scales: the Ozmidov scale LO, the Corrsin shear scale LS, and Lρ. This new model predicts that Rif varies from 0 to 0.5, which we test primarily against energetic field observations collected in 100 m of water on the Australian North West Shelf (NWS), in addition to laboratory observations. The field observations consisted of turbulence microstructure vertical profiles taken near moored temperature and velocity turbulence time series. Irrespective of the value of the gradient Richardson number Ri, both instruments yielded a median Rif=0.17, while the observed Rif ranged from 0.01 to 0.50, in agreement with the predicted range of Rif. Using a Prandtl mixing length model, we show that diapycnal mixing Kρ can be predicted from Lρ and the background vertical shear S. Using field and laboratory observations, we show that Lρ=0.3LE where LE is the Ellison length scale. The diapycnal diffusivity can thus be calculated from Kρ=0.09LES2. This prediction agrees very well with the diapycnal mixing estimates obtained from our moored turbulence instruments for observed diffusivities as large as 10−1 m2s−1. Moorings with relatively low sampling rates can thus provide long time series estimates of diapycnal mixing rates, significantly increasing the number of diapycnal mixing estimates in the ocean.

Destratification of thermally stratified turbulent open-channel flow by surface cooling

“…In KWAZ we demonstrated close similarities between the stratified turbulence in the central region of our channel flow with sheared stratified turbulence observed in large scale geophysical and environmental flows. In particular, our simulations demonstrated excellent agreement with various parameterisations for oceanic diapycnal mixing (Osborn & Cox 1972; Osborn 1980; Ivey, Bluteau & Jones 2018), a number of scalings based on Monin–Obukhov theory (Chung& Matheou 2012; Scotti & White 2016; Zhou, Taylor & Caulfield 2017) and the classic Monin–Obukhov stability functions determined by Dyer (1974) from atmospheric field data.…”

Asymptotic Dynamics of High Dynamic Range Stratified Turbulence

“…such that L 2 m /L 2 ρ = P r T . Therefore, since for our flows P r T ≈ 1 and P ≈ (1+Γ)ǫ, it is apparent that the density length scale at the heart of the model presented by Ivey et al [7] is coupled to the Corrsin scale by L ρ /L C = √ 1 + Γ, and the key parameter D ≡ χ/ǫ = 1/Γ has nearly fixed value.…”

Mixing Efficiency in the Presence of Stratification: When Is It Constant?

“…If L t is taken to be the Thorpe scale, the length scale of overturns seen in profiles, it could be calculated from microstructure data, although for weak stratification, it could be highly dependent on sensor noise. This would be worth exploring in the future, especially given that the Thorpe scale and the Ellison scale, the scale defined by time series of the fluctuating density (Ivey et al ), are closely correlated, thus facilitating comparable analysis of both types of data.…”