Search

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 Lρdependent on χ. We show the flux Richardson number Rifis 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 Rifvaries 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 Rifranged 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 Kρcan be predicted from Lρand the background vertical shear S. Using field and laboratory observations, we show that Lρ=0.3LEwhere LEis 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 10−1m2s−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. We introduce a new turbulent length scale Lρdependent on χ, and show Lρis proportional to the Ellison scale LERifis determined by the dimensionless ratio of LO, LSand Lρand varies from 0 to 0.5, consistent with field observationsThe model Kρ=0.09LE2Scan predict diapycnal diffusivity from conventional moored observations