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- Author or Editor: E. C. Itsweire x
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Abstract
High-resolution velocity shear, CTD, and microstructure measurements were made simultaneously from the research submarine Dolphin in Monterey Bay in October 1984. During three consecutive dives, the Dolphin cycled between the surface and 110 m along predetermined tracks 10 miles northwest of Monterey. Inside the seasonal thermocline, the vertical velocity shear appeared to be concentrated in layers 10 m thick extending several kilometers horizontally. The thickness of the shear layers is consistent with the typical size of turbulent patches encountered in the seasonal thermocline. In contrast, no large shear layers were observed below a 50 m depth. The depth levels at which the shear layers were observed were nearly constant throughout each dive, suggesting that the shear layers, with some unknown degree of intermittency, might extend horizontally over several square kilometers. The shear vector inside the seasonal themocline (at σ t = 25.5) rotated 360° over an inertial period, but did appear to propagate vertically over the 30-hour observation period. These observations suggest that the passage of a storm caused the upper thermocline to ring, creating a local jetlike flow below the mixed layer.
Abstract
High-resolution velocity shear, CTD, and microstructure measurements were made simultaneously from the research submarine Dolphin in Monterey Bay in October 1984. During three consecutive dives, the Dolphin cycled between the surface and 110 m along predetermined tracks 10 miles northwest of Monterey. Inside the seasonal thermocline, the vertical velocity shear appeared to be concentrated in layers 10 m thick extending several kilometers horizontally. The thickness of the shear layers is consistent with the typical size of turbulent patches encountered in the seasonal thermocline. In contrast, no large shear layers were observed below a 50 m depth. The depth levels at which the shear layers were observed were nearly constant throughout each dive, suggesting that the shear layers, with some unknown degree of intermittency, might extend horizontally over several square kilometers. The shear vector inside the seasonal themocline (at σ t = 25.5) rotated 360° over an inertial period, but did appear to propagate vertically over the 30-hour observation period. These observations suggest that the passage of a storm caused the upper thermocline to ring, creating a local jetlike flow below the mixed layer.
Abstract
Direct numerical simulations of the time evolution of homogeneous stably stratified turbulent sheer flows have been performed for several Richardson numbers Ri and Reynolds numbers Rλ. The results show excellent agreement with length scale models developed from laboratory experiments to characterize oceanic turbulence. When the Richardson number Ri is less than the stationary value Ri s , the turbulence intensity grows at all scales; the growth rate is a function of Ri. The size of the vertical density inversions also increases. When Ri ≥ Ri, the largest turbulent eddies become vertically constrained by buoyancy when the Ellison (turbulence) scale L Eand the Ozmidov (buoyancy) scale L O are equal. At this point the mixing is most efficient and the flux Richardson number or mixing efficiency is Rf ≈ 0.20 for the stationary Richardson number Ri s = 0.21. The vertical mass flux becomes countergradient when ε ≈ 19vN 2, and vertical density overturns are suppressed in few than half of a Brunt-Väisälä period. The results of the simulations have also been recast in terms of the hydrodynamic phase diagram introduced for fossil turbulence models. In this framework, buoyancy control of the energy-containing scales begins when ε ≈ 4DCN 2. This value is in good agreement with indirect laboratory observations and field observations. Careful examination et the individual components of the velocity and scalar dissipation tensors reveals that, for fully developed, stably stratified shear flows, these tensors are far from isotropic, implying that the isotropic formulas often used to calculate the dissipation rates ε and χ in the oceanic thermocline could underestimate these rates by factors of 2 to 4. Finally, the validity of the steady-state models used to estimate vertical eddy diffusivities in the thermocline is discussed.
Abstract
Direct numerical simulations of the time evolution of homogeneous stably stratified turbulent sheer flows have been performed for several Richardson numbers Ri and Reynolds numbers Rλ. The results show excellent agreement with length scale models developed from laboratory experiments to characterize oceanic turbulence. When the Richardson number Ri is less than the stationary value Ri s , the turbulence intensity grows at all scales; the growth rate is a function of Ri. The size of the vertical density inversions also increases. When Ri ≥ Ri, the largest turbulent eddies become vertically constrained by buoyancy when the Ellison (turbulence) scale L Eand the Ozmidov (buoyancy) scale L O are equal. At this point the mixing is most efficient and the flux Richardson number or mixing efficiency is Rf ≈ 0.20 for the stationary Richardson number Ri s = 0.21. The vertical mass flux becomes countergradient when ε ≈ 19vN 2, and vertical density overturns are suppressed in few than half of a Brunt-Väisälä period. The results of the simulations have also been recast in terms of the hydrodynamic phase diagram introduced for fossil turbulence models. In this framework, buoyancy control of the energy-containing scales begins when ε ≈ 4DCN 2. This value is in good agreement with indirect laboratory observations and field observations. Careful examination et the individual components of the velocity and scalar dissipation tensors reveals that, for fully developed, stably stratified shear flows, these tensors are far from isotropic, implying that the isotropic formulas often used to calculate the dissipation rates ε and χ in the oceanic thermocline could underestimate these rates by factors of 2 to 4. Finally, the validity of the steady-state models used to estimate vertical eddy diffusivities in the thermocline is discussed.
