Presenter: Henri Drake (Princeton / GFDL)
Description:
Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Measurements of in-situ turbulence reveal that mixing is bottom-enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. However, in-situ mixing estimates are indirect and the inferred vertical velocities have not yet been confirmed. Purposeful releases of inert tracers, and their subsequent spreading, have been used to independently infer turbulent diffusivities; however, these Tracer Release Experiments (TREs) provide estimates in excess of in-situ ones. In an attempt to reconcile these differences, Ruan and Ferrari (2021) derived exact buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. We show in a numerical simulation that tracer-averaged diapycnal motion is directly driven by the tracer-averaged buoyancy velocity, a convolution of the asymmetric upwelling/downwelling dipole. Diapycnal spreading, however, involves both the expected contribution from the tracer-averaged in-situ diffusion and an additional diapycnal shear dispersion term. Diapycnal shear dispersion, caused by correlations between buoyancy and buoyancy velocity anomalies, can either enhance or reduce tracer spreading. Diapycnal shear dispersion causes diapycnal stretching in the stratified interior, which is compensated by diapycnal contraction near the bottom; for simulations of the Brazil Basin Tracer Release Experiment, these two distinct shear dispersion effects nearly cancel by coincidence. By contrast, a numerical tracer released near the bottom experiences leading-order shear dispersion that varies in time. These results suggest mixing estimates from TREs are not unambiguous, especially near topography, and that more attention should be paid towards the evolution of tracers’ first moments.
More Information: hdrake.github.io
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Full list of Authors
- Raffaele Ferrari (Massachusetts Institute of Technology)
- Xiaozhou Ruan (Massachusetts Institute of Technology)
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Diapycnal motion, diffusion, and stretching of tracers above rough topography
Category
Scientific Session > PL - Physical Oceanography: Mesoscale and Larger > PL05 Ventilation, circulation and mixing of the deep ocean: Observing and modeling the deep and bottom limbs of the meridional overturning circulation
Description
Presentation Preference: Oral
Supporting Program: None
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