Estimating Suppression of Eddy Mixing by Mean Flows

Abstract

Particle-and tracer-based estimates of lateral diffusivities are used to estimate the suppression of eddy mixing across strong currents. Particles and tracers are advected using a velocity field derived from sea surface height measurements from the South Pacific, in a region west of Drake Passage. This velocity field has been used in a companion paper to show that both particle-and tracer-based estimates of eddy diffusivities are equivalent, despite recent claims to the contrary. These estimates of eddy diffusivities are here analyzed to show 1) that the degree of suppression of mixing across the strong Antarctic Circumpolar Current is correctly predicted by mixing length theory modified to include eddy propagation along the mean flow and 2) that the suppression can be inferred from particle trajectories by studying the structure of the autocorrelation function of the particle velocities beyond the first zero crossing. These results are then used to discuss how to compute lateral and vertical variations in eddy diffusivities using floats and drifters in the real ocean.

Lagrangian scale of particle dispersion in turbulence

“…The above conclusion about the dominance in the diffusivity of one scale, which is close to the forcing scale, is at odds with results obtained in several studies, which find that the diffusivity is determined by the energy-containing eddy scales [20][21][22][23] . In 2D turbulence, in the presence of the inverse energy cascade, these scales are in the lower-k part of the Eulerian spectrum.…”

“…In 2D turbulence, in the presence of the inverse energy cascade, these scales are in the lower-k part of the Eulerian spectrum. The most likely reason for such discrepancy is the fact that in most models of turbulence (for example, Klocker et al 23 ; Boffetta and Musacchio 24 ), forcing is defined as spatially periodic (localized in k), but random in time (white noise). In these experiments, both in electromagnetically driven and in the Faraday waves driven turbulence, forcing scale vortices are substantially more coherent than turbulent eddies at any other scales.…”

Energy pathways and structures of oceanic eddies from the ECCO2 state estimate and simplified models

“…The 1.5-layer model has been widely used by the community to explore eddy characteristics and eddy mixing (e.g. Early et al, 2011;Cushman-Roisin et al, 1990;McWilliams and Flierl, 1979;Jacob et al, 2002;Radko and Stern, 1999;Klocher et al, 2012a). Though greatly simplified, the transport properties/zonal propagation speeds of the nonlinear vortices and the non-dispersive line in the ω − k spectrum in the nonlinear 1.5-layer model resemble those from the altimetry (Early et al, 2011).…”

“…Many studies exist about the contribution of eddies and the mean flow to tracer transport and eddy diffusivity (e.g. Klocher et al, 2012a;Rypina et al, 2012;Shuckburgh et al, 2009). Striations are coherent in the quasi-zonal direction and can cause shear dispersion and thus anisotropic transport.…”

“…To our best knowledge, though the effect of mean flow on tracer mixing induced by the total eddies has been studied (e.g. Klocher et al, 2012a;Shuckburgh et al, 2009), the partition of the contribution of striations and high-frequency eddies to mixing has not been explored. Next we present one possible framework to split the contribution.…”