Properties and Origins of the Anisotropic Eddy-Induced Transport in the North Atlantic

Abstract

This study examines anisotropic transport properties of the eddying North Atlantic flow, using an idealized model of the double-gyre oceanic circulation and altimetry-derived velocities. The material transport by the time-dependent flow (quantified by the eddy diffusivity tensor) varies geographically and is anisotropic, that is, it has a well-defined direction of the maximum transport. One component of the time-dependent flow, zonally elongated large-scale transients, is particularly important for the anisotropy, as it corresponds to primarily zonal material transport and long correlation time scales. The importance of these large-scale zonal transients in the material distribution is further confirmed with simulations of idealized color dye tracers, which has implications for parameterizations of the eddy transport in non-eddy-resolving models.

Investigating the Eddy Diffusivity Concept in the Coastal Ocean

“…Note that this Lagrangian approach is different from the Eulerian approach, where eddy diffusivity is estimated using the ''eddy-only'' component of the flow and thus characterizes the local effects of eddies at a given geographical location rather than following the Lagrangian particle paths. This technique has been recently used to estimate eddy diffusivities of real and simulated drifters in the North Atlantic (Rypina et al 2012;Kamenkovich et al 2015). This method is also equivalent to the eddy diffusivities resulting from integrating the autocorrelation of the Lagrangian velocities along pseudotrajectories (i.e., of the eddy velocity component following full Lagrangian trajectories; Davis 1991;Sallee et al 2008;Griesel et al 2010;R.…”

“…Despite the wide use of the diffusion-based parameterizations, thorough tests of the applicability and limitations of lateral eddy diffusivity concept to real oceanic flows are rare (Kamenkovich et al 2015), and more work is needed to better understand the validity of diffusionbased parameterizations. In this paper, we make use of data collected during a focused field experiment involving drifters, dye, and a high-resolution (800 m) highfrequency (HF) radar system in the coastal ocean south of Martha's Vineyard, Massachusetts, to examine the nature of eddy diffusivity in a coastal environment.…”

“…Lateral eddy diffusivity estimation using single-particle dispersion tensor A convenient, although not unique, method for estimating diffusivity is based on calculating the singleparticle dispersion for an ensemble of Lagrangian particles relative to their center of mass (e.g., LaCasce 2008; Rypina et al 2012;Kamenkovich et al 2015).…”

Multi-Iteration Approach to Studying Tracer Spreading Using Drifter Data

“…Understanding and quantifying the effects of eddies versus mean advection on the evolution of the tracer field is key to many tracer dispersal problems (Kamenkovich et al 2015;Rypina et al 2012;Booth and Kamenkovich 2008;Henning and Vallis 2004;Sallee et al 2010Sallee et al , 2012FIG. 4.…”

Connection between encounter volume and diffusivity in geophysical flows

“…For example, in some cases it may be more feasible to count encounters rather than to measure interactions or property exchanges directly, whereas in other cases the number of encounters might be most pertinent to the process in question but difficult to measure directly. In many applications, including ocean turbulence, the most commonly used metric of mixing is the eddy diffusivity, κ, a quantity that relates transport of fluid elements by turbulent eddies to diffusion (LaCasce, 2008;Vallis, 2006;Rypina et al, 2012;Kamenkovich et al, 2015). The underlying assumption is that the eddy field drives downgradient tracer transfer, similar to molecular diffusion but with a different (larger) diffusion coefficient.…”