Global Patterns of Mesoscale Eddy Properties and Diffusivities

Abstract

Mesoscale eddies play a major role in the transport of tracers in the ocean. Focusing on a sector in the east Pacific, the authors present estimates of eddy diffusivities derived from kinematic tracer simulations using satellite-observed velocity fields. Meridional diffusivities are diagnosed, and how they are related to eddy properties through the mixing length formulation of Ferrari and Nikurashin, which accounts for the suppression of diffusivity due to eddy propagation relative to the mean flow, is shown. The uniqueness of this study is that, through systematically varying the zonal-mean flow, a hypothetical “unsuppressed” diffusivity is diagnosed. At a given latitude, the unsuppressed diffusivity occurs when the zonal-mean flow equals the eddy phase speed. This provides an independent estimate of eddy phase propagation, which agrees well with theoretical arguments. It is also shown that the unsuppressed diffusivity is predicted very well by classical mixing length theory, that is, that it is proportional to the rms eddy velocity times the observed eddy size, with a spatially constant mixing efficiency of 0.35. Then, the suppression factor is estimated and it is shown that it too can be understood quantitatively in terms of easily observed mean flow properties. The authors then extrapolate from these sector experiments to the global scale, making predictions for the global surface eddy diffusivity. Together with a prognostic equation for eddy kinetic energy and a theory explaining observed eddy sizes, these concepts could potentially be used in a closure for eddy diffusivities in coarse-resolution ocean climate models.

Connection between encounter volume and diffusivity in geophysical flows

“…A number of diffusivity estimates other than Okubo's have been made for the Gulf Stream extension region (e.g., Zhurbas and Oh, 2004;LaCasce, 2008;Rypina et al, 2012;Abernathey and Marshall, 2013;Klocker and Abernathey, 2014;or Cole et al, 2015). These estimates are based on surface drifters (Zhurbas and Oh, 2004;LaCasce, 2008;Rypina et al, 2012), satellite-observed velocity fields (Abernathey and Marshall, 2013;Klocker and Abernathey, 2014;Rypina et al, 2012), and Argo float observations (Cole et al, 2015), and they use either the spread of drifters or the evolution of simulated or observed tracer fields to deduce diffusivity.…”

“…These estimates are based on surface drifters (Zhurbas and Oh, 2004;LaCasce, 2008;Rypina et al, 2012), satellite-observed velocity fields (Abernathey and Marshall, 2013;Klocker and Abernathey, 2014;Rypina et al, 2012), and Argo float observations (Cole et al, 2015), and they use either the spread of drifters or the evolution of simulated or observed tracer fields to deduce diffusivity. The resulting diffusivities are spatially varying and span 2 orders of magnitude, from 2 × 10 4 m 2 s −1 in the most energetic regions in the immediate vicinity of the Gulf Stream and its extension, to 10 3 m 2 s −1 in less energetic areas, to 200 m 2 s −1 in the coastal areas of the Slope Sea.…”

“…The resulting diffusivities are spatially varying and span 2 orders of magnitude, from 2 × 10 4 m 2 s −1 in the most energetic regions in the immediate vicinity of the Gulf Stream and its extension, to 10 3 m 2 s −1 in less energetic areas, to 200 m 2 s −1 in the coastal areas of the Slope Sea. Diffusivity estimates vary significantly depending on the initial tracer distribution used (Abernathey and Marshall, 2013) and depend on whether the suppression by the mean current has been taken into account (Klocker and Abernathey, 2014). The diffusivity tensor has also been shown to be anisotropic, with a large anisotropy ratio near the Gulf Stream (Rypina et al, 2012).…”

A computational study of turbulent kinetic energy transport in barotropic turbulence on the f-plane

“…Their structure and energy vary widely over the global ocean, [9][10][11][12] and recent studies have suggested that mesoscale eddy energy transport may play a significant role in setting the large scale distribution of mesoscale eddy energy. 19,20,22 In long-running global climate models, these eddies are not resolved, and mesoscale eddy energy transport is not taken into account in most parameterizations of their effects.…”

“…Several parameterizations of κ GM are of the form κ GM ∝ ℓ 2 /T or κ GM ∝ V ℓ where V , ℓ, and T are velocity, length, and time scales of the eddies that are given as functions of the resolved large scale flow. 7,8 The characteristic length, time, and velocity scales of mesoscale eddies vary considerably over the global ocean, [9][10][11][12] and any successful model for κ GM should reflect this. In a recent diagnostic study of the effects of mesoscale eddies in an idealized channel simulation Bachman and Fox-Kemper 3 found that it was possible to construct a very accurate model of the form κ GM ∝ V ℓ provided that V was diagnosed directly from an eddy-resolving simulation, whereas available predictions of V based only on local properties of the large-scale flow were inaccurate.…”