Enhancement of Mesoscale Eddy Stirring at Steering Levels in the Southern Ocean

Abstract

Meridional cross sections of effective diffusivity in the Southern Ocean are presented and discussed. The effective diffusivity, K eff , characterizes the rate at which mesoscale eddies stir properties on interior isopycnal surfaces and laterally at the sea surface. The distributions are obtained by monitoring the rate at which eddies stir an idealized tracer whose initial distribution varies monotonically across the Antarctic Circumpolar Current (ACC). In the absence of observed maps of eddying currents in the interior ocean, the advecting velocity field is taken from an eddy-permitting state estimate of the Southern Ocean (SOSE). A threedimensional advection-diffusion equation is solved and the diffusivity diagnosed by applying the Nakamura technique on both horizontal and isopycnal surfaces. The resulting meridional sections of K eff reveal intensified isopycnal eddy stirring (reaching values of ;2000 m 2 s 21 ) in a layer deep beneath the ACC but rising toward the surface on the equatorward flank. Lower effective diffusivity values (;500 m 2 s 21 ) are found near the surface where the mean flow of the ACC is strongest. It is argued that K eff is enhanced in the vicinity of the steering level of baroclinic waves, which is deep along the axis of the ACC but shallows on the equatorial flank. Values of K eff are also found to be spatially correlated with gradients of potential vorticity on isopycnal surfaces and are large where those gradients are weak and vice versa, as expected from simple dynamical arguments. Finally, implications of the spatial distributions of K eff for the dynamics of the ACC and its overturning circulation are discussed.

On the observability of turbulent transport rates by Argo: supporting evidence from an inversion experiment

“…Finally, K σ in ECCO v4 (Fig. 11, middle panel) hints at a steering level effect (Green, 1970;Abernathey et al, 2010;Ferrari and Nikurashin, 2010) below 500 m in the ACC that is not seen in Liu et al (2012). Such differences are indicative of large overall uncertainty in inverse parameter estimates.…”

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…To assess the relative role of eddy stirring relative to vertical mixing, we consider previous estimates of from modeling studies. High-resolution modeling of the ACC system has been considered by a number of investigators (Abernathey et al (2010), Ferrari and Nikurashin (2010), Klocker and Abernathey (2014)). Estimates for the general ACC system show in the range (10 2 − 10 3 2 / ).…”

“…The vertical scales of intrusions are typcially in the 50 to 100m range, and the gradients of temperature and salinity on these scales are the link between the mesoscale variance production and the microscale variance dissipation. While these scales are too fine to be resolved in climate models, they are resolved in global-scale process models that are used to examine the Southern Ocean (Abernathey et al (2010), Klocker and Abernathey (2014)). Our estimates of could be used to prescribe diffusivities acting in the Southern Ocean intrusive watermasses of these models.…”

Estimating Suppression of Eddy Mixing by Mean Flows

“…This suggests that eddy transport is greatest near the mean current itself, as eddy mixing scales with eddy kinetic energy (EKE ;Prandtl 1925;Holloway 1986). However, there is a growing body of evidence suggesting that the mean flow also suppresses mixing (e.g., Bower et al 1985;Nakamura and Ma 1997;Haynes and Shuckburgh 2000a,b;Allen and Nakamura 2001;Marshall et al 2006;Abernathey et al 2010;Ferrari and Nikurashin 2010). Thus assessing cross-jet transport involves finding the balance between the enhancement of mixing by instability and the suppression imposed by the mean.…”