Suppression of Eddy Diffusivity across Jets in the Southern Ocean

Abstract

Geostrophic eddies control the meridional mixing of heat, carbon, and other climatically important tracers in the Southern Ocean. The rate of eddy mixing is typically quantified through an eddy diffusivity. There is an ongoing debate as to whether eddy mixing in enhanced in the core of the Antarctic Circumpolar Current or on its flanks. A simple expression is derived that predicts the rate of eddy mixing, that is, the eddy diffusivity, as a function of eddy and mean current statistics. This novel expression predicts suppression of the cross-jet eddy diffusivity in the core of the Antarctic Circumpolar Current, despite enhanced values of eddy kinetic energy. The expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations. This work suggests that the meridional eddy diffusivity across the Antarctic Circumpolar Current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current.

Southern Ocean deep convection as a driver of Antarctic warming events

“…The bipolar ocean seesaw mechanism proposes that the centennial-to-millennial-scale However, the ocean seesaw mechanism is challenged on physical oceanographic grounds by the difficulty of propagating anomalies between the South Atlantic and Antarctica; the problem is that the Antarctic Circumpolar Current (ACC) presents a dynamic barrier [Ferrari and Nikurashin, 2010] and there is no zonal boundary for wave propagation, leaving eddy fluxes of temperature and salinity or atmospheric teleconnections to propagate the signal across the ACC [e.g. Schmittner et al, 2003;Vettoretti and Peltier, 2015].…”

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

“…The mean advection can significantly modulate the eddy-induced transport. In particular, the meridional diffusivity is enhanced at steering levels (Green 1970;Killworth 1997) and is suppressed by zonal propagation of eddies relative to the mean zonal flow (Ferrari and Nikurashin 2010); meridional shear in zonal currents can cause shear dispersion (e.g., Taylor 1953;Young et al 1982;Smith 2005); and cross-jet transport barriers exist on strong currents such as the Gulf Stream and its extension (Samelson 1992;Rypina et al 2011) and alternating multiple jets (Haynes et al 2007;Berloff et al 2009). In addition, powerful mean currents, such as those within the western boundary regions and the upper-ocean Antarctic Circumpolar Current, can dwarf the along-stream eddy-induced transport.…”

Settling velocity of quasi-neutrally-buoyant inertial particles

“…It consists in rewriting the unknown as the product between the Gaussian weight and an expansion in a power series in y up to the order in question, in this case the first, with space-time-dependent prefactors (notice that in (20) an expansion up to order 0, i.e. no expansion at all, appeared):…”

“…This steady or periodic behavior of p(x, v, t) is due to the steady/periodic character of the fluid flow u(x, t), which is the only non-constant driving agent in the evolution equation (3). Keeping into account the expansions of the terms making up p starting from (20), definition (5) translates into:…”

“…The same technique can be extended to handle the case of a random, homogeneous, and stationary velocity field [14] with some non-trivial modifications in the rigorous proofs of convergence [15]. For an interesting investigation of the role played by mean currents on the eddy diffusivity of tracers, see, e.g., [16][17][18][19][20][21].…”