Diapycnal Mixing in the Antarctic Circumpolar Current

Abstract

The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 6 0.2) 3 10 25 m 2 s 21. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 6 0.07) 3 10 25 m 2 s 21. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%-50% above the Garrett-Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.

An analytical theory of the buoyancy–Kolmogorov subrange transition in turbulent flows with stable stratification

“…Applied in the oceanographic context, this result stipulates that the breaking internal waves contribute to the diapycnal scalar mixing by generating turbulence dominated scales of the order of O(k −1 O ). In agreement with the conclusions reached in Ledwell et al [113], the prevailing in the mid-depth oceanic interior values of N and yield the diapycnal diffusivity that appears to be an order of magnitude smaller than what is necessary to maintain the overturning circulation.…”

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

“…In particular, the first direct measurements of turbulent dissipation and mixing rates in the Southern Ocean were made as part of the DIMES study. Ledwell et al (2010), St. Laurent et al (2012), and Sheen et al (2014) showed that enhanced internal wave fine-structure and turbulence levels were not widespread, but limited to frontal zones where strong bottom currents collide with steep large-amplitude topography.…”

“…A description of the US2 study, as well as a discussion of the US2 data, has been previously presented by Ledwell et al (2010) and St. Laurent et al (2012).…”

Freshwater transport in the coupled ocean-atmosphere system: a passive ocean

“…Morerever, our ocean model employs a constant vertical mixing coefficient. Observations suggest that abyssal mixing varies greatly in space (i.e., Polzin et al 1997) although thermocline values appear uniformly low (Ledwell et al 1993(Ledwell et al , 2011. The dependance of the FWT on mixing is unclear.…”