Turbulence and Diapycnal Mixing in Drake Passage

Abstract

Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) is constricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10 28 W kg 21 at abyssal and middepths. The mixing enhancement in the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1 3 10 24 m 2 s 21 . This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smoother bathymetry. Outside of the frontal regions, dissipation rates are O(10 210 ) W kg 21 , comparable to the background levels of turbulence found throughout most mid-and low-latitude regions of the global ocean.

The Finite Element Sea Ice-Ocean Model (FESOM) v.1.4: formulation of an ocean general circulation model

“…Observational estimates indicate that the diapycnal diffusivity is of the order of 0.12 ± 0.02 − 0.17 ± 0.02 × 10 −4 m 2 s −1 (Ledwell et al, 1993(Ledwell et al, , 1998 in the subtropical Atlantic pycnocline and 0.15 ± 0.07 × 10 −4 m 2 s −1 in the North Pacific Ocean (Kelley and Van Scoy, 1999), and much smaller values were observed near the equator (Gregg et al, 2003). In the deep ocean the diffusivity is observed to be small (0.1 × 10 −4 m 2 s −1 ) over smooth topography and much larger (1 − 5 × 10 −4 m 2 s −1 ) near the bottom in regions of rough topography Toole et al, 1997;Ledwell et al, 2000;Morris et al, 2001;Laurent et al, 2012). A modified version of the Bryan and Lewis (1979) background vertical tracer diffusivity is used poleward of 15 • in the model formulation with FESOM (Fig.…”

Enhanced Diapycnal Diffusivity in Intrusive Regions of the Drake Passage

“…If shear-driven turbulence is dominant, it is expected that the two methods for computing diffusivity will produce equal values. However, in the case of double-diffusive instability contributing to turbulent mixing, the Osborn (1980) model fails, as the concept of mixing efficiency becomes ill defined (St. Laurent and Schmitt 1999). Thus, the Osborn and Cox (1972) method provides the reliable means for estimating diffusivity for double-diffusive scenarios.…”

Mixing Variability in the Southern Ocean

“…Observations of diffusivity in the thermocline waters determined by tracer release (Ledwell et al 1993(Ledwell et al , 1998, and microstructure studies (Gregg 1987), sparked controversy with values of diffusivity an order of magnitude lower than the canonical value. For the past 20 yr, observational estimates have shown diffusivity to be highly spatially nonuniform, with values above rough topography up to two orders of magnitude larger than the canonical value (Polzin et al 1996(Polzin et al , 1997Ledwell et al 2000;St. Laurent et al 2001;Naveira Garabato et al 2004b;Sloyan 2005 We present the first estimates of turbulent mixing from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats.…”