Abyssal Mixing in the Brazil Basin*

Abstract

One of the major objectives of the Deep Basin Experiment, a component of the World Ocean Circulation Experiment, was to quantify the intensity and spatial distribution of deep vertical mixing within the Brazil Basin. In this study, basin-averaged estimates of deep vertical mixing rates are calculated using two independent methodologies and datasets: 1) vertical fluxes are derived from large-scale temperature and density budgets using direct measurements of deep flow through passages connecting the Brazil Basin to surrounding basins and a comprehensive hydrographic dataset within the basin interior and 2) vertical mixing rates are estimated from finescale bathymetry and hydrographic data using a functional relationship between turbulent dissipation and bathymetric roughness, deduced from localized measurements of ocean microstructure obtained during the Deep Basin Experiment. The space-time mean estimates of vertical mixing diffusivities across representative surfaces within the Antarctic Bottom Water layer fell in the range ϳ 1-5(ϫ 10 Ϫ4 m 2 s Ϫ1) and were indistinguishable from each other within the estimation uncertainties. The mixing rates inferred from potential temperature budgets update, and are consistent with, earlier estimates that were based on less data. Mixing rates inferred from budgets bounded by neutral surfaces are not significantly different from the former. This implies that lateral eddy fluxes along isopycnals are not important in the potential temperature budgets, at least within the large estimation uncertainties. Unresolved processes, such as cabbeling and low frequency variability, which complicate inference of mixing from large-scale budgets, have been considered. The agreement between diffusivity estimates based on a modeled relationship between bathymetric roughness and turbulent dissipation, with those inferred from large-scale budgets, provides independent confirmation that the mixing rates have been accurately quantified.

High mixing rates in the abyssal Southern Ocean

“…A high value of diffusivity does not necessarily imply a large energy dissipation, if strati®cation is low. The dissipation we ®nd is 1:1 3 10 29 W kg 21 , similar to, but slightly smaller than, values deduced for the deep Brazil basin 12 . Taking the median thickness of the LWDSW layer to be 423 6 20 m, to mix the LWSDW into the overlying waters implies an energy dissipation of 1:8 6 0:5 3 10 9 W: small in comparison with the global requirements of 2:1 3 10 12 W (ref.…”

Abyssal mixing from bottom boundary effects in Mid-Atlantic Ridge flank canyons

“…These data reflect only mixing below 2000 m depth. Morris, et al (2001) , divides the area into similar topographic classes to estimate the total mixing in the Brazil Basin, and finds results consistent with this and with heat budgets like those in Hogg, et al (1982).…”

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.…”