Diapycnal diffusivity at the upper boundary of the tropical North Atlantic oxygen minimum zone

Abstract

[1] A deliberate tracer release experiment in 2008-2010 was used to study diapycnal mixing in the tropical northeastern Atlantic. The tracer (CF 3 SF 5 ) was injected on the isopycnal surface s Q = 26.88 kg m À3 , which corresponds to about 330 m depth. Three surveys, performed 7, 20, and 30 months after the release, sampled the vertically and laterally expanding tracer patch. The mean diapycnal mixing estimate over the entire region occupied by the tracer and the period of 30 months was found to be (1.19 AE 0.18) Â 10 À5 m 2 s À1 , or, alternatively, (3.07 AE 0.58) Â 10 À11 (kg m À3 ) 2 s À1 as computed from the advection-diffusion equation in isopycnal coordinates with the thickness-weighted averaging. The latter method is preferable in the regions of different stratification for it yields local diapycnal mixing estimates varying less with stratification than their Cartesian coordinate counterparts. Results of this study are comparable to the results of the North Atlantic tracer release experiment (NATRE). However, the internal wave-wave interaction models predict reduced mixing from the breaking of internal waves at low latitudes. Thus, the diapycnal diffusivity found in this study is higher than parameterized by the low latitude of the site (4 N-12 N).

On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

“…Most of the terms in the oxygen budget are associated with significant error, which particularly is the case for consumption and meridional eddy flux. Due to the TRE (Banyte et al, 2012) and repeated microstructure measurements , the error in the diapycnal oxygen supply is comparatively small. The diapycnal oxygen supply is strongest slightly above the deep OMZ core, where it accounts for about one third of the oxygen supply required to balance consumption.…”

“…In order to estimate the vertical or diapycnal oxygen supply, the diapycnal diffusivity K ρ as a measure for diapycnal mixing is required. From the diapycnal spread of the deliberately released tracer during GUTRE, a mean diapycnal diffusivity of (1.2 ± 0.2) × 10 −5 m 2 s −1 was derived (Banyte et al, 2012). The tracer was injected on the isopycnal σ θ =26.88 kg m −3 (about 330 m), corresponding to the oxycline above the deep OMZ.…”

“…The depth of release, of about 330 m, corresponds to the depth of the oxycline above the deep oxygen minimum. During the following 2.5 years, three tracer surveys were carried out to measure the vertical and horizontal spreading of the tracer (Banyte et al (2012), Table 1). During the second TRE, OSTRE (Oxygen Supply Tracer Release Experiment), in November 2012, 88.5 kg of the same tracer were released at 21 • W, 11 • N on the potential density surface σ θ = 27.03 kg m −3 , corresponding to about 500 m in depth, which is in the core region of the ETNA OMZ.…”

Coupled physical/biogeochemical modeling including O<sub>2</sub>-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

“…Indeed, between the "standard simulation" using the parameterization of Nevison et al (2003) (Test no. 4) and the "reference simulation" using the one from Suntharalingam et al (2000, 2012, mean N 2 O concentrations increased by a factor of 20 % in the intermediate layer (100 to 400 m depth) over the continental shelf and shelf break where the OMZ is developed (Fig. 12a).…”

“…"Original values" are from the original papers, and changed for the needs of our configuration. We recall the references used by Koné et al (2005) Fasham et al (1990) S00-12: Suntharalingam et al (2000, 2012.…”