Diapycnal diffusivity in the core and oxycline of the tropical North Atlantic oxygen minimum zone.

Köllner, Manuela, Visbeck, Martin , Tanhua, Toste and Fischer, Tim (2016) Diapycnal diffusivity in the core and oxycline of the tropical North Atlantic oxygen minimum zone. Journal of Marine Systems, 160 . pp. 54-63. DOI 10.1016/j.jmarsys.2016.03.012.

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Diapycnal diffusivity estimates from two Tracer Release Experiments (TREs) and microstructure measurements in the oxycline and core of the oxygen minimum zone (OMZ) in the eastern tropical North Atlantic are compared. For the first time, two TREs within the same area at different depths were realized: the Guinea Upwelling Tracer Release Experiment (GUTRE) initiated in 2008 in the oxycline at approximately 320 m depth, and the Oxygen Supply Tracer Release Experiment (OSTRE) initiated in 2012 in the core of the OMZ at approximately 410 m depth. The mean diapycnal diffusivity Dz was found to be insignificantly smaller in the OMZ core with (1.06 ± 0.24) × 10− 5 m2 s− 1 compared to (1.11 ± 0.22) × 10− 5 m2 s− 1 90 m shallower in the oxycline. Unexpectedly, GUTRE tracer was detected during two of the OSTRE surveys which showed that the estimated diapycnal diffusivity from GUTRE over a time period of seven years was within the uncertainty of the previous estimates over a time period of three years. The results are consistent with the Dz estimates from microstructure measurements and demonstrate that Dz does not vary significantly vertically in the OMZ within the depth range of 200–600 m and does not change with time. The presence of a seamount chain in the vicinity of the GUTRE injection region did not cause enhanced Dz compared to the smoother bottom topography of the OSTRE injection region, although the analysis of vertical shear spectra from ship ADCP data showed elevated internal wave energy level in the seamount vicinity. However, the two tracer patches covered increasingly overlapping areas with time and thus spatially integrated increasingly similar fields of local diffusivity, as well as the difference in local stratification counteracted the influence of roughness on Dz. For both experiments no significant vertical displacements of the tracer were observed, thus diapycnal upwelling within the ETNA OMZ is below the uncertainty level of 5 m yr− 1.

Document Type: Article
Keywords: Tropical Atlantic; Guinea Dome; Oxygen minimum zone; Diffusion coefficients; Tracers; Ventilation; Upwelling
Research affiliation: OceanRep > SFB 754 > A3
OceanRep > SFB 754
Kiel University
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.jmarsys.2016.03.012
ISSN: 0924-7963
Date Deposited: 15 Apr 2016 07:24
Last Modified: 23 Sep 2019 22:20
URI: http://oceanrep.geomar.de/id/eprint/32057

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