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doi:10.5194/os-2016-102
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
10 Jan 2017
Review status
This discussion paper is under review for the journal Ocean Science (OS).
Decadal oxygen change in the eastern tropical North Atlantic
Johannes Hahn, Peter Brandt, Sunke Schmidtko, and Gerd Krahmann GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
Abstract. Repeat shipboard and multi-year moored observations obtained in the oxygen minimum zone (OMZ) of the eastern tropical North Atlantic (ETNA) were used to study the decadal change in oxygen for the period 2006–2015. At the depth of the deep oxycline (200–400 m), oxygen decreased with a rate of −6.2 ± 3.8 μmol kg−1 decade−1, while below the OMZ core (400–1,000 m) oxygen increased by 4.1 ± 1.7 μmol kg−1 decade−1 on average. The inclusion of these decadal oxygen trends in the recently estimated oxygen budget for the ETNA OMZ showed a weakened ventilation of the upper 400 m, whereas the ventilation strengthened homogeneously over depth below 400 m. This resulted in a shoaling of the ETNA OMZ of −0.03 ± 0.02 kg m−3 decade−1 in density space, which was only partly compensated by a deepening of isopycnal surfaces, thus pointing to a shoaling of the OMZ in depth space as well. Shipboard, float and satellite observations of velocity and hydrography indicate different regional as well as remote changes in the circulation pattern to be responsible for the change in the ventilation of the ETNA. The reduced ventilation in the upper 400 m may have been induced by a southward shift of the wind-driven circulation or by a change of the composition of South Atlantic Central Water. There are hints that below 400 m, latitudinally alternating zonal jets have strengthened, thus contributing to the increased ventilation. Nevertheless, temporal changes in isopycnal eddy supply or diapycnal supply (diapycnal mixing as well as diapycnal advection) cannot be excluded in having contributed to the observed oxygen change.

Citation: Hahn, J., Brandt, P., Schmidtko, S., and Krahmann, G.: Decadal oxygen change in the eastern tropical North Atlantic, Ocean Sci. Discuss., doi:10.5194/os-2016-102, in review, 2017.
Johannes Hahn et al.
Johannes Hahn et al.

Data sets

Oxygen, hydrography and velocity in the eastern tropical North Atlantic (1999 - 2015)
J. Hahn, P. Brandt, S. Schmidtko, and G. Krahmann
https://doi.pangaea.de/10.1594/PANGAEA.869568
Johannes Hahn et al.

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Short summary
Recent studies have shown, that the eastern tropical North Atlantic is subject to a strong decrease of the oceanic oxygen concentration in the upper 1000 m from the 1960s to today. By analyzing a broad observational data set from different measurement platforms, this study found an even stronger oxygen decrease in the upper 400 m throughout the past decade, whereas an oxygen increase was found below at depth between 400–1000 m. This was associated with changes in the ventilation of this regime.
Recent studies have shown, that the eastern tropical North Atlantic is subject to a strong...
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