Abstract
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. Along 23° W between 6 and 14° N, oxygen decreased with a rate of −5.9 ± 3.5 µmol kg−1 decade−1 within the depth covering the deep oxycline (200–400 m), while below the OMZ core (400–1000 m) oxygen increased by 4.0 ± 1.6 µmol kg−1 decade−1 on average. The inclusion of these decadal oxygen trends in the recently estimated oxygen budget for the ETNA OMZ suggests a weakened ventilation of the upper 400 m, whereas the ventilation strengthened homogeneously below 400 m. The changed ventilation 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 (−22 ± 17 m decade−1). Based on the improved oxygen budget, possible causes for the changed ventilation are analyzed and discussed. Largely ruling out other ventilation processes, the zonal advective oxygen supply stands out as the most probable budget term responsible for the decadal oxygen changes.
Highlights
Over the past decades, tropical oceans have been subject to a conspicuous deoxygenation (Brandt et al, 2015; Stramma et al, 2008, 2012) at thermocline to intermediate depths comprising the upper 700 m of the ocean
Results will be shown from the combined analysis of moored, shipboard and float observations in the tropical Atlantic with a particular focus on the eastern tropical North Atlantic (ETNA) oxygen minimum zone (OMZ) in order to investigate and quantify decadal changes of oxygen www.ocean-sci.net/13/551/2017/
We found a migration of the OMZ core position in density space toward lighter water (−0.03 ± 0.02 kg m−3 decade−1), whereas no significant upward migration in depth space could be shown (−10 ± 25 m decade−1) from this estimate
Summary
Tropical oceans have been subject to a conspicuous deoxygenation (Brandt et al, 2015; Stramma et al, 2008, 2012) at thermocline to intermediate depths comprising the upper 700 m of the ocean These well-documented changes manifest themselves in a profound decrease of dissolved oxygen and a volumetric increase of open-ocean tropical oxygen minimum zones (OMZs). OMZs are located in the weakly ventilated shadow zones of the ventilated thermocline (Luyten et al, 1983) in the eastern tropical Atlantic and Pacific ocean basins off the Equator as well as in the northern Indian Ocean between 100 and 900 m (Karstensen et al, 2008) Their existence predominantly results from a weak oxygen supply to these sluggish flow regimes accompanied by locally enhanced oxygen consumption in proximity to coastal and open-ocean upwelling regions (Helly and Levin, 2004).
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