Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

Fischer, Tim; Banyte, Donata; Brandt, Peter; Dengler, Marcus; Krahmann, Gerd; Tanhua, Toste; Visbeck, Martin

doi:10.5194/bg-10-5079-2013

Cited by 52 publications

(68 citation statements)

References 58 publications

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“…Background vertical mixing is reduced in the tropical subsurface (20 • S-20 • N, 185-565 m), consistent with microstructure observations and tracer release experiments from the tropical Atlantic (Fischer et al, 2013). A two-dimensional, single level energy-moisture balance atmosphere and a dynamic-thermodynamic sea ice model are used, forced with prescribed monthly climatological winds (Kalnay et al, 1996) and ice sheets (Peltier, 2004).…”

Section: Physical Modelsupporting

confidence: 60%

“…We applied a lower vertical mixing rate of 1.5 × 10 −5 m 2 s −1 in tropical subsurface between 185 and 565 m based on microstructure and tracer release experiments in the tropical Atlantic (Fischer et al, 2013). Our chosen value is near their high-end mixing rate uncertainty range (0.8-1.4 × 10 −5 m 2 s −1 ), whereas SS16 assumed a constant value 0.3 × 10 −5 m 2 s −1 everywhere in the global ocean.…”

Section: Physical Modelmentioning

confidence: 99%

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A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

et al. 2017

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Nitrogen is a key limiting nutrient that influences marine productivity and carbon sequestration in the ocean via the biological pump. In this study, we present the first estimates of nitrogen cycling in a coupled 3D ocean-biogeochemistry-isotope model forced with realistic boundary conditions from the Last Glacial Maximum (LGM) ∼21,000 years before present constrained by nitrogen isotopes. The model predicts a large decrease in nitrogen loss rates due to higher oxygen concentrations in the thermocline and sea level drop, and, as a response, reduced nitrogen fixation. Model experiments are performed to evaluate effects of hypothesized increases of atmospheric iron fluxes and oceanic phosphorus inventory relative to present-day conditions. Enhanced atmospheric iron deposition, which is required to reproduce observations, fuels export production in the Southern Ocean causing increased deep ocean nutrient storage. This reduces transport of preformed nutrients to the tropics via mode waters, thereby decreasing productivity, oxygen deficient zones, and water column N-loss there. A larger global phosphorus inventory up to 15% cannot be excluded from the currently available nitrogen isotope data. It stimulates additional nitrogen fixation that increases the global oceanic nitrogen inventory, productivity, and water column N-loss. Among our sensitivity simulations, the best agreements with nitrogen isotope data from LGM sediments indicate that water column and sedimentary N-loss were reduced by 17-62% and 35-69%, respectively, relative to preindustrial values. Our model demonstrates that multiple processes alter the nitrogen isotopic signal in most locations, which creates large uncertainties when quantitatively constraining individual nitrogen cycling processes. One key uncertainty is nitrogen fixation, which decreases by 25-65% in the model during the LGM mainly in response to reduced N-loss, due to the lack of observations in the open ocean most notably in the tropical and subtropical southern hemisphere. Nevertheless, the model estimated large increase to the global nitrate inventory of 6.5-22% suggests it may play an important role enhancing the biological carbon pump that contributes to lower atmospheric CO 2 during the LGM.

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Section: Physical Modelsupporting

confidence: 60%

Section: Physical Modelmentioning

confidence: 99%

A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

et al. 2017

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“…One of the main results of the recent efforts is a first quantification of the oxygen budget of the deep ETNA OMZ Fischer et al, 2013;Hahn et al, 2014) that is extended here to 800 m in depth (Fig. 21).…”

Section: Summary and Discussionmentioning

confidence: 99%

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

et al. 2015

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Abstract. Ocean observations are analysed in the framework of Collaborative Research Center 754 (SFB 754) "ClimateBiogeochemistry Interactions in the Tropical Ocean" to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m in depth with its core region centred at about 20 • W, 10 • N and a shallow OMZ at about 100 m in depth, with the lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60 %) and vertical mixing (about 20 %, locally up to 30 %). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120-180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10 % of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal timescales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ, the eastern tropical South Pacific OMZ shows a similar structure, including an equatorial oxygen maximum driven by zonal advection but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

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“…Their results suggest that no direct water supply associated with the NEUC/nNECC, and in turn with SACW, takes place below this level. Indeed, oxygen concentrations decrease rapidly in the naOMZ below σ26.8 [ Fischer et al ., ], pointing to a weaker ventilation. The signal of NACW in this deep layer is especially relevant because (1) it points to a new pathway and source of oxygen for the naOMZ, not previously taken into account, and (2) reveals a vertical limit to the constraint for the equatorward transfer of mass from the subtropical gyre to the shadow zone [ Luyten et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Water mass pathways to the North Atlantic oxygen minimum zone

et al. 2015

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The water mass pathways to the North Atlantic Oxygen Minimum Zone (naOMZ) are traditionally sketched within the cyclonic tropical circulation via the poleward branching from the eastward flowing jets that lie south of 10 N. However, our water mass analysis of historic hydrographic observations together with numerical Lagrangian experiments consistently reveal that the potential density level of r h 5 26.8 kg m 23 (r26.8, approximately 300 m depth) separates two distinct regimes of circulation within the Central Water (CW) stratum of the naOMZ. In the upper CW (above r26.8), and in agreement with previous studies, the supply of water mainly comes from the south with a predominant contribution of South Atlantic CW. In the lower CW (below r26.8), where minimal oxygen content is found, the tropical pathway is instead drastically weakened in favor of a subtropical pathway. More than two thirds of the total water supply to this lower layer takes place north of 10 N, mainly via an eastward flow at 14 N and northern recirculations from the northern subtropical gyre. The existence of these northern jets explains the greater contribution of North Atlantic CW observed in the lower CW, making up to 50% of the water mass at the naOMZ core. The equatorward transfer of mass from the well-ventilated northern subtropical gyre emerges as an essential part of the ventilation of the naOMZ.

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Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

Cited by 52 publications

References 58 publications

A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

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

Water mass pathways to the North Atlantic oxygen minimum zone

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