On the role of circulation and mixing in the ventilation of the oxygen minimum zone of the eastern tropical North Atlantic Peter Brandt with contributions from H. W. Bange, D. Banyte, M. Dengler, S.-H. Didwischus, T. Fischer, R. J. Greatbatch, J. Hahn, T. Kanzow, J. Karstensen, A. Körtzinger, G. Krahmann, S. Schmidtko, L. Stramma, T. Tanhua and M. Visbeck GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany

Oxygen Depletion in the North Atlantic OMZ Oxygen data show a reduction of dissolved oxygen in the North Atlantic OMZ over the last 40 years. Stramma et al  mol/kg Motivation

Habitat Reduction for Pelagic Fishes Stramma et al. 2011

Main Questions  How are oxygen minimum zones ventilated?  Can we derive an oxygen budget for the ETNA OMZ?  Can we expect a further decline of oceanic oxygen in the future due to anthropogenic climate change? Motivation

Mean Circulation and Oxygen Distribution Complex zonal current system connects high-oxygen western boundary regime with sluggish flow in the eastern basin. Brandt et al Mean Oxygen Distribution

Measurement Programme Repeat ship section along 23°W; moored observations; microstructure measurements; tracer release Mean Oxygen Distribution Brandt et al. 2014

Mean 23°W Section Equatorial oxygen maximum Deep oxycline at about 300m or   =26.8 kg/m 3 OMZ is ventilated from the west by zonal currents

Oxygen Budget  Oxygen sink Heterotrophic respiration  Oxygen source or sink: Diapycnal mixing Meridional eddy fluxes Advection by latitudinally alternating zonal jets  Oxygen tendency Oxygen Minimum Zone

Respiration Estimates  AOUR: apparent oxygen utilization rate  Derived as the ratio of AOU and CFC11 ages  Exponential decay of AOUR downward is assumed Karstensen et al Heterotrophic respiration

Diapycnal Mixing  Microstructure measurements yield a diapycnal diffusivity, K, that is relatively constant in the depth range of the OMZ Ventilation Processes Fischer et al. 2013

Diapycnal Mixing Ventilation Processes Fischer et al. 2013

Hahn et al Meridional Eddy Fluxes Two Methods  Eddy correlation method applied to moored observations of oxygen and meridional velocity (here at 5°N, 23°W)  Flux gradient parameterization based on repeat ship sections Ventilation Processes

Eddy Flux Divergence Oxygen supply due to meridional eddy flux Ventilation Processes Hahn et al Meridional eddy diffusivity

Meridional Eddy Supply Ventilation Processes Hahn et al. 2014

Oxygen Depletion in the North Atlantic OMZ = Climate Change? Oxygen data show a reduction of dissolved oxygen in the North Atlantic OMZ over the last 40 years. Stramma et al  mol/kg Oxygen Minimum Zone

Possible Explanations for Oxygen Depletion  Increased stratification and a corresponding reduction of ventilation, or solubility changes associated with a warming of subducted water masses (Bopp et al. 2002; Matear and Hirst 2003)  Increase in heterotrophic respiration along the pathways of ventilating water masses due to excess organic carbon formed at higher CO 2 levels (Oschlies et al. 2008)  Increase of organic matter oxidization in shallow waters due to reduction of mineral ballast fluxes (Hofmann and Schellnhuber 2009)  Observations indicate circulation changes: weakening of zonal jets (Brandt et al. 2010) Oxygen Minimum Zone

m, 9-15°N, 20-26°W m, 9-15°N, 20-26°W

Summary  Advection dominates upper 300m ventilation  Deoxygenation represents a substantial oxygen imbalance of the oxygen budget (about 10%)  Deoxygenation associated with anthropogenic climate change might not be the dominant signal on regional scale Hahn et al. 2014