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
On the role of circulation and mixing in the
ventilation of the oxygen minimum zone of the
eastern tropical North Atlantic
Oxygen Depletion in the North
Atlantic OMZ
Motivation
Oxygen data show a reduction
of dissolved oxygen in the North
Atlantic OMZ over the last 40
years.
mmol/kg
Stramma et al. 2008
Habitat Reduction for Pelagic
Fishes
Stramma et al. 2011
Motivation
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?
Mean Circulation and
Oxygen Distribution
Mean Oxygen Distribution
Complex zonal current system connects high-oxygen western
boundary regime with sluggish flow in the eastern basin.
Brandt et al. 2014
Mean Oxygen Distribution
Measurement Programme
Repeat ship section along 23°W; moored observations;
microstructure measurements; tracer release
Brandt et al. 2014
Mean 23°W Section
Equatorial
oxygen
maximum
Deep
oxycline at
about 300m
or sq=26.8
kg/m3
OMZ is
ventilated
from the
west by
zonal
currents
Oxygen Minimum Zone
Oxygen Budget
Oxygen sink
• Heterotrophic respiration
Oxygen source or sink:
• Diapycnal mixing
• Meridional eddy fluxes
• Advection by latitudinally alternating zonal jets
Oxygen tendency
Heterotrophic respiration
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. 2008
Ventilation Processes
Diapycnal Mixing
Microstructure
measurements
yield a diapycnal
diffusivity, K, that is
relatively constant
in the depth range
of the OMZ
Fischer et al. 2013
Ventilation Processes
Diapycnal Mixing
Fischer et al. 2013
Ventilation Processes
Meridional Eddy Fluxes
Two Methods
Eddy correlation method applied to
moored observations of oxygen and
meridional velocity (here at 5°N,
23°W)
FO2 = v'O2 '
Flux gradient parameterization
based on repeat ship sections
dO2
F  Ke
dy
Hahn et al. 2014
Ventilation Processes
Eddy Flux Divergence
Oxygen supply
due to
meridional
eddy flux
Meridional eddy diffusivity
Hahn et al. 2014
Ventilation Processes
Meridional Eddy Supply
Hahn et al. 2014
Oxygen Minimum Zone
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.
mmol/kg
Stramma et al. 2008
Possible Explanations for
Oxygen Depletion
Oxygen Minimum Zone
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 CO2 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)
150-300m, 9-15°N, 20-26°W
350-700m, 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