On the role of circulation and mixing in the ventilation of the oxygen minimum zone of the eastern tropical North Atlantic
with contributions from
Marcus Dengler, Sven-Helge Didwischus, Tim Fischer, Richard J. Greatbatch,
Johannes Hahn, Johannes Karstensen, Arne Körtzinger, Gerd Krahmann,
Sunke Schmidtko, Lothar Stramma, Toste Tanhua, and Martin Visbeck
O2
SFB754
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. 2008
mmol/kg
Habitat Reduction for Pelagic Fishes
Stramma et al. 2011
5
Global Model Simulations
Oschlies, pers. comm. 2013
Annual mean oxygen [μmol/kg] at 300m in observations (WOA) and different state-of-the-art global models
Stramma et al. 2013
Mismatch between Observed and Modeled Trends
Pattern correlation between simulated (upper right) and observed (bottom) patterns of past oxygen change over the last 50 yr is negative
Oxygen (left,
μmol/kg) and oxygen trend (right, μmol/kg/yr) at 300m.
Outline
Structure of the
Eastern Tropical North Atlantic (ETNA) Oxygen Minimum Zone (OMZ)
• Mean structure
• Equatorial oxygen maximum
Oxygen Budget
• Consumption
• Diapycnal mixing
• Lateral mixing
• Advection
Long-term Oxygen Changes
Summary
Ventilated Thermocline
Transport processes at the boundary between ventilated and
unventilated thermocline:
advection (solid arrow) and diffusive flux (open
arrow) Luyten et al. 1983
FLAME simulation, C. Eden
Simulation of OMZs involve physical
processes from large to small scales: circulation, jets, eddies, filaments, turbulent mixing.
Oxygen Distribution at 600m [ccm/l]
Left: From METEOR expedition 1925/27
(Wattenberg 1939)
Right: From WOA’09 (same style, courtesy Florian Schütte)
Note, oxygen maximum at the equator
Wattenberg 1939 WOA‘09
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. 2015
Measurement Programme
Repeat ship section along 23°W; moored observations;
microstructure measurements; tracer release
Brandt et al. 2015
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 at Deep Oxygen Minimum
Deep OMZ (below 200m) located in the interior with slightly enhanced oxygen
concentration
toward the eastern boundary
Oxygen at Shallow Oxygen Minimum
Shallow OMZ
(above 200m) close to the eastern
boundary upwelling region
Single low oxygen events also in the region of the deep OMZ
Oxygen at CVOO Mooring
15
Oxygen at 40-60m (black), 140m (grey) and
oxygen saturation (red) Karstensen et al., 2015
CVOO
Passage of a Mode-Water Eddy at the CVOO Mooring
Low oxygen zones are created just below the mixed-layer, in the euphotic zone of high productive anticyclonic modewater eddies (oxygen at 42 and 170m, salinity, meridional velocity [m/s])
Karstensen et al., 2015 16
Equatorial oxygen and velocity distribution
Why there is on oxygen maximum at the equator?
Why it is largely missing in global Earth System Models?
Oschlies, pers. comm. 2013
Zonal Velocity in the Equatorial Atlantic at 23°W
Equatorial Deep Jets are a dominant flow feature below the Equatorial Undercurrent and oscillate with a period of about 4.5 years
Downward phase and
upward energy propagation
Equatorial Basin Mode
Greatbatch et al.
(2012) used a reduced- gravity model to
simulate regular high- baroclinic-mode
oscillations with a period of 4.5 years
Width of the EDJs could be correctly
simulated by including lateral eddy viscosity of about 200-300m2/s
Greatbatch et al. 2012
Advection-Diffusion Model
Model is forced by the velocity field of the equatorial basin mode
It includes a restoring to western boundary oxygen concentrations within a boundary layer and oxygen consumption (van Geen et al.
2006)
Simulation are performed until a constantly oscillating state is reached (about 160 yr)
Mean relative oxygen shows ventilation of the equatorial band due to basin mode oscillations
Equator
Brandt et al. 2012
Simulated Relative Oxygen Concentration at 23°W
Oxygen oscillates with the basin mode period (T0 = 4.5 yr) cycle having amplitudes of about 25% of western boundary values
Maximum oxygen concentration occurs after maximum eastward velocity (not in quadrature mean flux)
4.5-yr Deep Jet Cycle in Moored Observations at Equator, 23°W
Max O2
slightly after max zonal velocity
Larger O2 amplitude at 300 m than at 500 m
Ventilation of equatorial Atlantic by Deep Jets
Update of Brandt et al. 2012 22
Reduced-Gravity Model with EDJ and Mean Advection
a) Mean zonal flow field
b) Mean oxygen distribution
c) Oxygen anomaly along 23°W
d) Mean Oxygen along the equator
Equatorial Atlantic Ventilation
Eastward flow within NICC/SICC at 2°N/S, but longitudinal structure of these jets is largely unknown
Stacked jets at the equator superimposed on westward flowing Equatorial Intermediate Current (EIC)
East- and westward advection results in strong mixing between western boundary regime and eastern equatorial Atlantic
Mean advection together with the occurrence of stacked jets produces a broad oxygen maximum in the equatorial band between 2°S and 2°N.
Oxygen Budget of the ETNA OMZ
Oxygen tendency
Oxygen sink
• Heterotrophic respiration
Oxygen source or sink:
• Diapycnal mixing
• Meridional eddy fluxes
• Advection by latitudinally alternating zonal jets
¶O
2¶t = -C z ( ) + K
r¶
2O
2¶z
2+ K
y¶
2O
2¶y
2- u ¶O
2¶x + ...
Respiration Estimates
AOUR: apparent oxygen utilization rate
Derived as the ratio of AOU and CFC11 ages (data from the subtropics)
Exponential decay of AOUR downward is assumed
Karstensen et al. 2008
OUR from Different Tracer-Based Age Concepts
Mean age from the transit time distribution (TTD) is calculated by D/G=1, with D the width and G the mean age of the TTD
„classical“ tracer age is with D/G=0
Problems: very old water masses, mixing of different water masses
Large uncertainty
Mean age (TTD)
Mean age (TTD)
“classical”
tracer age
“classical”
tracer age
Mean 23°W Section
Equatorial oxygen maximum Deep
oxycline at about 300m or sq=26.8 kg/m3
Oldest water masses within OMZ
Diapycnal Mixing
Microstructure measurements yield a diapycnal diffusivity, K, that is relatively
constant with
depth in the depth range of the OMZ
Fischer et al. 2013
Tim Fischer,
PhD thesis 2000 m contour
Diffusivity K estimated from vmADCP
Diapycnal diffusivity derived from ADCP
estimated shear levels.
Enhanced mixing in the vicinity of
Sierra Leone Rise
Tracer Release Experiment
Diapycnal and lateral mixing estimated from tracer spreading:
Kr = (1.19±0.18) x 10-5 m2 s-1
Kx = 1200±600 m2 s-1, Ky = 500±200 m2 s-1
Banyte et al. 2012, 2013
Hahn et al. 2014
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
FO
2 = v'O2 '
dy K dO
F = - e 2
Mean Eddy Diffusivity Profile K
e33
… characteristic eddy velocity
… characteristic eddy length scale
Basic approach:
mean state mesoscale
A B
A B
following Ferrari and Polzin (2005), Eden (2007)
K
eµ U
eL
eU
eL
eL
e2 / ) ' '
( u
2v
2EKE
U
e= = +
2 2
'
O O L
e
s=
Mean Eddy Diffusivity Profile K
e34 Brandt et al. (2010)
NATRE: Ferrari and Polzin (2005)
GUTRE: Banyte et al. (2013) TNEA: Hahn et al. (2014)
Eddy Flux Divergence
Oxygen supply due to
meridional eddy flux
Hahn et al. 2014
Meridional eddy diffusivity
Meridional Eddy Supply
Hahn et al. 2014
Latitudinally Alternating Zonal Jets in the Tropical Atlantic
Mean zonal velocity from profiling and acoustically- tracked floats
Zonal jets
penetrating into the OMZ
Ollitrault et al. 2006
Brandt et al. 2010
Latitudinally Alternating Zonal Jets in the OMZ
Local oxygen maxima relative to background oxygen curvature at neutral density surface gn=27.1 correspond to eastward flow.
gn=27.1
Duteil et al. 2014
High-Resolution Ocean Models
Improvement in simulated
oxygen
distribution due to a stronger oxygen supply by a more
realistic
representation of the equatorial and off-
equatorial
undercurrents
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. 2008
mmol/kg
Ocean Deoxygenation
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)
Simulated global O2 changes in response to external forcing (90% confidence), but Atlantic O2 changes undistinguishable from internal variability (Andrews et al. 2013)
Observations indicate circulation changes:
e.g. weakening of zonal jets (Brandt et al. 2010)
43
Oxygen and Current Changes along 23°W
1972-1985 1999-2008 Brandt et al. 2010
350-700m, 9-15°N, 20-26°W
Summary
Advection dominates ventilation in the upper 300m
Deoxygenation associated with anthropogenic
climate change might not be the dominant signal on regional scale
Strong decadal
oxygen changes likely associated with
circulation variability
Mechanisms are still unknown
Trend 2006-2014
Acknowledgements
This study was supported by the German Science Foundation (DFG) as part of the
Sonderforschungsbereich 754 “Climate-
Biogeochemistry Interactions in the Tropical Ocean” and by the German Federal Ministry of
Education and Research as part of the co-operative projects “NORDATLANTIK”, “RACE”, and “AWA”.
Moored observations were acquired in cooperation with the PIRATA project.
46
