Transcript Peter Brandt with contributions from Marcus Dengler, Sven-Helge Didwischus, Tim Fischer, Richard J.

Peter Brandt
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
On the role of circulation and mixing in the
ventilation of the oxygen minimum zone of
the eastern tropical North Atlantic
SFB754
t
T ,S , v
N ,P ,F e
O2
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
Motivation
Global Model Simulations
Annual mean oxygen [μmol/kg] at 300m in observations (WOA) and
different state-of-the-art global models
Oschlies, pers. comm. 2013
5
Motivation
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
Stramma et al. 2013
Oxygen
(left,
μmol/kg)
and oxygen
trend (right,
μmol/kg/yr)
at 300m.
Outline
Structure of the
• Mean structure
Oxygen Budget
• Consumption
• Diapycnal mixing
• Lateral mixing
• Advection
• Equatorial oxygen
Long-term Oxygen
Eastern Tropical North
Atlantic (ETNA) Oxygen
Minimum Zone (OMZ)
maximum
Changes
Summary
Structure of the ETNA OMZ: Mean Structure
Ventilated Thermocline
Transport processes at
the boundary between
ventilated and
unventilated thermocline:
advection (solid arrow)
and diffusive flux (open
arrow)
FLAME simulation, C. Eden
Luyten et al. 1983
Simulation of OMZs
involve physical
processes from large to
small scales: circulation,
jets, eddies, filaments,
turbulent mixing.
Structure of the ETNA OMZ: Mean Structure
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
WOA‘09
Wattenberg 1939
Mean Circulation and
Oxygen Distribution
Structure of the ETNA OMZ: Mean Structure
Complex zonal current system connects high-oxygen western
boundary regime with sluggish flow in the eastern basin.
Brandt et al. 2015
Structure of the ETNA OMZ: Mean Structure
Measurement Programme
Repeat ship section along 23°W; moored observations;
microstructure measurements; tracer release
Brandt et al. 2015
Structure of the ETNA OMZ: Mean Structure
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
Structure of the ETNA OMZ: Mean Structure
Oxygen at Deep Oxygen
Minimum
Deep OMZ (below
200m) located in
the interior with
slightly enhanced
oxygen
concentration
toward the eastern
boundary
Structure of the ETNA OMZ: Mean Structure
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
Structure of the ETNA OMZ: Mean Structure
Oxygen at CVOO Mooring
CVOO
Oxygen at 40-60m (black),
140m (grey) and
oxygen saturation (red)
Karstensen et al., 2015
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Structure of the ETNA OMZ: Mean Structure
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
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Structure of the ETNA OMZ: Equatorial oxygen maximum
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
Structure of the ETNA OMZ: Equatorial oxygen maximum
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
Structure of the ETNA OMZ: Equatorial oxygen maximum
Equatorial Basin Mode
Greatbatch et al.
(2012) used a reducedgravity model to
simulate regular highbaroclinic-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
Structure of the ETNA OMZ: Equatorial oxygen maximum
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
Equator
ventilation of the
equatorial band
due to basin mode
oscillations
Brandt et al. 2012
Structure of the ETNA OMZ: Equatorial oxygen maximum
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)
Structure of the ETNA OMZ: Equatorial oxygen maximum
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
Structure of the ETNA OMZ: Equatorial oxygen maximum
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
Structure of the ETNA OMZ: Equatorial oxygen maximum
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
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
¶O2
¶ O2
¶ O2
¶O2
= -C ( z) + K r 2 + K y 2 - u
+...
¶t
¶z
¶y
¶x
2
2
Oxygen Budget: Consumption
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
Oxygen Budget: Consumption
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)
“classical”
tracer age
Oxygen Budget: Consumption
Mean 23°W Section
Equatorial
oxygen
maximum
Deep
oxycline at
about 300m
or sq=26.8
kg/m3
Oldest
water
masses
within OMZ
Oxygen Budget: Diapycnal Mixing
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
Oxygen Budget: Diapycnal Mixing
Enhanced mixing in the vicinity of
Sierra Leone Rise
Diapycnal
Diffusivity K estimated from vmADCP
diffusivity
derived from
ADCP
estimated
shear levels.
Tim Fischer,
PhD thesis
2000 m contour
Oxygen Budget: Diapycnal Mixing
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
Oxygen Budget: Lateral Mixing
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
F  Ke
dO 2
dy
Hahn et al. 2014
Oxygen Budget: Lateral Mixing
Mean Eddy Diffusivity Profile Ke
Basic approach:
following Ferrari and Polzin (2005), Eden (2007)
Ke µUe Le
Le
mean state
… characteristic eddy length
scale
Le 
O2 '
mesoscale
B
B
A
A
Le
 s O2
Ue
… characteristic eddy
velocity
Ue 
EKE 
(u '  v ' ) / 2
2
2
33
Oxygen Budget: Lateral Mixing
Mean Eddy Diffusivity Profile Ke
Brandt et al. (2010)
TNEA: Hahn et al. (2014)
GUTRE: Banyte et al. (2013)
NATRE: Ferrari and Polzin
(2005)
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Oxygen Budget: Lateral Mixing
Eddy Flux Divergence
Oxygen supply
due to
meridional
eddy flux
Meridional eddy diffusivity
Hahn et al. 2014
Oxygen Budget: Lateral Mixing
Meridional Eddy Supply
Hahn et al. 2014
Oxygen Budget: Advection
Latitudinally Alternating Zonal
Jets in the Tropical Atlantic
Mean zonal
velocity from
profiling and
acousticallytracked floats
Zonal jets
penetrating into
the OMZ
Ollitrault et al. 2006
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.
Brandt et al. 2010
gn=27.1
Oxygen Budget: Advection
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High-Resolution Ocean Models
Improvement in
simulated
oxygen
distribution due
to a stronger
oxygen supply
by a more
realistic
representation
of the equatorial
and offequatorial
undercurrents
Duteil et al. 2014
Long-term Oxygen Changes
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
Long-term Oxygen Changes
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)
Long-term Oxygen Changes
Oxygen and Current Changes
along 23°W
1972-1985
1999-2008
Brandt et al. 2010
43
150-300m, 9-15°N, 20-26°W
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 “ClimateBiogeochemistry 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.
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