in the oxygen minimum zone off West Africa

Tim Fischer, Marcus Dengler, Peter Brandt, Martin Visbeck, Donata Banyte, Gerd Krahmann, Johannes Karstensen, Annette Kock, Hermann W. Bange

Oxygen and nitrous oxide fluxes

in the oxygen minimum zone off West Africa

SOPRAN

SFB 754

Contact: IFM-GEOMAR, Düsternbrooker Weg 20, D-24105 Kiel www.ifm-geomar.de tfischer@ifm-geomar.de

Oxygen minimum zone (OMZ) off West Africa

Diapycnal fluxes and flux divergences of oxygen and nitrous oxide as part of the OMZ budget

Diapycnal fluxes by direct diffusive method during 3 cruises in 2008-2010.

K : diapycnal diffusivity

∇ c : vertical concentration gradient

K

= ^• 

 ^c

K K  c

O 2 downward flux into upper OMZ Inferences on oxygen budget of OMZ's upper part N 2 O upward flux from upper OMZ

 

 

from microscale velocity fluctuations

from ADCP acoustic survey of internal wave field,

a proxy for mixing intensity

from CTD and/or water samples Bottom topography Oxygen distribution at 400 m level Oxygen along 23 W meridional section

MSS:

ADCP:

10⁻¹⁰ 10⁻⁹ 10⁻⁸ 10⁻⁷ 10⁻⁶ 10⁻⁵ 10⁻⁴ 0

50 100 150 200 250 300 350 400 450

dissipation rate of turbulent kinetic energy

depth m

passive scatterers deduce velocity from Doppler- shifted echo

Enhanced flux at and

west of Sierra Leone

Rise

30^oW 25^oW 20^oW 15^oW

5^oN 10^oN 15^oN

station−depth−average of MSS derived K

m²/s

1e−6 1e−5 1e−4

Africa Cabo

Verde

Sierra Leone Rise

Our estimated diapycnal

supply is 20-50% of esti-

mated con- sumption.

Brandt et al.

(2010) estimate 15-40% from a

conceptual model.

longitude

latitude

diapycnal oxygen downflux at upper OMZ limit in nmol/m²/s

−30 −25 −20 −15

2 4 6 8 10 12 14 16

1 3 10 30

30^oW 25^oW 20^oW 15^oW

5^oN 10^oN 15^oN

nitrous oxide flux from OMZ in nmol/m²/s

1e−4 3e−4 1e−3 3e−3

Banyte, Tanhua, Visbeck, Wallace, Karstensen, Krahmann, Schneider, Stramma 2011: Vertical diffusivity at the upper boundary of the North Tropical Atlantic oxygen minimum zone [in prep.]

Brandt, Hormann, Körtzinger, Visbeck, Krahmann, Stramma, Lumpkin, Schmid 2010: Changes in the ventilation of the oxygen minimum zone of the Tropical North Atlantic J. Phys. Oceanogr. 40, 1784-1801 Karstensen, Stramma, Visbeck 2008: Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans Prog. Oceanogr. 77, 331-350

Stramma, Brandt, Schafstall, Schott, Fischer, Körtzinger 2008: Oxygen minimum zone in the North Atlantic south and east of the Cape Verde Islands J. Geophys. Res. 113, C04014

References

0 10 20 30 40 50

0 50 100 150 200 250 300 350 400 450 500

N2O in nmol/kg

depth m

= 1.0 · 10 ^-3 ± 0.4 · 10 ^{-3 nmol/m 2 /s} = 250 ± 100 ^{p mol/kg/a}

= 6.2 ± 0.7 ^{nmol/m 2 /s} = 1.7 ± 0.2 ^μmol/kg/a

K = 1.1 · 10 ^-5 ± 0.4 · 10 ^{-5 m 2 /s}

K = 1.2 · 10 ^-5 ± 0.2 · 10 ^{-5 m 2 /s}

30^oW 25^oW 20^oW 15^oW

5^oN 10^oN 15^oN

ADCP derived K

m²/s

1e−6 1e−5 1e−4

0 2 4 6 8

0

100

200

300

400

500

600

700

800

μmol / kg / a

depth m

supply diapycnal

consumption diapycnal fraction of total oxygen supply

50 100 150 200

0

100

200

300

400

500

600

700

800

O₂ in mol/kg

depth m

AAIW CW

typical oxygen profile in Guinea Dome Region



no diapycnal flux across this line

CW: Central Water

AAIW: Antarctic Intermediate Water

Consumption estimated for Atlantic and Pacific OMZs by Karstensen et al. 2008



Internal wave shear energy Acoustic Doppler Current Profiler

MSS Profiler with airfoil shear sensors

A large-scale Tracer Release Experiment (TRE) in 2008 to 2010 yielded K = 1.2 · 10

^-5

± 0.1 · 10

^-5

m

²

/s [Banyte et al. 2011]

0 100 200

0 50 100 150 200 250 300 350 400 450 500

O2 in mol/kg

depth m

0 20 40

0 50 100 150 200 250 300 350 400 450 500

N2O in nmol/kg

depth m

Typical concentration profiles

O 2 N 2 O

Direction of diapycnal flux as caused by concentration gradient

Depth ranges of K estimates

by method

MSS ADCP TRE

Constraints in depth range (method dependent) confine estimates of diapy- cnal fluxes to the upper half of the OMZ. The existence of a shallow oxygen minimum / N

2

O maximum in large parts of the OMZ decouples diapycnal fluxes at the OMZ upper border from the local mixed layer and atmosphere.

[

Stramma et al. 2008], modified.

[

Brandt et al. 2010], modified.

Cabo Verde

23 W- section

μmol/kg

black arrows: surface currents black lines: oxygen concentration isolines

white lines: density isolines m

OMZ