Ventilation of the Equatorial Atlantic

P. Brandt, R. J. Greatbatch, M. Claus, S.-H. Didwischus, J. HahnGEOMAR Helmholtz Centre for Ocean Research Kiel

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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 – stronger than in all other tropical regions.

Stramma et al. 2008

mmol/kg

Mechanisms of Oxygen Depletion

Anthropogenic forcing:Increased stratification and corresponding

reduction of ventilationSolubility 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)

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Stramma et al. (2013): Mismatch between observed and modeled trends in dissolved upper-ocean oxygen over the last 50 yr

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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

Atlantic Oxygen Distribution

Oxygen distribution at 600m depth as measured during the German Atlantic Meteor expedition 1925-27

Oxygen Minimum Zones (OMZ) in both hemispheres separated by an equatorial oxygen maximum

5Wattenberg 1939

Wyrtki (1962) concluded that the position and distribution of the oxygen minimum layer are determined by circulation

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Oxygen Minimum Zones

Ventilated thermocline: oxygen supply along isopycnal surfaces from the outcrop regions in the subtropics toward the equator and the western boundary (solid arrow).

FLAME simulation, C. Eden

mmol/kg

Mesoscale eddy field plays a dominant role in the ventilation of the OMZ (open arrow).

Luyten et al. 1983

SFB 754 Observations

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Shipboard and moored oxygen and current measurements along 23°W repeat section from 2006 - 2012

Tracer release experiment / microstructure measurements

Brandt et al. 2010 8

Mean 23°W Section 1999-2008

Vigorous equatorial zonal currents; latitudinally alternating zonal jets in the OMZNorth-South gradient in salinityEastward flow associated with increased oxygen

Consumption (Karstensen et al. 2008)Diapycnal supply (Banyte et al. 2012, Fischer et al. 2013) Meridional eddy supply from O2-rich equatorial region (Hahn et al. 2013, submitted)Oxygen tendency from 1972-1985 to 1999-2008 (Brandt et al. 2010)

Mean Oxygen Budget

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Hahn et al. 2013, submitted

The missing ventilation in the budget is largely due to zonal oxygen advection, diffusive zonal supply might play a minor role

Equatorial Ventilation

NICC/SICC at 2°N/S supplies oxygen to the eastern Atlantic

Strong mean westward flow at the equator below 380 m

Equatorial Deep Jets with downward phase propagation

Brandt et al. 2012

SICC

NICC

SEUC

300m

500m

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

11Update of Brandt et al. 2012

EDJ-like Equatorial Basin Mode

12Greatbatch et al. 2012

Zonal velocity of a basin mode solution from a single mode model

Strong east- and westward velocity along the equator represents exchange between well-ventilated western boundary and interior

Advection-Diffusion Model

Model forced by velocity field of equatorial basin mode, O2 source at the western boundary layer; O2 consumption

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Consumption × 0.2

Consumption × 5

Mean relative oxygen concentration shows ventilation of the equatorial band due to basin mode oscillations

Observed Mean Zonal Velocity and Oxygen along 23°W

Mean eastward currents are partly supplied out of the well-ventilated western boundary current

At 300 m depth: South Equatorial Undercurrent (SEUC) at 4°S and Northern Intermediate Countercurrent (NICC) at 2°N

At 500 m depth: Southern Intermediate Countercurrent (SICC) at 2°S and NICC

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SICC

NICC

SEUC

Advection-Diffusion Model

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Model forced by velocity field of equatorial basin mode and mean flow observed at 300 m depth

Mean O2 tongues associated with NICC and SEUC

NICC

SEUC

Advection-Diffusion Model

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Model forced by velocity field of equatorial basin mode and mean flow observed at 500 m depth

Mean O2 tongues associated with NICC and SICC

NICC

SICC

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Summary

Global models do not correctly capture location and distribution of low-oxygen layer in the tropicsOne obvious reason is the too weak mean and variable intermediate circulation in these simulationsObservations show equatorial oxygen maximum and strong poleward oxygen fluxes toward the OMZs that can be reproduced by idealized simulations with an advection-diffusion modelIf the circulation set the shape of the OMZs, long-term changes in the circulation might also contribute to long-term changes in oxygen concentration

after Brandt et al. 2010

Outlook

From 1972-1985 to 1999-2008 weakening of zonal circulation contributed to general O2 decline

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Oxygen and Current Changes along 23°W

1972-1985 1999-2008

Outlook

From 2006 to 2012 stronger changes with O2 decline at the deep oxycline

O2 increase and freshening (more AAIW) below

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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” and “RACE”.

Moored velocity and oxygen observations were acquired in cooperation with the PIRATA project.

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