1. Ventilation of the Equatorial Atlantic P. Brandt, R. J. Greatbatch, M. Claus, S.-H. Didwischus, J. Hahn GEOMAR Helmholtz Centre for Ocean Research Kiel 1

2. 2 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  mol/kg

3. Mechanisms of Oxygen Depletion Anthropogenic forcing:  Increased stratification and corresponding reduction of ventilation  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 CO 2 levels (Oschlies et al. 2008) 3 Stramma et al. (2013): Mismatch between observed and modeled trends in dissolved upper-ocean oxygen over the last 50 yr

4. 4 Global Model Simulations Oschlies, pers. comm. 2013 Annual mean oxygen [ μ mol/kg] at 300m in observations (WOA) and different state-of-the-art global models

5. 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 5 Wattenberg 1939 Wyrtki (1962) concluded that the position and distribution of the oxygen minimum layer are determined by circulation

6. 6 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  mol/kg Mesoscale eddy field plays a dominant role in the ventilation of the OMZ (open arrow). Luyten et al. 1983

7. SFB 754 Observations 7  Shipboard and moored oxygen and current measurements along 23°W repeat section from 2006 - 2012  Tracer release experiment / microstructure measurements

8. Brandt et al. 2010 8 Mean 23°W Section 1999-2008 Vigorous equatorial zonal currents; latitudinally alternating zonal jets in the OMZ North-South gradient in salinity Eastward flow associated with increased oxygen

9. Consumption (Karstensen et al. 2008) Diapycnal supply (Banyte et al. 2012, Fischer et al. 2013) Meridional eddy supply from O 2 -rich equatorial region (Hahn et al. 2013, submitted) Oxygen tendency from 1972- 1985 to 1999-2008 (Brandt et al. 2010) Mean Oxygen Budget 9 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

10. 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

11. 4.5-yr Deep Jet Cycle in Moored Observations at Equator, 23°W  Max O 2 slightly after max zonal velocity  Larger O 2 amplitude at 300 m than at 500 m  Ventilation of equatorial Atlantic by Deep Jets 11 Update of Brandt et al. 2012

12. EDJ-like Equatorial Basin Mode 12 Greatbatch 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

13. Advection-Diffusion Model  Model forced by velocity field of equatorial basin mode, O 2 source at the western boundary layer; O 2 consumption 13 Consumption × 0.2 Consumption × 5 Mean relative oxygen concentration shows ventilation of the equatorial band due to basin mode oscillations

14. 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 14 SICC NICC SEUC

15. Advection-Diffusion Model 15  Model forced by velocity field of equatorial basin mode and mean flow observed at 300 m depth  Mean O 2 tongues associated with NICC and SEUC NICC SEUC

16. Advection-Diffusion Model 16  Model forced by velocity field of equatorial basin mode and mean flow observed at 500 m depth  Mean O 2 tongues associated with NICC and SICC NICC SICC

17. 17 Summary  Global models do not correctly capture location and distribution of low-oxygen layer in the tropics  One obvious reason is the too weak mean and variable intermediate circulation in these simulations  Observations show equatorial oxygen maximum and strong poleward oxygen fluxes toward the OMZs that can be reproduced by idealized simulations with an advection-diffusion model  If the circulation set the shape of the OMZs, long- term changes in the circulation might also contribute to long-term changes in oxygen concentration

18. after Brandt et al. 2010 Outlook  From 1972-1985 to 1999- 2008 weakening of zonal circulation contributed to general O 2 decline 18

19. 19 Oxygen and Current Changes along 23°W 1972-19851999-2008

20. Outlook  From 2006 to 2012 stronger changes with O 2 decline at the deep oxycline  O 2 increase and freshening (more AAIW) below 20

21. 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. 21