Ventilation of the E quatorial Atlantic

1. Ventilation oftheEquatorialAtlantic P. Brandt, R. J. Greatbatch, M. Claus, S.-H. Didwischus, J. HahnGEOMAR Helmholtz CentreforOcean Research Kiel

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. mmol/kg Stramma et al. 2008

3. Mechanisms of Oxygen Depletion Stramma et al. (2013): Mismatch between observed and modeled trends in dissolved upper-ocean oxygen over the last 50 yr 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 CO2 levels (Oschlies et al. 2008)

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

6. Oxygen Minimum Zones Ventilated thermocline: oxygen supply along isopycnalsurfaces from the outcrop regions in the subtropics toward the equator and the western boundary (solid arrow). Luyten et al. 1983 Mesoscale eddy field plays a dominant role in the ventilation of the OMZ (open arrow). mmol/kg FLAME simulation, C. Eden

7. SFB 754 Observations Shipboardandmooredoxygenandcurrentmeasurementsalong 23°W repeatsectionfrom 2006 - 2012 Tracer releaseexperiment/microstructuremeasurements

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 Brandt et al. 2010

9. Mean Oxygen Budget 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) 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 SEUC NICC SICC Brandt et al. 2012 300m 500m 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

11. 4.5-yr Deep Jet Cycle in Moored Observations at Equator, 23°W Update of Brandt et al. 2012 Max O2 slightly after max zonal velocity Larger O2 amplitude at 300 m than at 500 m Ventilation of equatorial Atlantic by Deep Jets

12. EDJ-likeEquatorialBasin Mode 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 Consumption × 0.2 Consumption × 5 Mean relative oxygen concentration shows ventilation of the equatorial band due to basin mode oscillations Model forced by velocity field of equatorial basin mode, O2 source at the western boundary layer; O2 consumption

14. Observed Mean Zonal Velocity and Oxygen along 23°W SEUC NICC SICC 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

15. Advection-Diffusion Model NICC SEUC 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

16. Advection-Diffusion Model NICC SICC 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

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. Outlook after Brandt et al. 2010 From 1972-1985 to 1999-2008 weakening of zonal circulation contributed to general O2 decline

19. Oxygen and Current Changes along 23°W 1972-1985 1999-2008

20. Outlook From 2006 to 2012 strongerchangeswith O2declineatthedeep oxycline O2increaseandfreshening (more AAIW) below

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.