Richard Matear, and Andrew Lenton Jan 2009

1. Impact of historical climate change on the Southern Ocean carbon cycle and implications for future change Richard Matear, and Andrew Lenton Jan 2009 Wealth from Oceans Flagship and CSIRO Marine and Atmospheric Research

2. Key Question: How will the Southern Ocean Carbon Cycle respond to global warming? • Background • Importance of the Southern Ocean to the global carbon cycle • Review a recent studies of the response of the Southern Ocean carbon cycle to historical climate change • Simulations of how historical climate change influences Southern Ocean carbon cycle • Summarize modeling approach and the NCEP re-analysis forcing fields used to drive the ocean carbon model • Review how climate variability in heat and freshwater fluxes and wind stress alter the SO carbon uptake (both the natural carbon and anthropogenic CO2 uptake) • Discuss how to detect the changes in the Southern Ocean carbon cycle

3. Southern Annular Mode (SAM) • SAM calculated from NCEP-R1 850hPa geopotential height anomaly • A Positive SAM has stronger westerly winds • Robust feature of climate change projections (Fyfe 2007) Lenton & Matear 2007

4. Southern Ocean overturning circulation

5. Saturation of Southern Ocean carbon uptake (LeQuere et al 2007) • Rising CO2 in the atmosphere should drive an increased oceanic uptake of CO2 (red line) • Ocean uptake with climate variability (blue line) Increasing oceanic uptake

6. Enhanced Eddy: Natural Anthropogenic Control:NaturalAnthropogenic Zickfield etal 2007

7. Ocean General Circulation Model with biogeochemical cycles • Ocean model simulations based on the ocean component of the Mk3.5 CSIRO climate model • Approximately 1 x 2 degree north-south and east-west resolution • Light, mixed layer depth and phosphate formulation for export production of carbon and constant rain ratio for calcium carbon export (8%) • Simulations started from climatological T, S, DIC, Alkalinity and phosphate fields in year 1850 • Simulation run till 1948 after which the forcing fields are allowed to vary • Period of the 1940s used to diagnose the seasonal freshwater flux into the ocean to account for potential systematic errors in the NCEP freshwater fluxes

8. Model Experiments • Table . The daily forcing fields used to drive the model from year 1948 to 2002 for the various experiments. Variable forcing refers to the use of forcing fields from years 1948 to 2002 while 1948 denotes the use of forcing fields from only year 1948. For cases where the winds are allowed to vary (Total and Tau) the interannually varying wind speeds are used in the calculation of the gas exchange coefficient for the air-sea CO2 fluxes otherwise the 1948 year winds are used.

9. NCEP Atmospheric Forcing Changes FW HF • SO is south of 40°S • Increased heat and freshwater fluxes into the SO • Increased zonal windstress Taux Tauy

10. Zonal averaged windstress Taux max Latitude of Taux max • Stronger westerly winds which migrate south

11. Comparison between natural and anthropogenic carbon uptake Natural carbon • Natural carbon uptake dominates the climate variability response • Opposite response of natural and anthropogenic carbon uptake Anthropogenic carbon Total 1948 Tau Hflx Fflx

12. Correlation with the Annual mean South Ocean CO2 Fluxes • The correlation coefficient and regression value of the changes in the annual mean SO anthropogenic carbon uptake versus the change in the annual mean SO natural carbon fluxes. The changes in the carbon fluxes are determined by subtracting the fluxes from the 1948 experiment. The regression value gives the change in the anthropogenic carbon fluxes per unit change in the natural carbon fluxes.

13. Changes in export production • Southern Ocean averaged export production • Little simulated variability in export production (< 10%) • Affect of wind changes on export production for year 2002 - 1948 (total exp.) Total 1948 Tau Hflx Fflx

14. Ventilation of the Southern Ocean • Annual averaged surface density • Ocean ventilation based on the outcrop area of water denser than 27.1 (AAIW or denser) • Winds and freshwater fluxes have the largest impact on the ventilation of the Southern Ocean Total 1948 Tau Hflx Fflx

15. Relationship between carbon uptake and ventilation Natural carbon • High correlation with ocean ventilation variability FW HF Winds Total Anthropogenic carbon Ventilation Anomaly

16. Summary • High negative correlation between SO changes in natural and anthropogenic carbon uptake • For recent past and for next several decades the natural response will dominate the SO response • In the Southern Ocean, the response to the different forcing fields is complex • Model response is dominated by changes in ocean ventilation • Not clear that the SO carbon uptake is declining • Southern Ocean simulated reduction in oceanic uptake of carbon is sensitive to the forcing fields used in the ocean simulation (particularly the freshwater flux). Using sea surface salinity restoring would give results consistent with LeQuere etal 2007 • LeQuere et al 2007 conclusion that the SO uptake is declining is pre-mature

17. Detecting Change

18. Saturation of Southern Ocean carbon uptake (LeQuere et al 2007) • Rising CO2 in the atmosphere should drive an increased oceanic uptake of CO2 (red line) • Ocean uptake with climate variability (blue line) Increasing oceanic uptake

19. Trend in Southern Ocean Carbon Uptake • Southern Oceanic uptake is increasing in contrast to Le Quere et al 2007 decline Increased uptake Law et al., Science 2008

20. What can the surface pCO2 measurements tell us about the Southern Ocean Carbon sink? • Trend over years 1980 to 2004

21. pCO2 Trend (ppm/yr): Total and Tau experiments Total Tau

22. Dissolved Oxygen:zonally-averaged Pacific (umol/kg) DO minimum in the upwelled CPDW CPDW is also associated with a Dissolved Inorganic Carbon Maximum

23. Synthetic Inversion of Atmospheric O2 • Simulated sea-air oxygen flux (red line) • Inversion of simulated atmospheric oxygen data (two different networks) • The inversion can reproduce the interannual variability and trend • Dissolved oxygen changes in the ocean interior would also occur if there was an increase in the ACC Law and Matear In prep

24. Recent changes in SO stratification Eddy resolving simulations do not produce an increase In Antarctic Circumpolar Current and an increase in upwelling Bonning et al., Nature Geosciences 2008

25. Simulated ACC Transport Total 1948 Tau Hflx Fflx

26. Summary • High negative correlation between SO changes in natural and anthropogenic carbon uptake • For recent past and for next several decades the natural carbon cycle response will dominate the SO response to climate variability • Southern Ocean carbon response to climate change is not clear because: • the response to the different forcing fields is complex • Increase in Heat and Freshwater Fluxes into the ocean will counter act the increase in winds • How one prescribes the freshwater flux does influence the past variability in SO carbon uptake • Not clear how the SO will respond to a more positive SAM: - Greater Ekman transport or energy channeled into increased eddy activity • Biogeochemical observations of CO2 and oxygen could be used to detect change and determine how the SO responds to a more positive SAM • Thank you! Questions?

27. Thank you, Questions?

28. Box Model Representation of the SO

29. Metzl and Lenton in prep

30. Metzl and Lenton in prep

31. SAZ PF STF Antarctic Zone Southern Ocean Thermohaline Circulation Modified from Speer et al 2000

33. CO2 Uptake: Response to SSS restoring