Modeling Ocean Biogeochemistry

1. Modeling Ocean Biogeochemistry Galen A. McKinley University of Wisconsin - Madison ASP Summer School - Art of Climate Modeling June 7, 2006

2. What is Ocean Biogeochemistry? • Biology - micro-scale • Chemistry - organic and inorganic • Geology - interactions with solid Earth • Physical interactions • Air-sea exchange; Particle settling rates; Advection, diffusion, mixing • Individual processes and systematics are of key interest

3. Why include biogeochemistry in ocean models? • Carbon Cycle • Ocean carbon sink - past, present, future • Glacial / interglacial change • Trace gas emissions - Atm. chemistry • e.g. Dimethyl Sulfide (DMS): CCN, emitted by phytoplankton, theorized climate feedbacks

4. The ocean has absorbed 48% of anthropogenic CO2 emitted in last 200 yrs(Sabine et al., 2004)

5. Why include biogeochemistry in ocean models? • Carbon Cycle • Ocean carbon sink - past, present, future • Glacial / interglacial change • Trace gas emissions - Atm. chemistry • e.g. Dimethyl Sulfide (DMS): CCN, emitted by phytoplankton, theorized climate feedbacks

6. Temperature proxy

7. Why include biogeochemistry in ocean models? • Carbon Cycle • Ocean carbon sink - past, present, future • Glacial / interglacial change • Trace gas emissions - Atm. chemistry • e.g. Dimethyl Sulfide (DMS): CCN, emitted by phytoplankton, theorized climate feedbacks

8. Outline • Carbon cycle • Observations • Key chemistry • Large-scale processes • Modeling strategies and challenges • Selected results

9. The Carbon Cycle 1980’s estimates from Sarmiento & Gruber (2002)

10. Atlantic (A16) Dissolved Inorganic Carbon (DIC) (umol/kg)

11. Global sea-air CO2 flux Takahashi et al. 2002

12. Air-sea CO2 exchange • Air-sea exchange determined by air-water gradient of the partial pressure of CO2 (pCO2): pCO2 = pCO2air - pCO2water • And surface ocean turbulence • Parameterized with function of wind speed • pCO2water is determined by [H2CO3*],T,S

13. Carbon Chemistry in Seawater • Carbon reacts with water • DIC = Dissolved Inorganic Carbon = [CO2(aq)] + [H2CO3] + [HCO3-] + [CO3=] = [H2CO3*] + [HCO3-] + [CO3=] • At ocean pH, [H2CO3*] only ~0.5% of DIC. Thus, the balance of HCO3- and CO3= is key to setting pCO2water

14. What determines [HCO3-] + [CO3=]? • Both the electronic charge and the carbon content of a parcel • Ocean charge balance = Alkalinity

15. Alkalinity • Major Cations = [Na+] + [K+] + 2[Mg++] + 2 [Ca++] = + 0.606 eq/kg • Major Anions = [Cl-] + 2[SO4=] + [Br-] = -0.604 eq/kg • Global mean, difference is +0.002 eq/kg • Carbon is one of the few elements that can exist as ions of different charge, thus it adjusts to balance the charge

16. Alkalinity • ALK = [Na+] + [K+] + 2[Mg++] + 2 [Ca++] - [Cl-] - 2[SO4=] - [Br-] ≈ [HCO3-] + 2[CO3=] • i.e. a change in alkalinity will alter the distribution of the carbonate ions

17. Simplified balance equations • ALK ≈ [HCO3-] + 2[CO3=] • DIC ≈ [HCO3-] + [CO3=] • i.e. a parcel must both conserve both its charge and its carbon content

18. Rearranging and solving • [CO3=] ≈ ALK - DIC • [HCO3-] ≈ 2DIC - ALK • e.g. for constant DIC: if ALK, [CO3=] and [HCO3-]

19. What happens to [H2CO3*] and pCO2 as [CO3=] and [HCO3-]? • Full equation DIC = [H2CO3*] + [HCO3-] + [CO3=] • As balance shifts away from [CO3=], [H2CO3*] and pCO2water are increased

20. Carbon chemistry: Summary • pCO2 key for air-sea exchange • pCO2water a function of [H2CO3*], T, S • [H2CO3*] is a small part of total DIC, determined by balance with other ions • [HCO3-], [CO3=] set by alkalinity, or charge balance

21. Solubility Pump

22. Temperature influence Mean sea-air CO2 flux pCO2 pCO2

23. Upwelling Atlantic DIC

24. Biological Processes SeaWiFS - NASA

25. Organic Carbon Pump

26. Carbonate pump • Removes DIC to depth, but also reduces ALK • ALK, [CO3=] and [HCO3-] • pCO2water increased

27. Ecosystem Complexity • There are ~20,000 of identified species of phytoplankton in 4 major groups • Picoplankton • Diatoms (silicate shells) • Coccolithophorids (carbonate shells) • Dinoflagellites • Zooplankton - also great variety • Much variability in key aspects • Carbon to Nutrient, Carbon to Chlorophyll ratios • Sinking velocities • Growth rates, mortality rates, etc.

28. Best Modeling Strategy? Simple vs. Complex ?

29. Simple Equations

30. vs. Complex Equations

31. PROS Reduced computational cost More direct understanding of results CONS No species shifts Will it work for past or future climate? More difficult to compare to observations Simple Ecosystem

32. Modeling global CO2 flux (mol/m2/yr) Data MITgcm McKinley et al. 2004

33. PROS More realistic Allows species shifts with climate More direct comparison to data Enhanced process understanding CONS Computational costs increase by 10x’s Many unconstrained parameters More difficult to interpret Complex Ecosystem

34. Surface Chlorophyll Lima & Doney, 2004

35. Biogeochemical model intercomparison: North Pacific

36. Seven independent models * Preindustrial

37. Subtropical cycle: Station ALOHA Data, Takahashi et al. 2005, submitted

38. Station ALOHA pCO2 variability

39. High-latitude seasonal cycle: Kuril region

40. Modeling Challenges • Lack of data constraints • Ecosystem parameters • Variability at large-scale • High latitudes • Alkalinity / Freshwater fluxes • Remineralization controls • Gas exchange

41. Summary • Biogeochemistry of the carbon cycle is infinitely complex • How much complexity must be modeled? • Ongoing debate • Depends on timescales and questions of interest

42. Suggested reading • Text: Sarmiento and Gruber (2006) Ocean Biogeochemical Dynamics • Complex ecosystem models • Fasham et al. 1993 • Moore et al. 2002a,b; Lima and Doney 2004 • Dutkiewicz et al. 2005 • Simpleecosystem models • Najjar et al. 1992 • McKinley et al. 2004 • Dunne et al. 2005