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Constraining Fluxes at the Top:

Constraining Fluxes at the Top: Advances in Quantifying Air-Sea Carbon Dioxide Fluxes during the JGOFS Decade Rik Wanninkhof , NOAA/AOML Richard Feely, NOAA/PMEL commentator. Outline: Developments in measurement techniques for gas transfer

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Constraining Fluxes at the Top:

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  1. Constraining Fluxes at the Top: Advances in Quantifying Air-Sea Carbon Dioxide Fluxes during the JGOFS Decade Rik Wanninkhof , NOAA/AOML Richard Feely, NOAA/PMEL commentator • Outline: • Developments in measurement techniques for gas transfer • Parameterization of gas transfer with wind and other factors • Global pCO2 fields • Impact of parameterization on global fluxes • Approach in this presentation : • Discuss “state-of-the-art” before and during JGOFS • Provide assessment of methods and what remains to be done JGOFS Open Science Conf.

  2. The Abridged History of JGOFS and Gas Exchange “The Executive Committee has reviewed your proposal and concluded that it is not relevant to the objectives of GOFS” Neil Andersen, Oct. 1988 JGOFS Open Science Conf.

  3. The JGOFS Legacy JGOFS greatly enhanced research areas on the “periphery” of the program such as air-sea gas exchange. New techniques on local scale allow us to gain a greater mechanistic understanding of gas fluxes JGOFS Pre-JGOFS Future JGOFS Open Science Conf.

  4. Expression of Air -Sea CO2 Flux • Magnitude • Mechanism • Apply over larger space time domain k-transfer velocity From Sc # & wind speed S – Solubility From SST & Salinity F = k s (pCO2w- pCO2a) = K ∆ pCO2 pCO2a pCO2w From measurements and proxies From CMDL CCGG network JGOFS Open Science Conf.

  5. Pre-JGOFS: opportunistic tracers • The elusive search for relationships between gas exchange and environmental forcing: • Natural radioactivity • 222Radon • 14C • Nuclear bomb-tests • Mass balances of O2 (productivity vs. gas exchange) JGOFS Open Science Conf.

  6. Pre-JGOFS: 222Rn Wind speed Smethie et al. 1985 Status: 222Rn is an underutilized tracer. If measured and interpreted correctly this a a powerful tracer for gas exchange and vertical diffusivity below the mixed layer (Kromer and Roether, 1983) (Lagrangian mode, Many samples, 226Ra & 222Rn)) JGOFS Open Science Conf.

  7. Bomb -14C inventories: constraints over decadal timescales 200 Atlantic ∆14C 0 -100 Indian ∆14C Pacific ∆14C Broecker and Peng, 1983 Broecker et al., 1986 Status: Recent work suggests that 14C inventories might be overestimated by about 25 % (Hesshaimer, 1994 , Peycock, in prep. ). Need a re-assessment using recent WOCE/JGOFS data and regional wind speed and ocean transport models. JGOFS Open Science Conf.

  8. Bomb -14C inventories: status Re-assessment using recent WOCE/JGOFS data and regional wind speed and ocean transport models. Number of samples: (<1500 m) Pre-JGOFS ≈ 2000 JGOFS/WOCE≈10000 JGOFS Open Science Conf.

  9. JGOFS Gas Exchange Highlight #1 - Deliberate tracers- SF6 and 3He The first large scale deliberate perturbation studies: Manipulation rather than passive observations “gas exchange experiments - on demand.” k3He = h/∆t ln(Rt1/Rt2)/ [1-(Sc3He/ScSF6)n] JGOFS Open Science Conf.

  10. Deliberate tracers: Summary Locations: North Sea Georges Bank Equatorial Pacific North Atlantic South Pacific Southern Ocean South Atlantic Southern Ocean Nightingale et al.,GBC 2000 • General trend with wind in synthesis of all results • Strong dependence of wind for individual studies • Offsets between studies JGOFS Open Science Conf.

  11. Deliberate tracers: Status • Robust method • Measure gas transfer on 1-3 day timescales. • Applicable for wide range of locations • Lagrangian marker (“glorified marker buoy”) JGOFS Open Science Conf.

  12. JGOFS Gas Exchange Highlight #2 - Direct CO2 Flux Measurements (micrometeorological techniques) A breakthrough in gas exchange measurements- Measurement of fluxes on same time scale as fluctuations of forcing Eddy Correlation: F = w’ c’ + wc (need fast response sensors) Gradient: F = KC ∂C/∂z ≈ Y k U* z ∂Cav/∂z (need to characterize boundary layer) KC = eddy transfer coef, Y= stability parameter Eddy Accumulation: F= b Sw (Cu(av)-Cd)av)) (need real time valve switching) Sw = st. dev. w JGOFS Open Science Conf.

  13. Direct flux techniques:Issues-Small signal for CO2 Using very small gradients in concentration in the marine atmospheric boundary layer • ∆pCO2(3 m-20 m) ≈ 10-102 parts per billion compared to gradients between air and water of 10-102 parts per million. • The corrections are mostly significantly bigger than the signal. JGOFS Open Science Conf.

  14. Direct flux techniques: Issues - signal to noise • Co-Variance Measurements only possible in certain regions • ∆pCO2 air-water > 40 µatm • Flux > 2 mol m-2 yr-1 (global average ≈ • 0.5 mol m-2 yr-1) Broecker, W.S., J.R. Ledwell, T. Takahashi, L.M. R. Weiss, L. Memery, T.-H. Peng, B. Jähne, and K.O. Münnich, Isotopic versus micrometeorologic ocean CO2 fluxes: A serious conflict, J. Geophys. Res., 91, 10517-10527, 1986. JGOFS Open Science Conf.

  15. Direct flux techniques: Issues- two-way flux Gross (1-way) fluxes are about 50 times greater than net fluxes Eddy Correlation, w’c’ distribution Miami ASIST, Donelan pers. comm. • Need high frequency measurements • Small biases can change direction of flux JGOFS Open Science Conf.

  16. JGOFS Gas Exchange Implementation - Gas Ex-98 • Improved Technology • motion correction • CO2 measurements • Large Fluxes • Independent validations • Dual tracer • Mass Balance JGOFS Open Science Conf.

  17. Direct flux techniques: Results - • An order of magnitude more open ocean measurements in 1 month than in the previous 30 years • Scatter a factor of 2 • Good correspondence with • previous estimates and • relationships between gas • exchange and wind speed McGillis, pers. com. JGOFS Open Science Conf.

  18. Direct flux techniques: Results - parameterization with forcing (windspeed) The binned results follow the canonical square wind speed relationships but show bias at low and high winds suggesting a lower dependence at low winds and higher dependence at high winds. These results would not have been obvious for techniques averaging over longer time periods. JGOFS Open Science Conf.

  19. Direct flux techniques: Issues- • WHOI/ETL co-variance measurements have an uncertainty of about 1.5 mol m-2 yr-1 or 20% after bin averaging. • There appears to be a small bias at high winds of about 0.5 mol m-2 yr-1 • The current precision is largely limited by the measurements of w’ • Measurement should be done in process mode at one location • We cannot do good interpretation of chemical and biological measurements without good physical measurements JGOFS Open Science Conf.

  20. Gas Exchange and Environmental Forcing: Pre-JGOFS Why?- Applying results to larger regions Wind-wave tank results: 3 regimes • Smooth surface- k =f(u*) Sc -2/3 • Wavy surface k = f(u*) Sc -1/2 • Breaking waves k = f(bubble, u*) Sc -x All bets are off Issues: • Scaling • Tank (wall artifacts) JGOFS Open Science Conf.

  21. Gas Exchange and Environmental Forcing: Tanks Example: tank artifact- fetch Wind-wave and other laboratory tanks are ideal systems to study processes but absolute magnitudes generally cannot be applied to oceans Ocean estimate 100-m long tank 30-m long tank 10-m long tank Wanninkhof et al., 91, Donelan and Drennan JGOFS Open Science Conf.

  22. Gas Exchange and Environmental Forcing: Oceans • Most relationships of gas exchange use a subset of this data along with some a priori assumptions • The choice of relationships has a large effect on estimates of global air-sea CO2 flux JGOFS Open Science Conf.

  23. Gas Exchange and Environmental Forcing: Wind Liss & Merlivat, 1985: Assumed a functional relationship from wind-wave tanks and scaled it to a lake experiment at intermediate winds JGOFS Open Science Conf.

  24. Gas Exchange and Environmental Forcing: Wind Smethie et al. 1986: Linear fit through Radon TTO data forcing it through non-zero intercept based on wind-wave tank: low winds very little exchange (≈ 0) . High winds : no clue Use straight line. Later adapted downward through 14C point by Tans et al. 1990 JGOFS Open Science Conf.

  25. Gas Exchange and Environmental Forcing: Wind • Wanninkhof-92: • Gas exchange related to wind • stress ( U2) • Use global 14C constraint • Attempt to account for variability in wind JGOFS Open Science Conf.

  26. Gas Exchange and Environmental Forcing: Wind • Wanninkhof and McGillis, 1999 • Gas exchange related to energy input into waves (u3) • Scales as whitecap coverage (bubble-enhanced exchange) • Attempt to account for variability • Good fit to Gas Ex-98 data • Meets global 14C constraint JGOFS Open Science Conf.

  27. Gas Exchange and Environmental Forcing: Wind • Why are relationships different? • (or why do observations scatter ?) • Experimental uncertainty • Variability in forcing • Other parameters influence air-sea gas transfer JGOFS Open Science Conf.

  28. Gas Exchange and Environmental Forcing: Surfactants • Important in coastal waters • Probably of limited importance in open ocean • * Low surfactant concentrations • * Dispersed by wind Frew et al., 1997 JGOFS Open Science Conf.

  29. Gas Exchange and Environmental Forcing: Bubbles • Gas transfer is linearly related to whitecap coverage • Lower solubility gases more affected • Surfactants influence bubble mediated exchange Asher et al. , 1996 JGOFS Open Science Conf.

  30. Gas Exchange and Environmental Forcing: Remote Sensing Alternate parameterizations of air-sea gas transfer: 1. Whitecap coverage and radiometry: k = km + Wc kb Schluessel, pers com. JGOFS Open Science Conf.

  31. Gas Exchange and Environmental Forcing: Remote Sensing 2. Gas exchange and radar backscatter: Scatterometer and altimeter a. wind wave tank results b. global remote sensing. Frew and Glover, 2001 Issue: Since we infer wind speeds from the same sensors- do we gain anything? Wanninkhof and Bliven, 1991 JGOFS Open Science Conf.

  32. ∆pCO2 fields: Pre-JGOFS F = k s (pCO2w- pCO2a) Overall trends known: * Outgassing at low latitudes * Influx at high latitudes * Spring blooms draw down pCO2 * El Niños decrease efflux But: * Little seasonal information * Limited observations Tans, Fung,& Takahashi, 1990 JGOFS Open Science Conf.

  33. JGOFS Gas Exchange Highlight #4 - ∆pCO2 fields High resolution pCO2 measurements in the Pacific since Eq. Pac-92 Eq Pac-92 process study Cosca et al. in press JGOFS Open Science Conf.

  34. JGOFS Gas Exchange Highlight #4 - ∆pCO2 fields:Takahashi climatology Monthly changes in pCO2w JGOFS Open Science Conf.

  35. ∆pCO2 fields: Status • More measurements (≈10 degree • spacing per month) • Mechanistic understanding of • controls • Interpolation/modeling/assimilation • schemes to increase sample density • in time and space JGOFS Open Science Conf.

  36. Fluxes: JGOFS- Global monthly fluxes Combining pCO2 fields with k: F = k s (pCO2w- pCO2a) • On first order flux and ∆pCO2 maps do not look that different JGOFS Open Science Conf.

  37. Fluxes: Status • Parameterization of gas exchange • Methods of increasing pCO2 coverage • Improved atmospheric constraints-the oceanic imprint on the atmosphere • (O2/N2, 13C/12C) • On what time and space scales must fluxes be determined? • (averaging of non-linear and cross-correlated quantities). • Effect of averaging: Annual uptake using global wind speed distribution (Rayleigh distribution) k = 0.39 U2 = -1.95 Pg C/ yr k = 1.09U - 0.333U2 + 0.078U3 = -2.96 Pg C/yr Annual uptake using local monthly wind speed distribution k = 0.31 U2 *R2= -1.85 Pg C/ yr k = 0.0283 U3 *R3 = -2.33 Pg C/ yr JGOFS Open Science Conf.

  38. Fluxes: Status Do different parameterizations between gas exchange and wind matter? Global uptakes Liss and Merlivat-83: 1 Pg C yr-1 Wanninkhof-92: 1.85 Pg C yr-1 Wanninkhof&McGillis-98: 2.33 Pg C yr-1 Zemmelink-03: 2.45 Pg C yr-1 Yes! Global average k (=21.4 cm/hr): 0.3 Pg C yr-1 We might not know exact parameterization with forcing but forcing is clearly important JGOFS Open Science Conf.

  39. Where do we go from here? Courtesy K. Johnson, SoFex, 2001 JGOFS Open Science Conf.

  40. Big issues require big programs Big projects require big programs Small projects require big programs Build upon the knowledge gleaned in JGOFS to tackle global biogeochemistry issues in a coordinated fashion Ocean Projects in IGBP II JGOFS Open Science Conf.

  41. “Big programs require big people (heros)” Follow the footsteps of the JGOFS planners, organizers, and implementers to make the future programs a success. JGOFS Open Science Conf.

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