In order to constrain CO2 uptake by the ocean and terrestrial biosphere on seasonal time scales, it is essential to quantify the ocean-atmosphere and atmosphere-biosphere CO2 fluxes. Accurate knowledge of CO2 uptake by the world's oceans is critical for predicting future atmospheric CO2 levels and ocean acidification. Oceanic CO2 uptake together with the atmospheric growth rate and fossil fuel emissions also offer a robust constraint on terrestrial CO2 fluxes, which are singularly difficult to quantify due to their spatial and temporal variability. The principle of determining air-sea CO2 fluxes is straightforward. The flux, FCO2, is defined as:
FCO2= k α ΔpCO2
where k is the gas transfer velocity, a is the solubility of CO2, which is well quantified as a function of temperature and salinity [Weiss, 1974], and ΔpCO2 is difference in partial pressure of CO2 between the ocean and the atmosphere. The ΔpCO2 can be measured from a variety of platforms such as ships of opportunity [Cooper et al., 1998], drifters [Hood et al., 2001], and moorings [Friederich et al., 1995]. Currently planning is underway to optimally utilize these platforms, together with modeling, extrapolation algorithms, and remote sensing to determine global pCO2 fields on seasonal time scales. The first global climatology of ΔpCO2 has been produced by Takahashi et al. , and updated by Takahashi et al. , based on more than 20 years of data. Our long-term goal will be to obtain seasonal fluxes on annual time scale to discern interannual variability.
Results of 3He/SF6 dual tracer experiments conducted in coastal and shelf areas, as well as the open ocean. Solid symbols = open ocean experiments; open symbols = coastal and shelf experiments. The North sea data are from Nightingale et al. [2000b], FSLE data from Wanninkhof et al. , GasEx-98 from [McGillis et al., 2001], Georges Bank data are from Wanninkhof et al. , and reanalyzed by Asher and Wanninkhof , IRONEX data are from Nightingale et al. [2000a], SOFeX data are from Wanninkhof et al., , and SAGE data are from Ho et al . Also shown are wind speed gas transfer parameterizations of Wanninkhof and McGillis , Wanninkhof , Liss and Merlivat , Nightingale et al. [2000b], and Ho et al. . The SAGE and SOFeX data have been corrected for wind speed enhancement, assuming a quadratic relationship. The differences between these studies and scatter within each study suggest that relating gas transfer velocity to wind speed does not capture all the factors that affect gas exchange.
A US-led program to use process studies to improve quantification of air-sea CO2 fluxes and the gas transfer velocity was initiated in 1998. So far, two large-scale studies have been conducted, one in the North Atlantic (GasEx-98) and one in the Equatorial Pacific (GasEx-2001). One of the main goals of these efforts is to be able to quantify transfer velocities on regional scale from remote sensing such that, combined with regional ΔpCO2, global air-sea CO2 fluxes can be determined. A systematic approach will be followed to accomplish this goal that involves the following steps:
- Make direct flux measurements in the field to obtain short-term local CO2 fluxes/gas transfer velocities.
- Reconcile direct CO2 flux measurement with integrated measurements of gas transfer velocities using 3He/SF6 dual tracer technique.
- Understand the mechanisms controlling ocean mixed layer pCO2 on short time and space scales.
- Elucidate the forcing functions controlling gas transfer.
- Relate forcing functions to parameters that can be detected by remote sensing.
GasEx I: GasEx-98
GasEx-98 was conducted in a warm core eddy in the North Atlantic in 1998. During this study, direct CO2 flux measurement were made successfully for the first time in the open ocean, while factors controlling short time scale pCO2 variations were determined. The CO2 direct co-variance measurements gave robust estimates of gas exchange up to winds of 15 m s-1, and the measurements were in agreement with the flux profile measurements of CO2 and DMS in the marine boundary layer. The direct flux results were validated with the waterside measurements using the 3He/SF6 dual-deliberate tracer technique. An analysis of the CO2 co-variance data along with global constraints using bomb 14C invasion supports a cubic relationship with wind speed [Wanninkhof and McGillis, 1999; McGillis et al, 2001]. If such a relationship holds over most of the ocean it will have a major impact on estimates of CO2 uptake based on air-sea CO2 disequilibria by increasing oceanic CO2 uptake by 40 % due to lower exchanges in the outgassing regions that have lower winds and higher uptake in the windier high latitudes using a quadratic dependence.
GasEx II: GasEx-2001
The next process study (GasEx-2001)took place in the Equatorial Pacific in 2001. It offered a contrast with GasEx-98 in that the region is a strong CO2 source with relatively low wind speeds, while the North Atlantic has relatively higher winds and a large CO2 sink. Measurements of CO2 fluxes were made using the co-variance method and CO2 and DMS fluxes using the profile flux method (see publications in the GasEx-2001 JGR special issue). The direct flux results were reconciled with mass balance of TCO2 [Sabine et al., 2004]. During the experiment, processes controlling air-sea CO2 exchange under low wind conditions were examined. Diurnal changes in biogeochemistry of carbon dioxide and air-sea gas exchange rates were also determined. Furthermore, GasEx-2001 involved intensive measurements of biogeochemical and physical processes in the lower atmosphere and surface ocean mixed layer using a variety of innovative platforms (e.g., ASIS, LADAS, SPIP, SkinDeep). Studies included surfactant concentrations, surface wave roughness, and surface infrared imagery.
Results of Previous GasEx
The results of the two process studies differed. While GasEx-98 showed a strong wind speed dependence of gas transfer velocities at high wind speeds and a weaker dependence at lower wind speeds, GasEx-2001 showed the opposite, albeit over a narrower and lower wind speed range. These differences are attributed to the fact that factors other than wind-generated turbulence affect gas exchange over the open ocean. The high gas transfer velocities measured during GasEx-98 could be caused by bubble enhanced turbulence and exchange. The results from GasEx-2001 showing enhanced gas fluxes could be attributed to increased near-surface shear due to the diurnal heating effects (see e.g. McGillis et al.,  and other publications in the GasEx-2001 JGR special issue). This difference between GasEx-98 and 2001 raises the intriguing question of whether effects other than those that are parameterized by wind speed have a significant effect on gas transfer over the ocean, and calls for additional process studies in other regimes.
While the goal is straightforward, the implementation relies heavily on innovative measurements. Direct flux observations from moving platforms are extraordinarily difficult to make, and quantifying small-scale variability will require new approaches and new instrumentation. A brief summary of previous gas exchange experiments follows. Southern Ocean GasEx seeks to build on these experiments and extend previous results to a high wind environment in a globally significant region of CO2 flux.
References and Further Readings
- Asher, W. E., and R. Wanninkhof, The effect of bubble-mediated gas transfer on purposeful dual-gaseous tracer experiments, J. Geophys. Res., 103, 10555-10560, 1998.
- Cooper, D.J., A.J. Watson, and R.D. Ling, Variation of PCO2 along a North Atlantic shipping route (U.K. to the Caribbean): A year of automated observations, Mar. Chem., 60, 147-164, 1998.
- Friederich, G.E., P.G. Brewer, F. Chavez, and R. Herlien, Measurement of sea surface partial pressure of CO2 from a moored buoy, Deep-Sea Res., 42, 1175-1186, 1995.
- Ho, D. T., C. S. Law, M. J. Smith, P. Schlosser, M. Harvey, and P. Hill, Measurements of air-sea gas exchange at high wind speeds in the Southern Ocean: Implications for global parameterizations, Geophy. Res. Lett., 33, L16611. doi:10.1029/2006GL026817, 2006.
- Hood, E.M., R. Wanninkhof, and L. Merlivat, Short timescale variations of fCO2 in a North Atlantic warm-core eddy: Results from the Gas- Ex 98 carbon interface ocean atmosphere (CARIOCA) buoy data, J. Geophys. Res., v. 106, p. 2561-2572, 2001.
- Liss, P. S., and L. Merlivat, Air–sea gas exchange rates: Introduction and synthesis, in The Role of Air–Sea Exchange in Geochemical Cycling, edited by P. Buat-Ménard, pp. 113-127, D. Reidel, Dordrecht, 1986.
- McGillis, W.R., W.E. Asher, R. Wanninkhof, A.T. Jessup, and R.A. Feely, Introduction to special section: Air-Sea Exchange, J. Geophys. Res., v. 109, p. doi:10.1029/2004JC002605, 2004.
- McGillis, W. R., J. B. Edson, J. D. Ware, J. W. H. Dacey, J. E. Hare, C. W. Fairall, and R. Wanninkhof, Carbon dioxide flux techniques performed during GasEx 98, Mar. Chem., 75, 267-280, 2001.
- McGillis, W.R., J.B. Edson, C.J. Zappa, J.D. Ware, S.P. McKenna, E.A. Terray, J.E. Hare, C.W. Fairall, W. Drennan, M. Donelan, M.D. DeGrandpre, R.Wanninkhof, and Feely, R. A., Air-sea CO2 exchange in the equatorial Pacific, J. Geophys. Res., v. 109, no. C08S02, p. doi:10.1029/2003JC002256, 2004.
- Nightingale, P. D., P. S. Liss, and P. Schlosser, Measurements of air-sea gas transfer during an open ocean algal bloom, Geophys. Res. Lett., 27, 2117-2120, 2000a.
- Nightingale, P. D., G. Malin, C. S. Law, A. J. Watson, P. S. Liss, M. I. Liddicoat, J. Boutin, and R. C. Upstill-Goddard, In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers, Glob. Biogeochem. Cycle, 14, 373-387, 2000b.
- Sabine, C.L., R.A. Feely, G.C. Johnson, P.G. Strutton, M.F. Lamb, and K.E. McTaggart, A mixed layer carbon budget for the GasEx-2001 experiment. J. Geophys. Res., 109(C8), C08S05, doi: 10.1029/2002JC001747, 2004.
- Takahashi, T., R.A. Feely, R. Weiss, R. Wanninkhof, D.W. Chipman, S.C. Sutherland, and T.T. Takahashi, Global air-sea flux of CO2: An estimate based on measurements of sea-air pCO2 difference, Proc. Natl. Acad. Sci. USA, 94, 8292-8299, 1997.
- Takahashi, T., S.G. Sutherland, C. Sweeney, A.P. Poisson, N. Metzl, B. Tilbrook, N.R. Bates, R. Wanninkhof, R.A. Feely, C.L. Sabine, J. Olafsson, and Y. Nojjiri, Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects, Deep-Sea Res. II, 49, 1601-1622, 2002.
- Wanninkhof, R., Relationship between gas exchange and wind speed over the ocean., J. Geophys. Res., 97, 7373-7381, 1992.
- Wanninkhof, R., W. Asher, R. Weppernig, H. Chen, P. Schlosser, C. Langdon, and R. Sambrotto, Gas transfer experiment on Georges Bank using two volatile deliberate tracers, J. Geophys. Res., 98, 20237-20248, 1993.
- Wanninkhof, R., G. Hitchcock, B. Wiseman, G. Vargo, P. B. Ortner, W. E. Asher, D. T. Ho, P. Schlosser, M.-L. Dickson, M. Anderson, R. Masserini, K. Fanning, and J.-Z. Zhang, Gas Exchange, Dispersion, and Biological Productivity on the West Florida Shelf: Results from a Lagrangian Tracer Study, Geophys. Res. Lett., 24, 1767-1770, 1997.
- Wanninkhof, R., and W.R. McGillis, A cubic relationship between air-sea CO2 exchange and wind speed, Geophys. Res. Lett., 26, 1889-1892, 1999.
- Weiss, R.F., Carbon dioxide in water and seawater: the solubility of a non-ideal gas, Mar. Chem., 2, 203-215, 1974.