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Documentation (CT2008)
Biosphere Oceans Observations Fires Fossil Fuel TM5 Nested Model Assimilation
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1.   Introduction
The oceans play an important role in the Earth's carbon cycle. They are the largest long-term sink for carbon and have an enormous capacity to store and redistribute CO2 within the system. Oceanographers estimate that about 48% of the CO2 from fossil fuel burning has been absorbed by the ocean [Sabine et al., 2004]. The dissolution of CO2 in seawater shifts the balance of the ocean carbonate equilibrium towards a more acidic state (i.e., with a lower pH). This effect is already measurable [Caldeira and Wickett, 2003], and is expected to become an acute challenge to shell-forming organisms over the coming decades and centuries. Although the oceans as a whole have been a relatively steady net carbon sink, CO2 can also come out of the oceans depending on local temperatures, biological activity, wind speeds, and ocean circulation. These processes are all considered in CarbonTracker, since they can have significant effects on the ocean sink. Improved estimates of the air-sea exchange of carbon in turn help us to understand variability of both the atmospheric burden of CO2 and terrestrial carbon exchange.

2.   Detailed Description
Oceanic uptake of CO2 in CarbonTracker is computed using air-sea differences in partial pressure of CO2 inferred from ocean inversions, combined with a gas transfer velocity computed from wind speeds in the atmospheric transport model.

The long-term mean air-sea fluxes, and the uncertainties associated with them, derive from the ocean interior inversions reported in Jacobson et al. [2007]. These ocean inversion flux (OIF) estimates are composed of separate preindustrial (natural) and anthropogenic flux inversions based on the methods described in Gloor et al. [2003] and biogeochemical interpretations of Gruber, Sarmiento, and Stocker [1996]. The uptake of anthropogenic CO2 by the ocean is assumed to increase in proportion to atmospheric CO2 levels, consistent with estimates from ocean carbon models.

For CarbonTracker 2008, contemporary pCO2 fields were computed by summing the preindustrial and anthropogenic flux components from inversions using five different configurations of the Princeton/GFDL MOM3 ocean general circulation model [Pacanowski and Gnanadesikan, 1998], then dividing by a gas transfer velocity computed from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA40 reanalysis. There are two small differences in first-guess fluxes in this computation from those reported in Jacobson et al. [2007]. First, the five OIF estimates all used Takahashi et al. [2002] pCO2 estimates to provide high-resolution patterning of flux within inversion regions (the alternative "forward" model patterns were not used). To good approximation, this choice only affects the spatial and temporal distribution of flux within each of the 30 ocean inversion regions, not the magnitude of the estimated flux. Second, wind speed differences between the ERA40 product used in the offline analysis and the ECMWF operational model used in the online CarbonTracker analysis result in small deviations from the OIF estimates.

Gas transfer velocity in CarbonTracker is parameterized as a quadratic function of wind speed following Wanninkhof [1992], using the formulation for instantaneous winds. Gas exchange is computed every 3 hours using wind speeds from the ECMWF operational model as represented by the TM5 atmospheric transport model. Other than the smooth trend in anthropogenic flux assumed by the OIF results, interannual variability (IAV) in the first guess ocean flux comes entirely from wind speed effects on the gas transfer velocity. This is because the ocean inversions retrieve only a long-term mean and smooth trend.

The initial release of CarbonTracker (2007) used climatogical estimates of CO2 partial pressure in surface waters from Takahashi et al. [2002] to compute a first-guess air-sea flux. This air-sea pCO2 disequilibrium was modulated by a surface barometric pressure correction before being multiplied by a gas-transfer coefficient to yield a flux. Starting with CarbonTracker 2007B and in this 2008 release, the air-sea pCO2 disequilibrium is imposed from analysis of the OIF results, with short-term flux variability derived from the atmospheric model wind speeds via the gas transfer coefficient. The barometric pressure correction has been removed so that climatological high- and low-pressure cells do not bias the long-term means of the first guess fluxes. In either method, the first-guess fluxes have no interannual variability (IAV) due to pCO2 changes, such as those that occur in the tropical eastern Pacific during an El Niño. In CarbonTracker, this flux IAV must be inferred from atmospheric CO2 signals.

Air-sea transfer is inhibited by the presence of sea ice, and for this work fluxes are scaled by the daily sea ice fraction in each gridbox provided by the ECMWF forecast data.

The first-guess fluxes described here are subject to scaling during the CarbonTracker optimization process, in which atmospheric CO2 mole fraction observations are combined with transport simulated by the atmospheric model to infer flux signals. In this process, signals of terrestrial flux in atmospheric CO2 distribution can be erroneously interpreted as being caused by oceanic fluxes. This flux "aliasing" or "leakage" is evident in some regions as a change in the shape of the seasonal cycle of air-sea flux. Differences between CT2008 posterior air-sea fluxes and those of the OIF prior fluxes are minor, but do constitute an issue that we will be investigating in the future.

3.   Further Reading

  • NOAA Pacific Marine Environmental Laboratory (PMEL)
  • Ocean Acidification
  • Caldeira, K., and M. E. Wickett (2003), Anthropogenic carbon and ocean pH, Nature, 425365-365.
  • Gloor, M., N. Gruber, J. Sarmiento, C. L. Sabine, R. A. Feely, and C. Röbeck (2003), A first estimate of present and preindustrial air-sea CO2 flux patterns based on ocean interior carbon measurements and models, Geophysical Research Letters, 30, , 10.1029/2002GL015594.
  • Gruber, N., J. L. Sarmiento, and T. F. Stocker (1996), An improved method for detecting anthropogenic CO2 in the oceans, Global Biogeochemical Cycles, 10, , 809-837.
  • Jacobson, A. R., N. Gruber, J. L. Sarmiento, M. Gloor, and S. E. Mikaloff Fletcher (2007), A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: I. Methods and global-scale fluxes, Global Biogeochemical Cycles, 21, doi:10.1029/2005GB002556.
  • Pacanowski, R. C., and A. Gnanadesikan (1998), Transient response in a z-level ocean model that resolves topography with partial cells, Monthly Weather Review, 126, 3248--3270.
  • Sabine, C. L., R. A. Feely, N. Gruber, R. M. Key, K. Lee, J. L. Bullister, R. Wanninkhof, C. S. Wong, D. W. R. Wallace, B. Tilbrook, F. J. Millero, T. H. Peng, A. Kozyr, T. Ono, and A. F. Rios (2004), The oceanic sink for anthropogenic CO2, Science, 305, 367-371.
  • Takahashi, T., S. C. Sutherland, C. Sweeney, A. P. N. Metzl, B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely, C. Sabine, J. Olafsson, and Y. Nojiri (2002), Global air-sea CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects, Deep-Sea Research II, 49, , 1601--1622.
  • Wanninkhof, R. (1992), Relationship between wind speed and gas exchange over the ocean, Journal of Geophysical Research, 97, 7373--7382.