Radiocarbon Tracer Ocean Model Data: Readme file --------------------------------------------------------------------- NOAA Paleoclimatology Program and World Data Center A - for Paleoclimatology --------------------------------------------------------------------- NOTE: PLEASE CITE ORIGINAL REFERENCE WHEN USING THIS DATA!!!!! NAME OF DATA SET: Radiocarbon Tracer Ocean Model Data LAST UPDATE: 9/2000 (Original Receipt by WDCA Paleo) CONTRIBUTORS: Tom Guilderson, Harvard University and Lawrence Livermore National Laboratory IGBP PAGES/WDCA CONTRIBUTION SERIES NUMBER: 2000-061 SUGGESTED DATA CITATION: Guilderson, T.P., 2000, Radiocarbon Tracer Ocean Model Data, IGBP PAGES/World Data Center A-Paleoclimatology Data Contribution Series #2000-061. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA. ORIGINAL REFERENCE: Guilderson, T.P., Caldeira, K., and Duffy, P.B., 2000, Radiocarbon as a diagnostic tracer in ocean and carbon cycle modeling. Global Biogeochem. Cycles Vol. 14 , No. 3 , p. 887 (1999GB001192) GEOGRAPHIC REGION: Global PERIOD OF RECORD: 1972-1978 AD LIST OF FILES: readme.tracer14c.txt (this file), tracer14c.data.txt (tab-delimited ASCII format). DESCRIPTION: Radiocarbon Tracer Ocean Model Data Appendix 1, Guilderson et al. (2000): GEOSECS and model comparison of the penetration and uptake of bomb-radiocarbon. For definitions see Broecker et al., Global Biogeochem. Cycles, 9, 263-288, 1995. Penetration depth in meters, DD amplitude is in per mil ("), and specific column inventory (CI: 10E9 atoms-cm-2) of bomb 14C. These are equivalent model grid-box station comparisons. MODEL DESCRIPTION: Ocean model results are from the Lawrence Livermore National Laboratory's enhanced variant of the Geophysical Fluid Dynamics Laboratory Modular Ocean Model [Pacanowski et al., 1991]. In the runs presented here, the model was configured with 23 layers in the vertical, 7 of which were in the upper 300 m. The model includes the equation of state as well as equations for momentum, continuity, and tracer transport. Convection is represented by an adjustment scheme which mixes vertically adjacent grid cells when the potential density of the overlying cell exceeds that of the underlying one. The model has lightly smoothed bathymetry at a resolution of 2° latitude x 4° longitude. It represents flow through all major straits except for the Strait of Gibraltar, which is accounted for with a source of salt at the appropriate depth horizon. The simulations presented here include the "Gent-McWilliams" eddy parameterization [Gent and McWilliams, 1990]. Coefficients of vertical diffusivity are prescribed and depend on depth. Diffusivities increase from 0.2 cm2-sec-1 at the ocean surface to 1.3 cm2-sec-1 at the ocean bottom. In addition to treating the physical ocean circulation, the model also calculates concentrations and fluxes of the individual carbon isotopes. The model contains a simple "Redfield" biology model [Najjar et al., 1992] or more appropriately a biological chemical flux model with phosphate as the limiting nutrient. Fixation of silica by opal producers (diatoms) is allowed to outcompete calcium carbonate fixation (coccolithophorids), and as a consequence, the organic carbon to calcium carbonate rain ratio is not fixed a priori [e.g., Maier-Reimer, 1993]. Alkalinity is conserved and deep-sea carbonate dissolution occurs in waters that are undersaturated with respect to calcite or aragonite [Archer, 1991; Maier-Reimer, 1993]. Following the Ocean Carbon Model Intercomparison Project (OCMIP) standardization, gas exchange uses the Wanninkhof [1992] wind speed dependence and the solubility of Weiss [1974] and the model is forced with monthly climatological winds [Hellerman and Rosenstein, 1983]. This version of the model does not include an interactive sea-ice model. Instead, we used the climatological distribution of sea ice and sea ice inhibition of gas exchange [Zwally et al., 1983]. Surface salinities and temperatures are relaxed to the observed monthly climatology [Levitus and Boyer, 1994] with a time constant of 60 days. The model is spun-up in an accelerated mode; it was run for 3300 surface years with an acceleration factor of 7.5 (equivalent to ~25,000 years) in the deepest model level. We used a constant zero permil atmosphere and PCO2 of 280 ľatm, to allow for deep ocean 14C equilibration. This seemingly long spin-up is necessary because waters outcropping in the Southern Ocean must be returned to the deep ocean prior to complete isotopic equilibration with the atmosphere. After being spun-up, the model was then run with evolving atmospheric PCO2 and D14C starting in 1765 as documented by archives and observational networks [Boden et al., 1993]. Similar to Toggweiler et al., [1989], three zonal atmospheric bands are used for the postbomb atmospheric forcing.