How well do models simulate the carbon cycle? What processes need to be gotten right? What results from carbon cycle models are robust, and which ones are very sensitive?

The Ocean Carbon Model Intercomparison Project was designed to address these questions by developing a standard protocol for various biogeochemical cycles that could be run in a range of models. Anand Gnanadesikan at GFDL has played a significant role in the joint Princeton/GFDL contribution to OCMIP, in collaboration with Jorge Sarmiento's group at Princeton. In particular this work has centered around a suite of models in which the lateral and vertical diffusion were changed together so as to preserve the shape of the thermocline (Gnanadesikan et al., 2002). Additional runs were also made in which the boundary conditions in the Southern Ocean were changed so as to enhance ventilation of the deep waters (Gnanadesikan et al., 2004).

Important results from runs conducted as part of OCMIP include:

• The saturation state of calcite may change during the 21st century, with serious effects for marine organisms. (Orr et al., 2005)
• The rate of biological cycling is extremely sensitive to vertical diffusion. Increasing the vertical diffusion from 0.15 cm2/s to 0.6 cm2/s causes a doubling of productivity. Previous models have tended to have high rates of vertical diffusion and so may overestimate the rate of biological cycling (Gnanadesikan et al., 2002,2004).
• The rate of uptake of anthropogenic carbon dioxide is sensitive to changes in vertical mixing, but the range across models is much smaller (30%) than for biological production. (Matsumoto et al. 2004)
• The total rate of uptake of anthropogenic carbon dioxide is sensitive to Southern Ocean winds and eddies. (Mignone et al., 2005)
• Some studies have proposed sequestering carbon dioxide directly into the deep ocean. The location and efficiency of CO2 outgassing is strongly controlled by the vertical diffusion for shallow injection, and by the level of Southern Ocean convection for deep injection (Mignone et al., 2004).

  

  Average surface radiocarbon as a function of latitude compared with 7 OCMIP models run at GFDL/Princeton. (a) Models reported in Gnanadesikan et al. (2002) and represent changes in internal mixing. Models are denoted by vertical mixing+lateral mixing, so that HH has high mixing both in the vertical and lateral. Note that the high mixing models have too little radiocarbon in the tropics and that all models are too high in the Southern Ocean. (b) Two models reported in Gnanadesikan et al. (2004) in which surface boundary conditions are changed.

Papers resulting from the OCMIP work to date:

Doney, S.C., K. Lindsay, K. Caldeira, J.M. Campin, H. Drange, J.C. Dutay, M. Follows, Y. Gao, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, G. Madec, E.Maier-Reimer, J.C. Marshall, R.J. Matear, P. Monfray, R. Najjar, J.C. Orr, G.K. Plattner, J. Sarmiento, R. Schlitzer, I.J. Totterdell, M.F. Weirig, Y. Yamanaka, A. Yool, Evaluating global ocean carbon models: The importance of realistic physics, Global Biogeochem. Cycles , GB3017, doi:10.1029/2003GB002150, 2004.

Gnanadesikan, A., R.D. Slater, N. Gruber and J.L. Sarmiento, Oceanic vertical exchange and new production: A comparison of models and data, Deep Sea Res. II. , 49: 363-401, 2002.

Gnanadesikan, A., J.P. Dunne, R.M. Key, K. Matsumoto, J.L. Sarmiento, R.D. Slater and P.S. Swathi, Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity, Global Biogeochem. Cyclesi , GB4010, doi:10.1029/2003GB002097, 2004.

Matsumoto, K., J.L. Sarmiento, R.M. Key, J.L. Bullister, K. G. Caldeira, J.-M. Campin, S. C. Doney, H. Drange, M. J. Follows, Y. Gao, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, K. Lindsay, F. Louanchi, E. Maier-Reimer, R. J. Matear, P. Monfray, A. Mouchet, R. G. Najjar, J.C. Orr, G.-K. Plattner, C. L. Sabine, J. L. Sarmiento, R. Schlitzer, R.D. Slater, P.S. Swathi, I. Totterdell, M.-F. Weirig, M. E. Wickett, Y. Yamanaka, and A. Yool, Evaluation of ocean carbon cycle models with data-based metrics, Geophys. Res. Lett., 31, L07303, doi:10.1029/2003GL018970, 2004.

Mignone, B., J.L. Sarmiento, R.D. Slater and A. Gnanadesikan, Sensitivity of sequestration efficiency to mixing processes in the global ocean, Energy, 29, 1467-1478, 2004.

Orr,.J.C., O. Aumont, L. Bopp, S.C. Doney, V.J. Fabry, R.M. Feely, M. Follows, A. Gnanadesikan, A. Ishida, F. Joos, R.M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R.G. Najjar, G.K. Plattner, C.L. Sabine, J.L. Sarmiento, R. Schlitzer, R.D. Slater, I. Totterdell, M.F. Weirig, Y. Yamanaka, A. Yool, Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms, Nature, 437,681-686, 2005.

Mignone, B., A. Gnanadesikan, J.L. Sarmiento and R.D. Slater, Central role of Southern Hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon, Geophys. Res. Lett., doi:10.1029/2005Gl024464, 2006.

Marinov, I., A. Gnanadesikan, J.R. Toggweiler, and J.L. Sarmiento, The Southern Ocean Biogeochemical Divide, Nature, 441,964-967, 2006.

Papers in prep based on OCMIP