Climate of the Past www.clim-past.net 1814-9324 1814-9332 2 2 2006 10.5194/cp-2-91-2006 http://www.clim-past.net/2/91/2006/ http://www.clim-past.net/2/91/2006/cp-2-91-2006.html http://www.clim-past.net/2/91/2006/cp-2-91-2006.pdf 91 97 2006-09-19 On the importance of initial conditions for simulations of the Mid-Holocene climate H. Renssen hans.renssen@geo.falw.vu.nl E. Driesschaert M. F. Loutre T. Fichefet Faculty of Earth and Life Sciences, Vrije Univ. Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Institut d'Astronomie et de Géophysique G. Lemaître, Univ. Catholique de Louvain, 2 Chemin du Cyclotron, 1348 Louvain-la-Neuve, Belgium Three simulations of the Mid-Holocene (6 ka) climate were performed with the ECBilt-CLIO-VECODE coupled atmosphere-ocean-vegetation model to study the impact of initial conditions. These experiments were forced with identical 6 ka forcings (orbital parameters and atmospheric greenhouse gas concentrations) and differed only in initial conditions. Two simulations were designed as equilibrium experiments, with one being initialized with preindustrial conditions as required by the protocol of the Paleoclimate Modelling Intercomparison Project (PMIP), while in a second experiment early Holocene (9 ka) initial conditions were used. These equilibrium simulations were run for 2100 years with 6 ka forcings. The third experiment was set up as a transient simulation, also starting from early Holocene conditions, but forced with annually changing orbital parameters and greenhouse gas levels. The results of the last 100 years are compared and reveal no statistically significant differences, showing that in this model the initial conditions have no discernible impact on the 6 ka climate. This suggests that the PMIP set-up for 6 ka simulations is valid, with the condition that spin-up phase should be long enough (at least 550 years) to allow the deep ocean to adjust to the change in forcings. Berger, A. L.: Long-term variations of daily insolation and Quaternary climatic changes, J. Atmos. Sci., 35, 2363–2367, 1978. Brovkin, V., Claussen, M., Petoukhov, V., and Ganopolski, A.: On the stability of the atmosphere-vegetation system in the Sahara/Sahel region, J. Geophys. Res., 103, 31 613–31 624, 1998. Brovkin, V., Bendtsen, J., Claussen, M., Ganopolski, A., Kubatzki, C., Petoukhov, V., and Andreev, A.: Carbon cycle, vegetation and climate dynamics in the Holocene: experiments with the CLIMBER-2 model, Global Biogeochem. Cycles, 16, 1139, doi:10.1029/2001GB001662, 2002. Charney, J. G.: Dyamics of deserts and droughts in the Sahel, Quart. J. Roy. Meteorol. Soc., 101, 193–203, 1975. Charney, J. G., Stone, P. H., and Quirk, W. J.: Drought in the Sahara: a biogeophysical feedback mechanism, Science, 187, 434–435, 1975. Crucifix, M., Braconnot, P., Harrison, S. P., and Otto-Bliesner, B.: Second phase of Paleoclimate Modelling Intercomparison Project, EOS, 86, No 28, 12 July 2005. deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., and Yarusinsky, M.: Abrupt onset and termination of the African humid period: rapid climate responses to gradual insolation forcing, Quat. Sci. Rev., 19, 347–361, 2000. Gent, P. R. and McWilliams, J. C.: Isopycnal mixing in ocean general circulation models, J. Phys. Oceanogr., 20, 150–155, 1990. Goosse, H. and Fichefet, T.: Importance of ice-ocean interactions for the global ocean circulation: a model study, J. Geophys. Res., 104, 23 337–23 355, 1999. Hoelzmann, P., Jolly, D., Harrison, S. P., Laarif, F., Bonnefille, R., and Pachur, H.-J.: Mid-Holocene land-surface conditions in northern Africa and the Arabian peninsula: a data set for the analysis of biogeographical feedbacks in the climate system, Global Biogeochem. Cycles, 12, 35–51, 1998. Joussaume, S. and Taylor, K. E.: The Paleoclimate Modelling Intercomparison Project, in: PMIP, Paleoclimate Modeling Intercomparison Project (PMIP): Proceedings of the third PMIP workshop, Canada, 4–8 October 1999, edited by: Braconnot, P., WCRP-111, WMO/TD-1007, p 9–25, 2000. Joussaume, S., Taylor, K. E., Braconnot, P, et al.: Monsoon changes for 6000 years ago: results of 18 simulations from the Paleoclimate Modeling Intercomparision Project (PMIP), Geophys. Res. Lett., 26, 859–862, 1999 Opsteegh, J. D., Haarsma, R. J., Selten, F. M., and Kattenberg, A.: ECBILT: A dynamic alternative to mixed boundary conditions in ocean models, Tellus, 50A, 348–367, 1998. Peltier, W. R.: Ice age paleotopography, Science, 265, 195–201, 1994. Prentice, I. C.: The climate and biomes of Europe at 6000 yr BP: comparison of model simulations and pollen-based reconstructions, Quat. Sci. Rev., 17, 659–668, 1998. Prentice, I. C., Jolly, D., and BIOME 6000 participants: Mid-Holocene and glacial-maximum vegetation geography of the northern continents and Africa, J. Biogeogr., 27, 507–519, 2000. Raynaud, D., Barnola, J.-M., Chappellaz, J., Blunier, T., Indermühle, A., and Stauffer, B.: The ice record of greenhouse gases: a view in the context of future changes, Quat. Sci. Rev., 19, 9–17, 2000. Renssen, H., Brovkin, V., Fichefet, T., and Goosse, H.: Holocene climate instability during the termination of the African Humid Period. Geophys. Res. Lett., 30, 1184, doi:10.1029/ 2002GL016636, 2003. Renssen, H., Goosse, H. Fichefet, T., Brovkin, V., Driesschaert, E., and Wolk, F.: Simulating the Holocene climate evolution at northern high latitudes using a coupled atmosphere-sea ice-ocean-vegetation model, Clim. Dyn., 24, 23–43, 2005a. Renssen, H., Goosse, H., Fichefet, T., Masson-Delmotte, V., and Koç, N.: The Holocene climate evolution in the high-latitude Southern Hemisphere simulated by a coupled atmosphere-sea ice-ocean-vegetation model, Holocene, 15, 951–964, 2005b. Renssen, H., Goosse, H., and Fichefet, T.: Contrasting trends in North Atlantic deep-water formation in the Labrador Sea and Nordic Seas during the Holocene. Geophys. Res. Lett., 32, L08711, doi:10.1029/2005GL022462, 2005c. Renssen, H., Brovkin, V., Fichefet, T., and Goosse, H.: Simulation of the Holocene climate evolution in Northern Africa: the termination of the Afican Humid Period, Quat. Int., 150, 95–102, 2006. Wiersma, A. P., Renssen, H., Goosse, H., and Fichefet, T.: Evaluation of different freshwater forcing scenarios for the 8.2 ka BP event in a coupled climate model, Clim. Dyn., in press, doi:10.1007/s00382-006-0166-0, 2006.